let's discuss backpressure!

[bcrazy]

I know that if you have a too free exhaust you can drag the fuel/air mixture out of the cylinders and thereby reducing performance. Let's say you wan't to build a free flowing exhaust for a NA volvo red block. All the cams for the red block are mild designs with little overlap. (except perhaps the H and K) On the B230FB that i will soon build an exhaust for the exhasut duration and lift is less that that of the intake. So if you have an extremely good flow exhaust with a good header with good scavengin you still wouldn't pull very much of the air/fuel out of the cylinders because there is so little overlap? In my experience wiht vw aircooled engine i have learned that as little backpressure as possible gives the best power. This applies to totally stock beetle engines with less than 60 hp and a tuned 2.0L producing 150 hp. More backpressure produces more heat and less power. At least this i what i have experienced over the years. Anyone who has any input? If I am correct when you look at NA racing engines the all run free exhaust, or close to it after the collector?

[EricF]

Well, it depends on where you want to make your power, and how concerned you are with torque down low. If I made exhaust for an NA car (trying to design one for my friend's Altima right now), I would run relatively small pipe size (2.25" for a red block?), no cat (God bless Florida emission regulation.. or lack thereof), and a high-flowing dynomax turbo or magnaflow muffler. This setup should yield decent low-end power on red blocks, and good mid and upper range... Low turbulence and high velocity due to the lack of major obstructions and smaller piping size... Though match your cam profile, if you run a K cam or similarly aggressive, higher-revving cam, aim for making the power higher in the rev range, i.e. bigger piping etc...

There's some basic stuff, I'll let the more seasoned people delve deeper into things.

[Bready]

Not sure how you are dragging the fuel air mixture out of the cylinders as a result of a free flowing exhaust. I think there is some confusion with improperly tuned manifold or header scavenging and cam design - rather than an actual exhaust issue (and I think for purposes of aftermarket exhaust applications for most on the board we are talking 'cat back' scenarios).

The issues addressed in both Bcrazy and Erics posts seem to be more cam and manifold related specs than they are exhaust design, which is pretty easy to confuse - not slagging anyone here :wave: .

A short thought on the matter of the need for back pressure: I would like to see ANY dyno chart that demonstrates back pressure being beneficial in any exhaust design scenario that we are likely to be employing.

The need for back pressure is one of the great folk-wisdoms of tuning.

"Back pressure in an exhaust system is evil" - Corky Bell.

JB

[245gti]

[quote:07fffe8f12]"Back pressure in an exhaust system is evil" - Corky Bell. [/quote:07fffe8f12]

Remember, Corky was writing strictly with regards to turbocharged engines, after the turbo. That's not what we're talking about here....

Backpressure and scavenging ability can easily get confused. If you build a system with good scavenging ability, you'll have a system with some backpressure. If you build a system with no backpressure, you'll end up with a system with no scavenging ability. The trick is to find the fine balance between the two. The bottom line is on a normally aspirated engine, too big results in poor performance....practical experience has told me that.

I'm sure everyone has differing opinions on this one....

[Bready]

Fair enough Dale.

Can you speak a bit about the scenarios where you have found oversized exhaust (cat back) degrading performance? Think your experiences would be welcome additions to the thread - at least from this end.

Best

JB

[hayzcustoms]

i have a red block na. i have tried many setups but found keeping the stock piping size, gutting the cat, piping through the first muffler and putting a high flow muffler on the rear with the stock tail pipe too work quite well. i used a summit muffler as it was cheap and i perfer the slightly louder tone. but that is just my opinion.

[245gti]

One of the T'Bricks members here in Calgary (Volvo88) has a beautiful '87 244. Shortly after he got it he put a high flow down pipe and full length 3" exhaust system. His first complaint was that he didn't think it had as much power as the original factory system although it was free flowing and relatively loud. Fortunately (for him) a shop crushed the system when they put it on a hoist. He got it replaced with the factory downpipe and a 2 1/4" (maybe 2 1/2"?) system. He was amazed at the difference it made. Much more power. Even more so than the factory system. I may be a little wrong with my dimensions simply because it wasn't my car but I do know that when he went to the smaller system his performance improved drastically. It didn't take any more evidence than that to convince me that bigger isn't necessarily better in a normally aspirated application.

There may be a certain point in the length of the system where the size will no longer be a factor. Scavenging has much to do with size and length of header runners but the size of the system beyond that may have some substantial impact as well. I'm sure there are others who will have some valuable input here as well....

[Bishop]

Cylinder Scavenging, backpressure etc all has to do with manifold/extractor/header design, once your past that point, you want as much flow as possible even in an NA car.
What some seem to overlook though, is that often header design needs to extend to well under the car depending on where in the rpm band you want to make good power.

[Bready]

Ya - Bishop is echoing my perspective as well.

I can see where going with too large a pipe straight out of the NA manifold may impact scavenging and alter the OEM exhaust tuning (I think I recall that OEM Volvo exhaust mani and related initial piping is fairly well engineered for a production vehicle), but I agree with Bishop - once you get to the cat flange and back I think it would be tough to find a need for any sort of concern for a 'tuned runner' type of scenario, and any sort back pressure on the exhaust in general - doubt that even if there were to pulse tuning on the exhaust itself, that the wave/pulses could get past the cat.

My guess is that the large pipe straight out of the manifold was probably impacting the scavanging capacity resulting in the decrease in performance rather than limited 'back pressure' from the large exhaust diameter per say. Pulse tuning is a bit different than just adding back pressure, and I would venture to guess that a more restrictive exhaust is going to have an impact on the scavenging effect of the manifold and downpipe/primary pipe as well.

I agree that different diameters should be considered when it comes to the downpipe in NA vs Turbo, since the scavenging is going to be less of a concern in the Turbo app a larger downpipe is the way to go. After the primary exhaust stage however I think the bigger the better.

Just some thoughts there - appreciate the dialog.

JB

[Mike]

I simply suggest that the evil gremlin of backpressure in the exhaust system (the exhaust system = anything added after the optimum header or optimum charger outlet plus a possible optimum downpipe) will, and does, in general, limit the overall possible performance potential, unregarding if it might be a NA, or a boost application.

And I also suggest that whatever features/specs that might be added behind the/a optimum format header on a correctly combined, built & set engine, you seldom find any gains...
The key issue is simply to have a design that allow for the least overall losses.

[Even the sort of conservative Volvo oem engineers suggested that a 60mm exhaust on a NA-mode 112bhp B23A was worth +10bhp some 10-15 years ago...]

In normal NA-race applications we're in general quite happy when our customers accepts to build a as straight as possible 100mm x100mm (4"x4") "tunnel" after the header (adapted in the apropriate manner) which, after the rear end, exits into a carefully designed 2 x 3" pipes with equally carefully selected 3"muffler (100dB noise race regulations are common here in Sweden...) and then finally exiting the exhaust system.
This format has proven to yield small-to-none torque/bhp losses, and when trying with, and without, the exhaust system are the needed/optimum a/f & ignition settings typically unchanged.

In a boost application I simply suggest that as soon as the last "wheel" has been passed in the charger has the "exhaust system" started.
Hence does the format of the exit of the charger matter as well as both the radii as well as the dia of the "down-pipe" matter, as well as the rest of the system.

So, in general, and obviously unregarding such practical issues like noiselevels, heat and exhasut fumes, etc; after the best format of the charger exit and the needed (?) optimum format of dia & radii of the added downpipe there are rarely no gains to be found, mainly losses.

Obviously with the proper settings of fuel & ignition vs load & boost in EACH comparisson case.

And the noble art of avoiding this correct m.o. comparison procedure is often the case when oem or smaller (read "too small") exhaust systems seemingly produce better performance than systems with less pressuredrops...

Best regards

Mike

M.Aaro@mail.bip.net

Luleå (northern part of Sweden)

NEW sept '03 files: Gr10/Engine-kit, Gr11/Head & the EMS/T-file [Zip + PDF]

[bcrazy]

I agree with most of you guys. If you have to large primary pipes in the header you loose velocity and therefore there is less vacum created in the collector at low revs. So if you have a header that is well matched to the cam and rpm range of the engine, any backpressure you add after the collector will hurt performance. So what you want to to is get the noise level down without adding too much backpressure. When working on my 1600 cc beetle engine i noticed that running a 2.5" straight trough mufflers right off the collector reduced combustion temperatures and allowed me to run more compression and more igniton advance. Performance increase was very noticeably improved over running a chamber style muffler, even before i upped the compression or altered ignition timing. I expect the same applies to all 4 stroke engines.

[Anonymous]

But we shouldn't get confused by automatically assuming that bigger pipes bring lower backpressure. Too-high gas speed will cause more backpressure in a constant flow situation, but since we're talking about a pulsing flow, anti-reversion properties play a role in the consideration of pipe size. Sudden changes in pipe diameter, or inappropriately sized pipe, can bring about high-pressure pockets that will reduce flow potential.

Managing the importance of flow with anti-reversion is the key. As Mike rather confusingly illustrated, NA race engines like to see a gradually decreasing pipe cross section, and the reason is because as the exhaust cools, the gas becomes more dense and requires less cross-sectional area to maintain velocity and pulse strength. With a turbo engine, anti-reversion (for the purpose of scavenging) properties are less important and bulk flow becomes the focus, so a larger exhaust is necessary, but I still believe that incorrectly sized pipe WILL increase backpressure, even if the pipe is on the large side of "correct".

Most of the myth that backpressure is REQUIRED to make an engine run right comes from the "carburated" era. Increasing exhaust flow brings about an increase in intake flow, and that requires different carb jetting and ignition events. Most people just toss on a bigger exhaust and when it doesn't improve power or when it brings about driveability problems they assume it doesn't work, but they don't play with mixture or timing to optimize the package. Believe it or not, the factory optimized all the systems to work properly with one another, with the compromizes assumed. When you mess with one aspect, you've got to mess with ALL of them.

Of course, one other thing that reinforces the need for backpressure are two-stroke engines - since by nature the overlap of the transfer ports and the exhaust ports are HUGE (by 4 stroke standards), and the air/fuel charge is actually supercharged by the crankcase, a certain amount of backpressure is required to keep the charge in the combustion chamber. The expansion chambers make power by allowing the high-pressure blowdown gasses to exit smoothly, but when the exhaust pulse hits the back wall (10-15 degree converging cone) it reflects back towards the exhaust port, "shutting off" the flow at a certain speed. When a 2 stroke comes "on the pipe", the exhaust system is tuned so the pressure wave hits the exhaust port just in time to maximize pressure within the combustion chamber. Since the crankcase compresses the air/fuel and forces it into the combustion chamber, exhaust scavenging isn't as important on 2 stroke engines, provided they have single or divided cylinders.

Supercharged engines are also very sensitive to backpressure - because all camshafts have overlap, added exhaust backpressure will always increase boost pressure. Whether or not this increases horsepower depends on how much boost is gained by how little backpressure. Radical cams have huge overlap most of the time, and that overlap allows plenty of air/fuel to flow out the exhaust ports, wasting fuel and horsepower. A little backpressure keeps the air/fuel in the CC, and it also allows a certain amount of pressure to develop in the chamber while both valves are open.

If your exhaust is TOO open, and your engine runs with an O2 sensor, you may get some fresh air drawn up the exhaust pipe, which will completely change the reading going to your ECU. Or, as has happened to me, the O2 sensor can run too cool which basically shuts it off. This is more evident in low-speed operation, but can certainly be confused as a lack of low-end torque.

Now, what pipe size is right for our engines? Your guess is as good as mine! :wink:

[jjam184]

It is really a balancing act. You want a good smooth flow but if you reduce back pressure to much tourqe will suffer.
I was always looking for a way to get next to zero until we dynoed a harly I helped build.
We put several different exhaust comvos on it an found that some of them flowed to much and we lost low and mid range tourq.
We tinkered with it and ended up threading a 1/2 inch bolt into the tail pipes, which created just the right amount of pressure.

[Bready]

[quote:0710a74178]Too-high gas speed will cause more backpressure in a constant flow situation[/quote:0710a74178]

Need that one explained just a bit more if you could Matt.

JB

[Anonymous]

Before I explain, I should say that I see what you're saying, Jerome - once the exhaust gas exits into the cat, the effectiveness of the tuning pulse is lost and there's no reason to attempt to "tune" anything past the cat. I gotta believe that you're wrong, but I have to admit that the POWER gains to be had by tuning the rest of the exhaust are minimal. I believe that by tuning the lengths of the pipes between cat and muff #1, and between muff #1 and muff #2 (if used), and between muff #2 and the end of the exhaust, you can increase the effectiveness of the mufflers and quiet the system without adding backpressure. However, I have NO IDEA how to calculate this!!!

All I'm saying in your quote is that it takes a certain amount of energy to force gas down a pipe, and the faster you try to drive that gas the more energy is needed. Increasing it beyond a certain amount (some say 200 ft/min in an exhaust pipe, some say 350 ft/min) drives up the energy required sharply. I don't know how you'd derive what velocity is high enough to keep the gas moving in one direction while still keeping backpressure to a minimum, but I also don't know how to derive how much exhaust gas volume is coming from a given engine at a given time, so I guess it doesn't mater... Hey, it's just a discussion, right?

So let's toss this out for discussion: Why are collectors necessary on normally aspirated racing headers (where they exit to the atmosphere right after the collector) but zoomies are effective on supercharged motors? If collectors are necessary on NA engines why wouldn't they be needed on supercharged motors, and vice-versa? For now, forget about the need to connect the header to any exhaust system...

[linuxman51]

collectors help scavenge exhaust out by putting a vacum on the other runners( if properly designed) think about the venturis on a carburator.

on the supercharged car, the volume of gases exiting the chamber is usually larger than that of the NA car, but as for the lack of a collector, I would assume that in that case it creates some sort of back pressure problem or perhaps the duration on the cam is such that a collector isnt needed (i.e. with a little bit of blow thro in mind)

[Bready]

Thanks Matt. Shaping up to be a fun discussion.

Well let me make sure that in the course of this discussion of NA exhaust systems we aren't confusing "back pressure" with "exhaust tuning" or designing the manifold and related primary exhaust components to maximize the effect of the high pressure waves that you are trying to take advantage of in the NA exhaust scenario. They are two very different things, and I wonder if they are getting confused.

It may be better to split the discussion into two seperate focuses: what happens UP TO the end of the header/manifold and related tubing; and what happens AFTER that point.

Back pressure, per se, is probably an issue that we on this board concern ourselves with AFTER the tuned exhaust components (unless we are talking about designing headers and manifolds etc...which very few of the folks on this board are probably considering). Point being that AFTER those tuned components the least amount of back pressure the better [running with open pipe would be best performance scenario if it were legal and tolerable from a sound and emmisions stand point], and pulse tuning as it applies to the cat-back components are a pretty moot point.

If we are talking about the primary NA exhaust components (from the end of the header or manifold and related tubing forwrd to the head) then we are talking about pulse tuning to maximize the effect of 'finite-amplitude-waves' as they act to draw out exhaust gasses in the combustion chamber. But these waves are not 'back pressure' they are a very different animal.

I mention that for clarity only, not to suggest that no one in this thread understands the difference.

Regarding the supercharger [we can toss in T-chargers too for this] and the reason you don't rely on collectors etc - is that you are dealing with an intake charge that is already compressed by the s or t charger - as such you have a pressurized intake charge before combustion so as soon as you crack the ex-valve the pressurized exhaust is ready to blow out of the chamber, so the pulse tuning is a moot point - that is the reason that you design the cam differently as well - with very little overlap to prevent reversion when the intake valve opens while in an NA app you WANT the valve overlap to use the intake charge WITH the high-pressure waves to clear the cylinder.

That is why I think that the problems Dale mentioned in Volvo88's scenario were the result of pulling that tuned pipe from the manifold rather than too large an exhaust. Changing the pipe diameter at the manifold reduced the capacity of the high pressure waves to clear the cylinder, resulting in degraded performance.

My thoughts, Long winded as always - apologies. NEXT

JB

[Captain Bondo]

With a long exhaust system, the gases naturally lose energy as they travel away from their poiunt of origin- in terms of kinetic energy as well as thermal. This causes a loss of velocity both due to the exhaust volume decreasing as well as loss of kinetic energy due to friction, turbulence, etc.

What this means is that what might have had good velocity in a pipe with a 3" cross section right after the collector will be essentially at a standstill halfway down the system. Now you've got gasses that have actually stopped inside the pipe (well, it's all dynamic, more exhaust stacks up behind it and pushes it through so notheing actually "stops" except in an instananeous, mathematical sort of way). But basically you're back to forcing exhaust out, rather than it carrying itself out via its own energy.

So, apart from possible benefits from pulsetuning on the exhaust side (which is indeeed acheivable by some sort of complex calculation most likely similar to the inverse of intake runner tuning) the goal is to decrease the cross-sectional area of the pipe in PROPORTION to the overall energy loss of the exhaust, therefore keep velocity CONSTANT, since a change in velocity respresents a pressure change/drop which is as already stated, undesireable.

Almost like the good ol' days....

[Anonymous]

May I?

like JB, I await with bated breath for clarification on your too high speed statement, Matt...that should be good...

I will agree that the cat acts as what is sometimes referred to as a pressure wave terminator...meaning that the cat basically kills the pressure wave aspect of pipe tuning ...

...if I understand Mike's comment on the 4x4 pipe, it sounds like that is being utilized as both a pressure wave terminator and as a pressure recovery accumulator...to basically result in a quieter exhaust noise level...[interesting layout]

I would agree with the assertions that the old adage of too little backpressure hurts torque is from the carburetor era; and that that is because of messing with only certain parts of the factory system, and not sufficiently matching the rest of the components to work with those that were changed...it does become a bit of a vicious circle at times...
and I would also agree that reducing backpressure can still have negative effects in FI apps for the same reasons: mismatching of components...

what is the best pipe size? that depends on several things...it keys off the diameter of the exhaust valve; and proceeds from there: NA has its complicating factors; boosted has its own factors...

JB, I agree that from what was described, Volvo88's probs stemmed primarily from a mismatch of components...

...and unfortunately, I have to agree that most do not concern themselves with backpressure issues before the DP [turbo], or with the proper sizing of the primaries of the exhaust system at the ports [NA]...for the reasons you gave...[and JB, I am in the process of designing and building a turbo header in 2 configurations: stubby S/R; 4 tube pulse T4/T3]

I have been experimenting with both 2.5 and 3 inch systems; NA and turbo...with some interesting results...2.5 works well on the NA apps with stock exhaust manifolds; and I suspect that 2.5 would still be fine on those apps if a tri-y header were installed...

...Garrett equipped turbos appear to derive more benefit from 3inch than do those with the mitsu's...I suspect that is because the mitsu's are so restrictive on the hot side to begin with...in fact, I just ripped off a 3inch from a mitsu equipped turbo, installed a 2.5 system, and the power output and performance improved considerably: much smoother acceleration....[subjective observations, admittedly; but the car's owner heartily concurs]

sidenote: on a 21FTi with Garrett .63T and 3inch, I just installed a 90+ manifold opened up to flow well into the garrett housing...the results were impressive: boost threshhold lowered about 100rpm to below 2400rpm; 5psi reached easily by 2450-2500rpm; manifold and turbine housing glow after a hard pull was reduced in size and duration; and the limitations of the T cam are now very apparent: the engine runs out of cam well before 4000rpm...

I have noticed one complicating factor in the pipe sizing situation that can seriously affect effectiveness: size; design[configuration]; and location of the muffler(s)/tailpipe...I have observed considerable variations in performance just by changing the muffler/tailpipe [the location was not changed]; I was surprised at how much of a difference that made on a turbo-with-3inch system...and suspect similar variations would also result on a 2.5 system...so you can't always blame it all on the pipe size...

back to component mismatch: too often, people get things backwards: they throw in a cam, then try to match the rest of the car to it...the cam is the last item to be decided upon...

...and on pipe sizing: for NA, I have not seen any benefits with pipes over 2.5; and on turbos, 3inch does not like tiny turbines...YMMV...

[Anonymous]

Kenny - You've got the NA aspect right, and you're touching on what I understand to be the reason on the supercharged aspect of things. The supercharger is all the scavenger the engine needs - it creates a higher pressure in the intake than is in the exhaust, so during overlap when a conventional engine would rely on a good header to scavenge the exhausts, the supercharged intake just "blows" the exhaust gasses out. Like a NA engine, a turbo has higher pressures in the exhaust than in the intake, so it must rely on scavenging (if possible) to clear the cylinders.

Other than that, I'm not exactly sure why I brought it up... I was going to compare the importance and function of the collector to that of the exhaust pipe, pointing to sufficient gas speed being required to keep the exhaust moving at maximum energy while not creating so much pressure drop that the engine loses power, but I think the focus has shifted more to "after-cat" exhaust function and backpressure versus velocity versus pipe size, so the header and collector isn't really a valid arguement.

Jerome - Good explanation WRT "backpressure" and "exhaust tuning", however I believe they *sort of* go hand-in-hand. A strong tuning pulse comes from a pipe that is sized small enough that there is SOME resistance to flow velocity changes, and that seems usually to be small enough to bring a little backpressure as well. Notice that in some SBC applications, a 1 7/8" header outperforms a 1 5/8" header almost everywhere on the dyno, but the 1 5/8" header will outpull the bigger one on the track. Major generalization, and brings up the discussion of dyno testing versus real life plus the importance of torque versus all-out horsepower, but it suggests that the smaller pipe, which doesn't "flow" as much as the bigger pipe, makes a stronger pressure pulse. The fact that the exhaust starts moving through the primary pipe during the blowdown phase, when it's under several hundred PSI and is able to send the exhaust ports to near (or up to) sonic velocities, means that the "resistance to flow velocity change" of the smaller pipe is not a big deal, and it pays off later in the amplitude of the pressure pulse. Did I explain that right???

Now: with your supercharger and turbocharger header explanation... I wouldn't be so hasty to lump the turbocharger engine in with the supercharger engine like this. The turbocharger engine can still benefit from a scavenging exhaust pulse before the turbo, while the supercharger can't. The only problem with a turbocharger header/collector arrangement as I see it, is that instead of a vacuum, the header/collector might only be able to create a lower pressure pulse, and the mathematics involved hasn't been well-documented. Pressure's up in the manifold, which increases the wave speed of the pulse, temperature's up, which lowers the wave speed (I think...), etc. Anyway, I'm sure it can (and has) been done.

Cappy - pretty much what I've been sayin, but can you help me explain WHY keeping gas speed up helps reduce backpressure? Oh wait - I think you did.

Tom - I tried explaining it to Jerome earlier, and I'm not sure it worked out so well. Are you just busting my balls here or do you disagree? :wink: If so, speak up!

I agree - Mike's exhaust sounds like there's more trickery in the 4x4 section than just flow - probably allowing the pulses to cancel and soften so the muffler can easier deal with them, plus it simulates an "open header" design by providing a huge volume for the header to dump into. Very interesting and innovative, and a good catch on your part - I'm not sure I would have noticed the significance of the noise requirement.

The cat being a pressure wave terminator - well, I guess so, but there are still pressure waves coming out of your tailpipe, aren't there? (I, admittedly, haven't had a catalytic converter on any car I've owned in at least a decade... maybe they affect more than I remember.) Maybe if the body of the cat were redesigned it would allow the pressure waves to continue on downstream, but the problem is that the pipes entering and exiting the cat are usually so abruptly transitioned, they don't do anything good for pressure pulses OR flow.

I think the rest of your post backs up what I'm saying fairly nicely, thank you - NA engines don't like exhausts that are too big, restrictive turbos don't like exhausts that are too big, freer-flowing turbos appreciate the bigger exhausts. Pretty much boils down to how much HP (and therefore exhaust gas) the engine is making and how much energy is there to drive it. As well, turbos don't seem to like chambered mufflers, but they like the Ultra-Flow or Magnaflow style just fine. I believe that you've designed and tested these exhausts fairly - now we should be examining WHY you found what you found.

Sorry guys - I don't mean to be dominating the discussion, especially since I don't have nearly as much practical experience as a couple of you do. I'm also not as familiar with physics as I need to be to be an expert in this field - I'm just arguing my position as best I can, and I'll be willing to listen to (and hopefully learn from) other arguements. In my heart of hearts I believe that high-but-not-too-high gas velocity is important even BEHIND the cat, but I am missing one or two key pieces of knowledge to effectively prove it. However, I still could be wrong - I have been before!

[bitjockey]

I agree that with a smaller turbocharger, a larger exhaust is detrimental.

Removing the exhaust after the downpipe with my TD04HL-13C turbo pretty much removed 90% of my torque, and caused a drop in top end power as well. Re-attaching the exhaust and running a 2.5" system with one catalytic converter and no mufflers was the way to go.

However with my TD04HL-15G turbo, the less backpressure the better. I am running 3" system turbo-to tail now, with no catalytic converter and one Warlock 3" muffler with a bypass. With the bypass open, the car just loves to rev, boost threshold is lowered and top end power is increased.

I believe Matt has a good grasp of the gas laws and physics involved behind this. I have to go, rather, on what I have gathered in my personal experiences.

Great discussion guys, I miss threads like this.

[Anonymous]

Matt...nah, I was not trying to give you grief; was curious if you were setting yourself up to fall into a trap...and you did not...yet...

keeping the exhaust gas speed up, in the exhaust primary tube for each cylinder, takes advantage of the blowdown energy you mentioned; and also improves the scavenging of that particular cylinder...and if properly configured: assists other cylinders as well. If you use too large a diameter for the primary tube, you kill that velocity; which then slows everything down...in my experience, smaller primary tube diameter headers compensate considerably for deficiencies elsewhere in the exhaust system...

optimizing the utilization of the blowdown energy requires correct exhaust valve opening timing...[tying in cam profile choice into the mix]; properly sized diameter of the primary tube to maintain that blowdown velocity; and sufficient volume of the primary tube--ie length--to allow the blowdown pulse to empty the cylinder and self-scavenge...

the key starting point in determining the 'proper' diameter for the primary tube is exhaust valve diameter--your SBC example fits nicely: SBCs normally have 1.6inch exhaust valves; 1 5/8 tubes are 1.625 OD: the blowdown velocity is maintained...[the 1 3/4 tubes hurt that, and only help at the really high rpms]--....then follows volume of that tube: you want the length of the primary tube to be able to 'contain' the displacement volume of the cylinder, and hopefully more than....[tri-y's do that artificially]...and go from there...

I won't belabor the NA aspects further: y'all are doing fine; and I've been wrapped up in the turbo side of it..

...on the turbo side of the approach, too many people are not aware, or have forgotten, what Hugh McInnes wrote re turbos and headers: that a header is just as important in a turbo app as in an NA app.....this neglect of headers for turbos has been compounded by factory approaches that use log manifolds...giving the impression that logs are fine for turbos...wrong: logs are lousy...and stifle turbo performance as bad as logs hurt NA performance...[BTDT on NAs; am working on it to be able to say BTDT on turbos]

Matt, referring to the trap I mentioned at the top...if you agree that maintaining blowdown velocity is important; and that sizing of the primary tube affects that; then can you [or Cappy, or anyone else who wants to give it a shot] explain why it is somehow 'good' to hog out the exhaust runner in the cyl head into some huge hole of a tunnel and expect that big hole to maintain exhaust velocity?

...I realize that I am getting a bit OT with that; but it does tie in...and if one visualizes the exhaust system as starting at the exhaust valve seat, then sizing from that point on does affect everything....comments?

[Hank Scorpio]

Ditto Robins commments. I applaud you guys for an n/a discussion. I vote to sticky ;)

You guys are dead on the money for alot of this, especially a cat. So, if you could what would your ultimate n/a exhaust system be? (For sake of arguement, go catless).

[TurboBrick940]

My own personal experiences (I have dyno charts for most of these setups, but couldn't find enough of them to make it worth posting):

Stock 2.5" Downpipe 2.25" exhaust - 13C
Standard overall performance, sandblaster exhaust note.

Stock 2.5" Downpipe 3" catback exhaust - 13C
Much better top end performance

Stock 2.5" Downpipe 3" Cat 3" Catback exhaust - 13C
More responsive midrange with slightly better top end

3" Open Downpipe - 13C
Garbage...sucked....awful...No....

3" Downpipe 3" Cat 3" catback exhaust - 13C
Somewhat better spool, very responsive throughout

3" Downpipe 3" Cat 3" Catback exhaust - 15G with 2-1/4" outlet
Good spool,good top end, extremely responsive

3" Downpipe 3" Gutted Cat 3" Catback - 15G with 2-1/4" outlet
Still better spool, stronger through midrange and up top

3" Downpipe 3" Gutted Cat 3" Catback - 15G with 2-3/4" updated outlet
Psychotic boost response, a totally different ride, top end pulls VERY hard.

[Anonymous]

Tom - I don't understand your question. Rephrase it if you would...

[Anonymous]

Robin and Eric...thank you for the results/observations...am I correct in assuming that these results were achieved with the late style exhaust manifold; and that with the larger turbos, the manifold outlet was enlarged to better match the turbine housing inlet?

Matt...a rephrase...

the question posed: if:
... (a) exhaust gas velocity, getting its impetus from the blowdown pressure pulse at EVO, is important to maintain in order to achieve cylinder emptying and self-scavenging, and if
...(b) sizing of the primary tube from that cylinder affects that velocity : negatively if too large--'large' being XX% larger than EV diameter--...and maintains or positively increases the velocity if the primary tube diameter is small--'small' being EV diameter or X% smaller than EV diameter--, and if
...(c) the exhaust system starts at the exhaust valve seat: making the exhaust runner in the head a part of the primary tube....then

(d) I would like to know why hogging out the exhaust runner in the head is considered "good" or the way to go...

...from my perspective, enlarging the exhaust runner in the head into a huge hole contradicts maintaining velocity efforts...

in other words, I am questioning the conventional wisdom of opening up the exhaust port/runner in the head...and am suggesting that [based on a,b,and c above: if velocity is important] then only doing any mods or grinding or hogging out of the exhaust runner to maintain/increase the velocity are to be desired...

this does get into flow and volume and CFM etc...I understand that...my contention is that, for useable power across the rpm range, optimizing/maintaining the velocity of the exhaust gas flow trumps volume [and I am NOT dissing volume ala flowbench measurement and modding to improve; just questioning its deification and application]...and that that starts at the exhaust valve seat...

...and I was asking for info to either confirm my contention or to show me where I err...

[and I was trying to pose the question in a way to get people to consider velocity v flow in a different way]

...a bit OT, but related to the topic in a causal manner, from the mismatch end, I think.

...did this help, Matt? or did I over-rephrase?

[I won't be able to respond for a few days...to the far-flung saltmines I go]

[bitjockey]

Robin and Eric...thank you for the results/observations...am I correct in assuming that these results were achieved with the late style exhaust manifold; and that with the larger turbos, the manifold outlet was enlarged to better match the turbine housing inlet?

Yes, you are. I removed the 'lip' section on the late model manifold to match the ported turbine housing. Also did some mild blending/porting of the runners and where they come together in the center.

[quote:6bee933c14]...from my perspective, enlarging the exhaust runner in the head into a huge hole contradicts maintaining velocity efforts...[/quote:6bee933c14]

At what point do you consider it necessary to in fact open the exhaust runner in the head to increase flow? Since that is a very common modification in an aftermarket/worked cylinder head, is it often misguided?

[quote:6bee933c14]my contention is that, for useable power across the rpm range, optimizing/maintaining the velocity of the exhaust gas flow trumps volume [/quote:6bee933c14]

While I agree with this in some ways, if less backpressure is needed for the turbo to spool, and in turn creating useable power across the rpm range, wouldn't volume be just as important? Perhaps on an N/A engine, but since the turbo is such an essential piece of the puzzle on a boosted motor, shouldn't it be given careful consideration?

[Anonymous]

I think you over-rephrased, Tom... as usual! :lol:

Anyway, I cannot argue your point, as I believe very strongly along the same lines.

So should we discuss critical flow, you think?

[Anonymous]

Robin...thank you; and some of your comments/questions will require considerable time to sit and type out my response...will do so asap...I look forward to that.

Matt...critical flow sounds good; and I'll let you start that...heehee...just to get a handle on where you want to go with it...should be very interesting...

A thought: maybe we need to define some of the terms we are using...I fear that I may have created some confusion by not defining the terms I'm using: so will try to correct that....

this IS an important area of discussion [in some ways, I see it/them as KEY areas]; and I will try to define my terms better...the subjects involved are tough enough to discuss without compounding it by lack of uniform terminology....[this is aimed at myself moreso than at anyone else: I too often am guilty of not explaining my terms]...so, I'll work on that...

[and, yeah, Matt, that means that I'm asking you what you mean by 'critical flow'...I think I understand what you are referring to; but will ask for your definition so as to be better able to follow your thoughts.....not trying to be dense: it sometimes occurs spontaneously...a 'blonde' moment; please pardon that]

[Mach375]

I figured I was going to keep quiet on this, but then I remembered why I was going to open my mouth (keyboard) in the first place.
Any backpressure you may want/need is provided by the turbine impeller. So the engine is getting plenty of backpressure to scavenge. After the turbo, you want as little as possible. The motor wants a little backpressure, yes. The turbo, however, wants absolutley NO backpressure. So what you are actually doing when upgrading exhaust on a turbo car is not reducing backpressure on the motor, but opening up any restriction to the turbo. The turbo needs to breathe freely. The engine, with that turbo on there, is getting exactly what it wants: some backpressure.
Think of it this way: You are not freeing up the motor. You are adding a better-flowing exhaust system to a turbocharger.....which happens to be hanging off a motor (preferrably yours :badboy: ).
This is why NA cars need no-so-huge exhaust systems: no turbo sitting there to create backpressure. So you need something to create it, and since the only thing there is the exhaust system, you need it in that.

[Bready]

Ah great thread, shades of yesteryear...

...all we need now is some groveling.

[quote:b00e4d3915]So the engine is getting plenty of backpressure to scavenge[/quote:b00e4d3915]

This is one of the areas where I think we are confusing terms - 'backpressure' in and of itself is not the same as 'pulse tuning' exhaust components - so I think it is important to clear that up. Simply adding 'back pressure' doesn't necessarily mean you are engineering good scavenging characteristics, in fact often the reverse is true.

My two cents, gonna sit back and groove on Matt and Tom for the time being.

Weeeeeeeee!

JB

[bcrazy]

Let me try and explain what i think would be the best N/A exhaust, starting at the head, ending at the back fender. Arrest me when i'm wrong. :wink:

Good 4-2-1 header with primaries not to big that gives good exhaust velocity in the rpm range you wan't. Let's say 1.5k-6k for a street NA red block. Don't know much about designing headers so will leave that up to those who know... Then after the collector ends somwhere under the car: as free as possible 2.5" mandrel bent. High flow 2.5" cat or no cat at all. One resonator type muffler in place of the original stock item. And as the last damper: resonator/straight trough 2.5" as big as will fit in the stock location. Hopefully this will give a nice deep exhaust note that want be too loud ad part throttle. Will probably make some noise when flat out... but not to bad?

[Anonymous]

Eeewww... sticky. :nono: If you keep doing that, you'll go blind!

Tomorrow I'll try to throw some thoughts down on Critical Flow, Sonic Flow, and try to come up with a list of definitions to clarify what we're talking about. I think we're getting some people confused...

crazy - that sounds like a decent plan for an exhaust, though I'm not a big believer in 4-2-1 headers. At least, not until someone can adequately explain the benefits to me. I had a 2.5" with a glasspack-style resonator and a Dynomax Ultra Flow on my B21FT, and I thought it was quite noisy, but I'm getting old and noisy exhausts don't appeal anymore.

[iadr]

I don't believe in 4-2-1 headers at all, for the same reason I don't believe in wide lobe center cams. For a 1.5K to 6K rev range- WTF? I use not much more than an 1800 RPM range in serious driving, and I want things to come together hard there. Anyway, it's a separate discussion.

My ideal NA exhaust- 4-1 with really long tube- 39" / 1M min, tube size per:
One cylinder's displacement in cubic inches times RPM ,
Divided by 88200 = optimum pipe cross-sectional, inside area in square inches.
This formula is empirically (by trial and error) derived, and is all around online.

Then into to the collector, which is of the necked down merge type. http://www.carcraft.com/techarticles/0304_merg/ or less likely but still worthwhile considering, a 4-2-1 collector: Flowmaster 4-2-1 collector (http://www.tognottisautoworld.com/search_result.asp?CATEGORY=ALL&MANUFACTURER=ALL&DESCRIPTION=&PRODUCT_ID=FLOC158214300)

From there into a few feet of 3.5" (or 4" on real high hp- 2650+cc and high compression) heavy truck tubing- ovalled for ground clearance, if needs be. David Vizard recommends a more radical build- a full tapered section box of considerable volume at the end of the collector. Robin @ http://www.beardmorebros.co.uk/ has more onsite and a sweet example (on the widened car). This is similar in function to Mike Aaro's 4" box. The entrance wants to be a simple butt connection (assuming 2.75 or 3" collector) but the exit needs to be really smooth.

Go down into 2.5" from mid car back.
I'd go into a Flowmaster, then into a Magnaflow/Dynomax, or maybe the Flowmaster second, not sure.

I'm split on whether the idea of under axle has merit: within reason, when using mandrel bend, the exhaust flow can follow the tubing over axle better than the noise can.

Note: theory point- as exhaust flow slows you need a bigger pipe, not smaller. Anyone remember the looking down off an overpass metaphor? If the cars slow to half speed, then you need twice as many lanes to pass the same traffic.

-edited flowmaster collector link to remove horizontal scrolling.
-m

[iadr]

further tech-heavy post, of debatable relevance- mostly of intake application

I was on Larry Widmer's board and asked:[quote:fe03c22dea] "I've heard a number of people refer to ft/sec specs, usually in the range of 270-330 ft/sec.

What I'm having problem placing is how that is measured- say for purposes of arguement- does that carry the unspoken statement that that is at full valve lift & @ 28" of water? Basically are those 270ft/sec-up to-mach1 speeds achieved on the bench? Obviously these are bench derived figures, as the actually running flow's velocity has almost a sine-wave velocity if graphed, correct?

I *really* felt I learned a lot from the article by David Vizard I have scanned and hosted @ http://www3.telus.net/public/iadr/V/porting%20pics/vizard2.jpg (change the last digit in the URL to 1, 3, 4,5 to see rest)
That was one of the first mentions actual quantified velocity I saw. First mention of computing your own average port cross section (and rule of thumb for a # to aim for in that regard), as well.
[/quote:fe03c22dea]

and got the very useful, if techie reply from one of the members (not Larry Widmer), reminding me it was mean (aka average over 360* crank)port velocity not peak that was refered to ... which is something that (seems to me) got muddled earlier in this thread.... :

The formulas and assumptions I'm currently using to estimate peak torque RPM are:

Peak torque RPM occurs when port velocity [he did not state mean, - I assume mean??-Ian] reaches 240-260fps.

Mean port velocity in fps = piston speed in fps X piston area in square feet / port area in square feet.

Mean piston speed in feet per minute = RPM X stroke in inches / 6.

Mean piston speed in feet per second is as above, then / 60.

I use a pitot tube to check for pressure differentials, in order to asses port flow, rather than to make guesstimates of overall port velocity.

Port velocity on the flowbench can be easily found by the formula:

velocity in feet per minute = CFM / area in square feet.

[Anonymous]

Note: theory point- as exhaust flow slows you need a bigger pipe, not smaller. Anyone remember the looking down off an overpass metaphor? If the cars slow to half speed, then you need twice as many lanes to pass the same traffic.

(For anyone who doesn't know, Ian's my favourite debating buddy! He's an ENORMOUS wealth of information, and he keeps amazing me with tidbits of it on our weekly chats. Some good examples are shown here... Also one of his strengths is REMEMBERING what he's read and where he's stashed it - I've read that article by Vizard (he sent it to me already) and I forgot I had it. Vizard is a GOD, by the way...)

I'm not going to argue any of your other points, but I have a problem with that metaphor - how do you figure you need twice as many lanes? When cars slow down by half, the space between them is usually cut in half as well, so the net result is that you need the same number of lanes to pass the same amount of cars. In OUR case, when the exhaust gas cools, the space between the molecules naturally shrinks and the gas becomes more dense. To pass the same mass of gas at the same velocity, you have to make the pipe smaller as the gas cools - not larger.

[iadr]

Thanks Matt. :wave:

As far as the overpass metaphor, I was more directing that as a alternative way of looking at things, to balance Kenny's post (well reasoned and probably correct) back on the first page, in regards to kinetic energy loss.
I guess we could say... it depends if the thermal or kinetic energy gets 'used up faster'?? hmm, so hypothetically: hot, slow air needs a big pipe, because if you tried to speed it up you'd have a restriction & cool fast air needs a small pipe, because you don't stand to gain much by slowing it down sending it to a larger pipe. Something like that. A lot of pulling-up-by-the-bootstraps logic straining my brain. It doesn't help a guy is going at a *very* loud video game- (yeah, at work) behind me.

On a related note, I tend to believe that the exhaust energy gets a small boost from the fact it is flowing into a region of lower pressure. That effect is best ..uhh effected by a large area of tailpipe.

Again both thoughts may be overwhelmed by other factors already covered but they are still factors...

[Anonymous]

Actually I would argue that slow moving air needs a larger pipe - slow moving air COMES FROM having a pipe that's too large - it doesn't need a larger pipe. Also, I would think that (for pulse tuning, anyway) the hotter the gas the less dense it is, so you need to make it flow faster for the same effect and the less of a price you pay for having the gas "restricted" because it would take less energy to compress it a little bit.

Agreed on the high-pressure-to-low-pressure flow theory.

Sorry - I'm a bit distracted too. You've got the loud video game, I've got the knowledge that I'm being sent down to our main office in Houston for some training and by sheer coincidence my two favourite bands are playing during the week that I'm there. Tickets are still available... :boink:

Anyway, back on topic: I dragged out my Fluid Mechanics textbook and tossed it next to our bed, so for the next few days I'll re-read that and maybe I'll have a stronger grasp of what we're talking about.

[JohnLane]

Some interesting points are being made in this thread. Kudos.
Something that should be stressed is the HUGE difference between N/A applications for scavage versus turbo application.
Do keep in mind that the Manufacturer went to a certain amount of effort at making things sized just so in that exhaust manifold for Standard levels of boost. We ask a lot more of it (turn up the boost) and one can quickly get away from efficient exhaust flow and be "leaving power on the table" by the thing having to try harder to get that larger volume of exhaust through the same size holes.
In my scanning of the thread I have not seen mention of using larger exhaust valves.......Where they can be fit a couple of MM in increased size will flow a LOT more volume without doing naughty things to velocity as the size of the port/exhaust manifold will be the deciding factor there.
If ya look at most true RACE turbo installations one will see that they are still using what looks to be 'shorty' headers, and ALWAYS with an external wastegate. I'd sure be liking to know the formula the race guys use for determining primary tube sizes and lengths. Any of you guys know this? I am thinking of rearranging the exhaust in my toy.
It may have to get that turbo replaced with a supercharger. That could be really amusing.
Naturally exhaust strategy is wayyyyy different for Supercharging versus N/A too. Yikes!!! More math. Maybe I shouldn't have been stoned through Hi-Screwl. :-P
Thoughts?
JohnLane.

[Captain Bondo]

I'm just not on enough to keep up to y'all. LOL

OK, on "hogging out exhaust ports":

The tightrope a porter is always walking on is wether the decrease in restrictiveness of a given channel (straighter flow path, increased cross-sectional area, decreased turbulence, etc) will result in a net gain when offset by the loss of gas speed.

IMO it's as simple and as complicated as that, does the physical geometric improvement of the shape of the channel outweight the loss of velocity. This is where things get hard when it comes to practical application, but I think that is the basic overall concept.

On loss of Kinetic vs. loss of thermal:
Loss of kinetic energy of a flowing fluid or gas is largfely dependent on its viscosity (and it velocity proportional to the pressure difference between its start and end point in this case.

The amount of kinetic energy is very veyr small compared to the amount of thermal energy a volume of gas contains.

Kinetic energy is largely dependant on mass, and gas is not very massive (although it may be possible to have massive gas... but anyways...) I think the formula for kinetic energy is mass x velocity (or mv^2, one or the other). so basically there isn't a ton of kinetic energy to lose, and gas isn't very viscous so assuming a stainght path with little restriction I don't believe much kinetic energy is lost.

However, the temperature difference between the exhaust and ambient is huge, so in my resoning a lot of energy is lost via heat tranfer (in fact this is one of the major culprits for overall combustion engine inefficiency as we all probably know).

So, I think the thermal component of the gases' total energy is much larger than that of the kinetic and is also lost much faster, which would explain the decreasing volume and therefore decreasing pipe size.

Hopefully that made sense.

[Anonymous]

just dropped in...hmmm: 'sticky'...ok, will try to make it worthy...catchin up:

JB....jeez...you get grovelling one time and now you expect regularly...you be bad as my one pet cat: I give it some kippered herring one time; now every time I get out the can opener...but that's ok...if grovelling does become necessary, I will expect a tall cold one to go along with....[and you be buyin']

...and I agree that we will need to discuss the terms further...I want to reply to both Robin's and Shane's comments on backpressure...which unfortunately will have to be early next week, customer probs/needs come first...

Shane...if you would not mind; I would ask for a further listing of the mods on your car...so as to enable me to respond on some things in the context of your car...no intention of dissing or anything; just would like to have a context to discuss some things in; and your listed mods look like a good way to do that...so, if you would...

John...I agree: larger exhaust valves, and the bennies to be derived from that mod, haven't been brought up yet...your comment should be expanded upon...Mike Aaro recommends larger EVs; the turboford guys do that a lot; Philip Bradley did that to the head he just had done this yr...and the why's and bennies of larger EVs deserve further scrutiny and discussion here....go for it, sir...

...re wastegates and other things, I hope we can get into those areas...and why SC?...on your V6, I think twinning would be better than going SC...depending on what it is that you are trying to address or rectify...am curious as to why SC appeals to you...

bcrazy...we got into the 4-2-1s in the 'na vs turbo' thread somewhat...the tri-y that slomo242 has is a 4-2-1; but is more correctly ID'd as a tri-y...
...tri-y's work...as slomo242 testified...and such a choice would be fine...[I have some further suggestions to offer next week]

...I can understand Ian's and Matt's viewpoint...it depends some on just what you want and how you want it...[a familiar dilemma?]...I would go for a true 4-2-1 [for an NA] if I were going to build one; but to buy a good off the shelf unit for your purposes, one of those tri-y's from KGTrimning or Unitek or .... would be quite sufficient...

Ian...good to hear from you...look forward to examining your thoughts and debating your views...and your view of a long primary tube is along my own [as part of a true 4-2-1]; just that I do think the tri-y is a viable compromise that works...

*******

I am approaching this overall subject from more of a 'what can I do to optimize what I already have to improve flow and reduce backpressure and increase power without messing with the stock cylinder head...then, when I have optimized all else, I'll turn my attention and efforts to enhancing the head' approach/viewpoint/strategy:

...firstly, because I am of the opinion that the SOHC head is an excellent design and layout out of the box...
...secondly, because I am also convinced that, because of the first opinion [based on study, CFD reports and analyses, and experience], until the bolt on appendages are optimized for my purposes, working on the head is getting it bass-ackwards: the stock head is sufficiently well engineered and made, that to mess with it before I get the exhaust system up to snuff to the point that the head is now the limiter; or until I get the induction system up to the same level of challenging the head to do better; and until the camshaft used cannot give me anymore because the head is a slacker; I would basically be urinating UP a hawser...(not one of my favorite activities)
...and thirdly, because I don't care to get rained upon in such a manner; and because I am a lazy as well as crotchety old fart, I try to get the most bang for the buck; and try to do things the smart and easy way...[but cannot claim to have succeeded often at either]

...those are my opinions, and that is my approach....am stating them not so as to dis anyone, or what they are doing, or have done...I follow people's excursions into various mods: sometimes intrigued; sometimes incredulous; sometimes bemused...all of which is ok....part of what makes life so much fun...[and I hope that I return in kind]

I am stating my approach, so as to be like defining a term: everyone will know from whence I come...and then be able to "see" what I mean easier.

I am glad to see those posting on this topic...and hope to read from some more...[how about it, Rhys?]

And I hope that we will disagree on some things...iron sharpens iron...

Matt...we do need to get the basic def's down...and verify that we all agree on the def's...

Cappy....figured that the hoggin out comment would gitcha started...good...(heehee)

Thomas Fritz
...the stealth FTi

[JohnLane]

Thomas........
Superdupercharging rather then a pair of smaller turbos cuz I'm lazy and a pair of turbos would require a pair of oil supply/return sets of plumbing to be made by me (sounds too much like work) along with all the rest of the plumbing to go with it. We are all too well aware of just how excited the automotive aftermarket has been about supplying all manner of exciting bolt-on goodies for the Fabulous PRV V-6 are we not? :-P
Superdupercharging cuz for the rallycar it will give RIGHT NOW throttle response with big power without the exhaust headaches (turbo exhaust systems are more complicated and tough to work on in a service area with limited time).........
Another little tidbit..........I got a feller who 'may' be interested in helping certain parts find their way into my paws as we develop a 'kit' to make 'em go a lot faster. Well, there ya go. We are all whores when we get right down to it aren't we? :roll: OK then, I am anyways.
Bigger valves?? You betcha........Bigger exhaust valves for your turbothang that runs more boost then stock won't hurt a one thing. N/A guys want to get the exhaust out, but must make the intake as nice-nice as possible cuz they don't have a compressor helping. We want to get that compressor going as fast as we can in our turbo-toys; thus it makes a lot of sense for us to work at optimizing exhaust. Make it easy for that turbo to get up to speed as fast as possible. Sure would be cool if a VATN were ever reliable. But they aren't.
Thoughts?
JohnLane.

[Canuckvolvo]

Kinetic energy is largely dependant on mass, and gas is not very massive (although it may be possible to have massive gas... but anyways...) I think the formula for kinetic energy is mass x velocity (or mv^2, one or the other).

Don't forget vat zee old german guy zed: E=mc^2

massxvelocity is momentum.

That's about the only thing I can add to this excellent thread, and kudos to all the contributors. I'll be taking Tom's approach with my B23FT- should have the head all assembled by next week and I'll just be doing minor work to it: smooth out ridges in the CC, clean up the ports, and a little polishing all around. I think the main thing I'll be doing is trying to tighten up the squish...

[Anonymous]

John...I understand what you're saying...it is good to find someone with similar enough objectives so that supplying needed components to you results in bennies for both...wish you well on that.

since I have a bit of waiting time, I thought that I would stir it up a bit:

BACKPRESSURE (or BACK PRESSURE): the pressure that results from restricting the free flow of the exhaust gases from the cylinder on its way to the outside of the vehicle via the exhaust system. This pressure is measureable on most, if not all, exhaust systems used in cars and trucks, whether NA or boosted [we are talking OE systems as the basis; but A/M systems are similarly afflicted].

the primary reason that there is some measureable backpressure is that the exhaust system has to do more than just route the exhaust gases away from the passenger area: it has to do that job quietly. When the EV opens, the hot gases come out in a rush; this pulse of hot gas generates considerable noise: it's speed is such that the noise generated is like a firecracker going off...aka explosion...

...this explosion-like rush of hot gas has, and creates, its own sound waves and frequencies...which we hear very loudly...

the exhaust system is usually designed to slow down this rush and disrupt or muffle these noisy pulses by restricting the flow via routing the exhaust gases through small diameter tubes...and often by mixing and blending the pulses to break up the frequencies and pulses...aka log manifolds...and then further disrupting the frequencies and pulses to change the multiple pulses into a uniform flow via the use of a muffler.

[yes...an over-simplified explanation...indulge me...]

because the OE makers have to do exhaust systems to achieve low noise levels, they utilize flow restriction/flow disruption methods to accomplish that purpose: a cheap way to get the noise down...

...and because the OEs know that what they are doing causes backpressure, they "tune" the engine to run with that level of backpressure...by "tune", I refer to the fuel metering decisions; the spark timing decisions; and to the valve event timing decisions...aka camshaft profiles/cam timing...that the OE makers use to allow the engine to perform acceptably with tha amount of exhaust backpressure the engine has to endure due to the design/configuration of the exhaust system to meet noise and, in the last 30 yrs, emissions standards...

If a person takes a car so equipped and so tuned, and then removes the restrictions in the exhaust system --in other words, removes all backpressure--, the car will probably not run as well as it did before the backpressure was eliminated. And because this poorer performance is the usual result of removing the exhaust backpressure, the myth has been born that engines need backpressure in order to run well...or even worse, that an engine needs backpressure in order to run...at all.

that myth is exactly that: a myth....resulting from a lack of understanding of the true cause of the poorer performance...and due to the perpetuation of mis-analyzed anecdotal evidence.

Changing the backpressure that the engine has to deal with also changes the "tuning" requirements listed above. If the engine can adapt its tuning specs to the new backpressure levels, it will run just fine with NO backpressure. If the tuning parameters cannot be adjusted to the new reality of no backpressure, the engine will run like crap. No backpressure is not the cause of poor performance...poor tuning is.

An engine does not need ANY exhaust backpressure in order to run well...if it is tuned for such operating conditions.

I repeat: backpressure is NOT needed for an engine to run...in any way, shape, manner , form, or amount....if the engine is tuned for no backpressure. And this is true for NA and boosted...

Allow me to state it as I mean it:

BACKPRESSURE STINKS!!!!! IT IS NOT YOUR FRIEND!!! YOU DO NOT NEED IT!! BACKPRESSURE HINDERS POWER!!!
[that was the edited for family viewing version]

Was that sufficiently clear?

*****

There is a problem, though, in the real world: it is kinda hard to totally eliminate backpressure.

hmmm...what to do? We deal with it; minimize it; and do what we can to turn our enemy into a useful enemy....hopefully.

...more as time allows...thoughts?

and...Robin and Shane...no offense intended; I have to do this in parts; I wanted to get the definition of backpressure stated and started; my position stated/explained; and further explanation/discussion re your comments will have to wait...my apologies for that...[back to the saltmine...]

Canuckvolvo....kudos on tightening up the squish!! more details when available please...

[CNGBrick]

BACKPRESSURE STINKS!!!!! IT IS NOT YOUR FRIEND!!! YOU DO NOT NEED IT!! BACKPRESSURE HINDERS POWER!!!
[that was the edited for family viewing version]

Was that sufficiently clear?

Just thought I'd throw this one in:

Watson and Janota in Turbocharging the Internal Combustion Engine state something to the effect that some back pressure is desirable in a pulse type turbo exhaust manifold. The reason stated is that a significant amount of pulse energy is lost during critical flow at the EVO (sonic flow condition). Having backpressure reduces the pressure differential before/after the valve seat so that the flow drops out of sonic earlier and preserves the pulse energy to drive the turbo. They claim the energy contained in the pulse travels through the runner with little loss and the result is a net gain in efficiency.

A suggested way to do this is to use smaller diameter runners in a pulse type manifold than would be used in a constant pressure design. They suggest runner flow area to be only slightly larger than exhaust valve flow area at maximum lift. Corky Bell appears to allude to this when he states that smaller is better when you have the choice of two sizes for your exhaust manifold runners.

This being said I haven't experimented with manifold designs so can't back this up with actual dyno testing.

Good meaty topic, it's been a while.

Richard Thomas

[Bishop]

In a engine designed for maximum flow, I agree with stealthfti in that no actual backpressure should ever be needed even in the headers.

Motorbikes are part proof you just don't need backpressure in a exhaust system period, if you look at length and design of bike systems, they are basically running a set of headers with a can on the end to kill some sound.
Bikes even like to use very long primaries to maximise flow, but then the engines have the correct state of tune to begin with.

There are other factors though in exhaust design, keeping exhaust gas velocity up is very important, more so in engines that spend a lot of their time at less than full rpm, street engines are that kind of engine.
A 2.5 inch system will flow great on a NA engine at 6000rpm, but what happens at 2000rpm?

I'm not 100% sure of the total concept here, but I'll try and state it how I see it.
A lot of motorbikes now run an exhaust valve, that is a actual valve in the exhaust system that opens and closes depending on engine rpm.
Generally the theory there is not to restrict the exhaust flow in any way, but to keep the exhaust gas velocity high in the primary part of the exhaust system, so that it's kind of reducing backpressure at low speeds by reducing turbulence in the system, keeping exhaust gas flow high.
And if you look into new tech stuff for cars, you will find that some car makers are looking into this tech for their cars.

About the only point I'm trying to make here, is that some have stated that some cars need backpressure because they put a huge system on car X and it did not work as well as a smaller system on the same car.
Well, my answer there is that it's the change in exhaust gas velocity that is causing the problem, and it's not actually backpressure that is the issue as such.
By oversizing the pipe, you reduced the exhaust gas velocity, and while the issue there does get fuzzy because you could almost say you increased backpressure, I don't think you have, you have just slowed the flow.

I'm confusing myself now, but I think you will get the general drift.
It's why I never recommend people go too big a system on NA or turbo engines, and why so many modern day turbo exhausts drop from 4 inch to 3 inch then to 2.5 at the back of the car, and still produce great power.
And some aftermarket exhaust systems for bikes that don't have an exhaust valve often have a removable restriction in the end of the muffler, if your riding the bike at full rpm it will effect the power, so you leave it out, but at lesser rpm riding it feels better to leave it in, it's not really creating backpressure at low rpm, it's keeping up the exhaust gas velocity out of the system.

I think exhaust gas velocity vs backpressure is kind of the issue here, but somewhere between the 2 is the perfect exhaust system.
Did that make any sense? it sounded ok in my head...

[Anonymous]

Richard...good points...and the fact that backpressure can be used to a good end does not change the validity of my contention: backpressure is not needed for an engine to run.
[[final preview note: I appreciate your points, and think they supported my suggestion to use what we cannot totally eliminate as a tool...and inspired the following...]]

I also suggested that since it is very difficult to eliminate backpressure, then the effort should be to minimize it, and to try to use it to accomplish what is needed for an engine to run. I did not stipulate what that is, or even hint at what "it" is...hadn't gotten that far. In my little tirade, I was trying to dispel the myth of backpressure being some necessary thing for engine operation. I may have been perhaps too simplistic in my explanation, admittedly.

To try to remedy that, allow me to say:

...if the myth of backpressure being a required or needed condition for engine operation can be seen for what it is: a myth; then the confusion that that myth creates regarding the understanding of what actually goes on during the 4 stroke cycle--with emphasis on the exhaust stroke; and the transition to the intake stroke--can be removed.

What do I mean by 'confusion'? Well, if backpressure is necessary for engine operation, that would mean that pumping losses incurred due to backpressure are somehow required: if the piston has to work harder to empty the cylinder because this backpressure is so necessary, then that must be a good thing...Huh??? the notion that making the piston work harder to empty the cylinder is desirable or somehow 'good' is nutzoid...but it is the logical follow-up to the notion that backpressure is required.

I cannot see any validity to the notion that more pumping losses is good. The objective of the exhaust stroke is to empty the cylinder of the burned and spent air/fuel mix as completely as possible, and to prepare for and assist the refilling of the cylinder during the intake stroke...and to do all that as efficiently and as easily as possible...by 'easily' I mean with the least amount of energy being expended by the piston and crank to accomplish the objective. Any such energy required over the minimum to do the job is a pumping loss: the piston is working harder than it should have to.

That is one side of the confusion caused by the myth of backpressure being required for engine operation.

Another side of the confusion created by the myth of backpressure being required is that the myth clouds, obscures and misleads the understanding of what really is needed for an engine to run...and to run well.

...and that would be flow...good flow of the exhaust gases at the velocities that promote good running and build power.

Backpressure does not make torque...correct exhaust gas velocities makes torque. If the velocities are right for the operating conditions, the flow is right; and the power is made. To say that it is the backpressure that builds the torque is a mis-analysis of the facts...controlling the exhaust flow to achieve the optimal gas velocities for the operating conditions is what makes the torque, not the backpressure that may be the result from that controlling of the flow. Using a smaller diameter primary pipe to control the flow to achieve the desired velocity of the exhaust gases will probably result in some measureable backpressure; but that measured backpressure is not the cause of the proper velocities that results in the torque being generated. It is the correct gas velocities, achieved by controlling the flow, that makes the torque. The means of controlling the flow...a smaller diameter pipe...which causes a restriction of the flow--resulting in measured backpressure--is exactly that: the MEANS to achieving the desired RESULT: the correct gas velocities.

now...just what this 'good flow' is, and what these 'correct velocities' are are part of what this discussion is about, and are part of where I hope we go with it...

I also hope that we can get into discussing the differences between the pulse type and constant pressure type of manifolds for the boosted side of the topic...our OE manifolds on the turbobricks are of the constant pressure type...it works...but I think pulse type is better...do let's get into that...

****
Bishop...well stated...and yes, you made sense...thank you...
****

...thoughts?

[Bishop]

stealthfti, cool, that's just where I was partly trying to go, backpressure is the great myth to developing torque in engines, because it's good exhaust gas velosity that is the key.
Although a side effect can be backpressure at high rpm, sort of, but you have a better grasp of that than I.

Yamaha have been using the "EXUP" valve in the exhaust sytem of their bikes for years, but even in the motorcycle scene people missunderstand it's purpose, with many people claiming it created backpressure in the system to improve low to midrage torque.
The reality is that Yamaha used it to keep exhaust gas velosity high at low to mid rpm, meaning they had 250-1000cc bikes in the 80's that ran rings around the compitition, at low to mid rpm.
In fact aftermarket exhaust makers back then were all confused by the system, they kept pulling it off the bikes to replace them with full systems, only to find that the bikes were slower, so in the end they gave up and only offered exhausts that replaced the system from the EXUP valve back.
(Note that Suzuki are now using the same tech in their new bikes, they call it something else, but it's the same system.)

[CNGBrick]

stealth,

I agree with the general direction you are taking and suspect you will be bringing up other points for the benefit all interested.

Backpressure certainly increases pumping losses during the exhaust cycle and can be problematic during valve overlap. Applying the same reasoning one could also argue that increasing intake charge pressure increases pumping losses and that we should aim at reducing that as well :wink: . (We all know that the benefits of increased intake charge pressure far outweigh the increased loss.)

The basic idea here is to look at the overall result. Backpressure is not "bad", it just "is". It all depends on your goal. The turbocharger needs "high quality" energy to operate and the quality of this energy is greatly improved by a small increase in pumping losses (both on the intake and exhaust portion of the cycle).

In this case, pressure in the exhaust manifold (however achieved) reduces the amount of time the exhaust gas spends in sonic flow at the exhaust valve. Sonic flow produces a shock wave and results in an unrecoverable loss of energy in the exhaust gas -- energy that could have been used to drive the turbocharger.

Just my way of looking at things :wave: .

RT

[Captain Bondo]

Nice to see Richard in on this too!

Great points. One comment on the backpressure within the manifold is that when gases are supersonic, and you are dealing with 4 differently timed pulses interacting things are less straightforward. Also, the exhaust still has "work" do do. The goal here is not to have the gases leave in the most efficient manner, well, it is a secondary goal to havingthem spin up the turbo as efficiently as possible. Once they've done that and have exited the turbo then all of the said rules apply again.

John,

I'm sure you've thought through everything carefully, but I was wondering if, instead of completely reinventing the wheel, you changed to a newer-tech, Garrett GT series ballistic ballbearing turbo and did some headers.

Those turbos are supposed to drop boost curves by 1000 rpm on their own. Between that and some headers you'd be approaching the supercharging with better compressor efficency, and only need a small re-map. Should be cheaper too- depending on your sources I guess.

Thoughts?

[bcrazy]

stealthfti

Totally agreee with you on the backpressure theory.

Questions:

1)
If you remove backpressure as much as possible, will the 2.4LH-JET be able to tune the engine to run good with very little backpressure?

2)
Is it also a myth that 4-2-1 or tri-y make more low down torque that 4-1 headers?

[Anonymous]

Sorry I haven't contributed in a while - been busy. However, I found this article laying around that some might like to read. I forget where I found it, or if Ian just sent it to me one day...

http://www3.telus.net/public/dupuis10/Cylinder%20Head%20Tech.doc

[Anonymous]

Hey, a good way to think of exhaust in a non turbo is siphoning gas. At high rpm the pistons have to expel the gas by stealing some energy from the crank, right? If the exhaust/header pipes are just small enough to build velocity in the gas, the inertia will help draw gases out of the pistons. Any extra gases drawn out by siphoning effect means the engine doesn't waste energy forcing it out. Well, when you siphon gas you use a small hose so the velocity of the gas is high enough so the momentum will keep drawing the gas out of the tank. Try siphoning gas with a 3 inch hose, assuming it would fit down the fill hole!
dave new dave-new.com

[Anonymous]

Hey, a good way to think of exhaust in a non turbo is siphoning gas. At high rpm the pistons have to expel the gas by stealing some energy from the crank, right? If the exhaust/header pipes are just small enough to build velocity in the gas, the inertia will help draw gases out of the pistons. Any extra gases drawn out by siphoning effect means the engine doesn't waste energy forcing it out. Well, when you siphon gas you use a small hose so the velocity of the gas is high enough so the momentum will keep drawing the gas out of the tank. Try siphoning gas with a 3 inch hose, assuming it would fit down the fill hole!
dave new dave-new.com

[Steve C]

[quote:cc65324136]when you siphon gas you use a small hose so the velocity of the gas is high enough so the momentum will keep drawing the gas out[/quote:cc65324136]

Absolutely wrong. That isn't what makes a siphon work. Any hose will do once it is primed.

[Anonymous]

unfortunately this has to be fast...

bcrazy...2 very good questions...

...can LH2.4 adapt to a near total elimination of BP? I do not have a definitive answer, so I cannot say for certain that it can or cannot. But I will give a qualified yes in answer...I recently assisted in a B230F/LH 2.4 enhancement in a 92 240...V15 cam and 2.5 exhaust from the end of the twin pipes of the front pipe on back with a hi-flow 2.5in cat and resonator and single muffler...the improvement in launch acceleration; in-traffic accel; and on-ramp merge accel were very noticeable; the customer was quite pleased with the results. The 2.4 system adapted to that configuration adequately: no errors detected/no codes set...fuel economy same or improved [no feedback on that yet]...the only further enhancement I could have/would have suggested would have been a 4-2-1 header; with one of the tri-y's we have discussed priorly being a recommended alternative to a custom built 4-2-1...for that app and the customer's intended use, I was disinclined to recommend a 4-1 header...

...on that car, I was relieved that the 2.4 did adapt to the mods done...but would have been happier if I could have been able to play with base timing...[but 2.4 does not allow that]...there is room for improvement if the timing could have been played with; of that I am confident.

[I think 2.2 is a better choice for some things...greater adjustability is built in...]

The above vehicle ended up with some reduction in BP; but not as much of a reduction in BP as you are considering. But, in actuality, the exhaust system you outlined is not BP free...more of a BP minimized/utilized layout...so my conclusion is that 2.4 could handle that; but with timing being non-adjustable, dialing it in is gonna be left up to the computer.

...I hope that helps your analysis...

#2...no, it is not a myth...

to clarify that reply, let's ask it this way: which makes better bottom end: 4-1 or 4-2-1 or tri-y?...
...hmmm...the answer is and will be debatable/debated. My reply is: a properly sized 4-2-1 would give the most; a properly sized tri-y would be second and a properly sized 4-1 would be third...[again, my opinion keyed on an inline engine, not a V motor]

...you'll notice that I said 'properly sized'...that's the joker in the deck...the 4-1 will give the most at the top end; but the tri-y and 4-2-1 will outdo the 4-1 on the bottom end, if dialed in size-wise for the app/use...more on that as I can steal an evening to write up some things for the topic...

davenew...your comments show good understanding of velocity/pumping losses...but Steve is correct: any size will work once primed...the tuffie is to have enough lungpower to prime a big hose...

JohnLane...I think I did err some in suggesting twinning...I looked at the pics oppositelock has uploaded on the SPE...and I have some more complete thoughts and suggestions for you on that...to assist my efforts: is the displacement at 5100CC? how stock are the exhaust manifolds that you are using currently? do the rules allow headers? and what T04 are you using? single or twin scroll? I am not going to presume to tell you what to do; but I have some better suggestions to offer...if you would care to hear them.

Matt...figured you was having fun in Houston...

RT...you be not off the hook just yet..heehee...

[...'priorly' is probably not a valid word...but like Corbin Dallas, I only speak 2 languages: english and bad english...my deficiency]

[Anonymous]

Core... Bin... Dah... Lass? Big Bang Boom! Heheh!

Houston is in two weeks, but I'm still Jazzed about seeing SCOTS and The Rev.

Let's talk about headers, baby! I need to hear what you have to say, Tom. You challenged everything I understand about header design, so I'm gonna hold you to a cohesive and coherant answer, okay?

I also need for you to distinguish between "tri-y" and "4-2-1". They're basically the same, and I imagine that you're using the terms in reference to the length of the primaries... However, someone mentioned seeing a header design that combined pairs of cylinders that fire 180 and 540 degrees apart - i.e., 1&2 and 3&4. Doesn't make much sense to me, and that person couldn't show me an example of what he saw (or thought he saw), so I have to believe he was mistaken.

I'm going to expose my possibly-faulty logic on the subject, and give you the chance to enlighten me: Long tube headers resonate primarily at one RPM - there are harmonics that help other RPM ranges but are generally not very large. 4-2-1 headers resonate at the frequency corresponding to the tube length of the fork and the leg (primary and secondary, if you will) of each Y, and also resonate at the frequency corresponding to just the fork (primary). However, since the longer path - which resonates at lower frequencies (and therefore RPM) has to share "volume" with the second primary, the resonant pulse at low RPM is weaker than that of a long tube, 4-1 header. Also, since the secondary header leg needs to flow two cylinders' worth of exhaust, it should also be bigger so each pulse is going to be weaker as well.

However, I can also see that with the 4-2-1 header, the scavenging wave generated from #1 exhaust can also affect #4 exhaust, which allows a resonant pulse to occur at half the RPM one would expect it to with the same length 4-1 header. This could give the 4-2-1 header the advantage at low RPM, provided the pulse is strong enough to make use of. Also, careful combination of primary and secondary leg lengths could set up reinforcing resonances to make bigger scavenging pulses. (One caveat - whenever you have two point sources for pulsing signals of different frequencies, there WILL be cancellation effects. The header designer must try to minimize the effect of these wave cancellations because they would act in exactly the OPPOSITE way we want a header to behave)

I've heard and read arguements both for and against long tube, equal length headers, and for me they seem the safest bet. That's just because I don't know enough about harmonics and thermodynamics to intuitively design a 4-2-1 header, I'm sure... Most of the header experts are old V8 guys who cut their teeth in the late 50s/early 60s, and only a couple of them know or care anything about 4 cylinder engines. I'm also a bit loathe to trust anyone who has designed a header within the past 15 or 20 years, simply because marketing and individuality plays such a big role in what products are successful, rather than good design.

What's the difference? With a V8, you don't have evenly-spaced exhaust pulses on each side of the engine. Therefore, getting two pairs of cylinders on each side to "talk" to one another is difficult at best. My feeling is that long tube, equal length 4-1 headers are easiest to design for a specific torque improvement, while 4-2-1 headers are spreading the scavenging frequencies around all over the RPM range, giving a general improvement. That's my feelings about that subject on a V8 engine specifically, but there is some truth to that for the 4 cylinders as well.

Good reading on both sides of the arguement comes from Ed Henneman (Headers By Ed) and Doug Thorley (Doug's Headers), with additional old-tyme experience provided by Jere Stahl (Stahl Headers). Henneman is passionate about the 4-1 design, while Thorley believes in the Tri-Y design. Stahl builds 'em both. All three were successful drag racers in their day, where Ed and Doug were more known for their headers and Stahl for driving a Dodge.

The one consession Ed makes to the Tri-Y design (and I don't know if he's aware he's making it) is with Pontiac engines. They used siameesed center-cylinder exhaust ports, which fire #3 and #5 cylinders (and #4 and #6 on the other side of the block) down the same pipe. He found huge, repeatable success from sizing the center pipe according to the flow of the combined pipes, rather than on a per-cylinder basis like everyone else was doing. He kept the length the same as the other pipes, though... Would more torque have been produced if the center pipe were twice as long as the outboard pipes, due to the fact that it's seeing twice as many pulses as the others, or did the close spacing (180 crankshaft degrees) apart preclude that?

Richard and Kenny made good points on "sonic" and "critical" flow, and some examples how reducing backpressure in an exhaust might reduce the torque produced in a turbocharged engine. Interestingly, one other way of getting the flow to drop out of the sonic range earlier is to have larger runners before the turbo...

Someone mentioned split turbine housings... They are supposed to help low RPM turbine efficiency, so long as the exhaust header is split as well, to deliver equally-timed pulses to both sides of the turbine housing. This is done by keeping the effective volume smaller by splitting the exhaust systems into two right up until the turbine wheel, so each pulse has less room to expand and has a stronger impact on the turbo.

I think Corky Bell basically said, however, that once you start cramming too many cylinders into each half of the turbine housing, you reduce the need for the split housing. Same goes with higher RPM - you'll reduce the need for the split housing, and you could even be reducing the overall output versus a single-entry housing. On an engine like a V8, with the inability to get even pulses on each bank, does the split turbine housing even work? Higher RPM 4 cylinder engines probably flow enough exhaust that a split housing isn't required or even desiarable, either.

You'll notice that on the late-model Turbo exhaust manifolds, the design directs exhaust gasses towards the turbine with internal "ramps". This is an attempt at ducting the pulse directly into the turbo, rather than the earlier design where all the runners are allowed to collect and even out in the plenum before entering the turbine housing. Is this why the late-model manifolds work better?

Enough for now - that gives everyone something to chew on for a while, I just hope it doesn't get spit back at me!

Matt

[CNGBrick]

Interestingly, one other way of getting the flow to drop out of the sonic range earlier is to have larger runners before the turbo...

Just a clarification: I was not refering to the flow being sonic in the manifold but at the choke point between the valve and seat when the pressure ratios across the seat are great enough to produce sonic flow (also known as choked flow).

The flow drops out of sonic when the pressure ratio is sufficiently reduced by blow down of the cylinder pressure and pressure build-up on the downstream side of the valve. With a large volume downstream of the valve the pressure takes longer to build up.

On another note, the pressure pulse apparently makes its way through the turbine and exits downstream. That brings up the point of pulse tuning after the turbo. The turbo speed responds to the instantaneous pressure differential across the turbine. Pulse reflection on the downstream end would then affect upstream tuning. Oh well, more headaches.

RT

[Anonymous]

Matt...ok, I will put together an answer for you...but that will take a few days...

RT...interesting on the pulse getting past the turbine wheel...had not heard it put quite that way before...but I see what you are saying...

I think that using a divergent nozzle at the turbine discharge--ie opening up the diameter of the downpipe right after the flange--would resolve that problem; and resolve another problem with the flow at the turbine discharge: spiral flow...belling out the diameter of the DP to one larger than the turbine discharge helps disrupt the spiral flow of the gases and transforms them into a linear flow, reducing the turbulence of spiral flow and improving the velocity of the flow...and thereby reducing BP at the turbine discharge...this reduction of turbulence of the gas flow downstream of the turbine discharge, primarily due to the spiral nature of the flow at the turbine discharge, is also the reason why it is usually recommended that when using an external wastegate, the gases should not be routed back into the DP for at least 14-20 inches after the turbine discharge...the further on down the DP the gases can go before they are interrupted by the gases from the external wastegate, the less turbulence will be created...the spiral flow has a better chance to change to linear flow before being further disturbed by the gases from the external wastegate...a smoother merge; less turbulence; better flow; less BP...

the divergent nozzle/larger diameter DP won't necessarily eliminate the pulses, but I think it reduces the strength of those pulses to the point that we don't have to specifically address them via configuration...[but I may be in error on that]...if the d-n/l-d DP can straighten out the spiral flow, I think it also acts, to a perhaps limited extent, as a pressure wave terminator due to the larger diameter...acting like a resonator...how well it does that would be affected by the shape of the diverging nozzle and how large the diameter of the DP is...

...thoughts?

[bcrazy]

I've just installed a 2.5" exhaust. Since i thouhg that straight trough all the way would be a bit to loud, i ordered a back muffler from a Simons 2.5" catback system made for Volvo 940(na) To my big surprice the back muffler is straight trough but not fully 2.5", mabye 2.25" I used this together with a short glasspac type 2.5" in place of the stock resonator. I used 2.5" piping from the original header and back. I will put in a high flow 2.5" cat as soon as possible. Performance is notably increased at low and mid revs. From 5K-6K performance is the same as stock, but thats probably because the engine doesn't make any power here anyway? Engine seems to run much more "free" and i can lug along at low revs easier. Sound is not too loud a nice deep "rumble" at low revs that changes into a bit raspier at mid and high revs, not ricey.

[linuxman51]

RWE (real world experience) Had 2.5 pipe run from the downpipe back (stock downpipe), shallow bends over the axle (altho it hits, so its going back to the shop), this is significantly quieter both out the back and in the car than just open down pipe (never mind that infernal flange rattle). Car seems to move a little better as well, but thats all butt-dyno and perhaps a bit of tuning. It certainly hasn't lost any power, and this might be completely unrelated, but it seems as though with a free flowing exhaust one is able to run more timing both on and off boost... I went from 37 degrees at normal cruising speeds to around 43, and gained 3-4 mmhg, constant tps, it was click click click click burn vaccccum.. (in order to maintain a constant load, we were actually accelerating slightly)
how about some thoughts on that? The exhaust has a definite roar to it under wot, but its certainly not unpleasant for me (and whoever's behind me can just deal with it) 8-)

[Anonymous]

All tri-y's are 4-2-1s...most 4-2-1s are definitely NOT tri-y's...and some headers that are called 'tri-y' aren't really tri-y; regardless of the maker's claims.

Most of the 4-2-1s that you find available in the marketplace are cosmetic headers: they look nice...and they probably do improve power 'some' if compared to the cast iron manifold being replaced...but essentially they just look fast.

On a V8 or V6, you cannot really "do" a tri-y....the closest that you can come to a tri-y on a 'V' motor is what is known as a '180 degree' header: where you pair the cylinders that are 180 opposed in the firing order [think of the distributor cap: pair the cylinders that are across from each other in the cap]...another way to describe a 180 degree header is to look at it as pairing the cylinders that are "1.00 revolutions" apart in firing...the resulting header, besides being like a pile of snakes, will actually be a 4-2-1 header assembly...that entwines itself down the sides and underneath and around the engine...it is much too integrated to be called a 'pair' of headers; it is an assembly. The Doug Thorley's Tri-Y's are not true tri-y's: they are 4-2-1s that pair cylinders that are not opposite in the firing order...close, but no cigar.

To keep this as clear as I can, I am going to premise the rest of this on an in-line 4 cylinder [I4] configuration...

What is a 4-2-1? it is an exhaust system configuration/layout that pairs the '180 out' cylinders...1-4; 2-3 on our redblocks...these two sets of paired cylinders are then merged into secondary pipes: one for each pair...and then finally merged into a single pipe. The stock exhaust system on our NA redblocks is a 4-2-1 configuration.

What is so great about a 4-2-1? In brief: it flows a lot better than a log style manifold...less interference/turbulence between cylinders...and is still reasonably compact and cost-effective....[more on this in part two]

[a very good source of information on all of this, backed up with scientifically obtained data/results/observations, is "Scientific Design of Exhaust and Intake Systems" (3rd Ed.) by Philip H. Smith and John C. Morrison...I highly recommend the book.]

The stock version of a 4-2-1 that we find on our redblocks works ok. Actually, it works pretty darn good...the horsepower per cubic inch output on our NA redblocks is a very respectable number for a factory stock production NA 2v/cyl engine. And the 4-2-1 exhaust contributes to that accomplishment.

But it could be better. Sizing of the length and diameters of the primary and secondary pipes are how you optimize a 4-2-1...and a cast iron primary "pipe" section where none of the four "tubes" are the same length is definitely something that can be improved upon.

What is a 'tri-y'? It is a 4-2-1 that is configured in such a way as to optimize the paired primary tubes to improve flow and velocity...incorporating pulse tuning and wave tuning calculations and formulas etc etc to get good flow and velocity at the low and mid rpm ranges.

OK...sounds just like a 4-2-1...true...but a tri-y does something else...it uses the primary paired tubes differently...in a tri-y, the paired primary tube is used as a fake-out: a tri-y design uses the paired primary tube to make the exhaust gases think and act as though the primary tube for that cylinder is twice as long as it actually is. And a tri-y design also uses the paired primary tube to generate an interference/reflected wave action phenomena that gives the exhaust gases from #1 and extra kick in the butt to keep moving.
...example: #1 cylinder's primary tube use #4's primary tube as a blind alley...the exhaust gases come flying out of #1...rushes down the primary tube to the merge...most of the gases continue on past the merge; but some goes up #4's primary tube [the twice as long fake-out]; and since #4's exhaust valve is closed: it's a dead end. Not only is #4's primary tube a dead end for the gases from #1; it is --just as importantly-- also a dead end for the pressure waves that came out of #1 cylinder; these pressure waves also travel up #4's primary tube. When these pressure waves hit the dead end and reflect back, it creates what Smith and Morrison describe as an interference action phenomena that uses the reflected waves to give the gases an extra push...or you could look at it like a second push past the merge...

...and to get that just right requires some serious engineering and development effort...diameters; lengths; angles of merge...lots of fun things to get right...

Chapter 5 of Smith and Morrison's book really gets into the nitty-gritty of this; and explains it much more thoroughly than my crude attempt here.

Bottom line: true tri-y's DO work...very well...esp in the low and mid ranges when compared to a 4-1.

*****

the rest of this answer [to Matt's question] as to why I stated that a properly sized 4-2-1 does more than a tri-y or a 4-1 for the low and mid range will have to wait for part two...

[245gti]

No offense, Tom - but I'm not buyin' that "dead end" theory just yet. Maybe it's their attempt to explain something else? I guess I'd have to read the book. I'm not slagging you at all: it just sounds a little "mystical" that the dead end makes the primaries seem twice as long as they really are, in the way you explained. I can see how the reflected pressure pulse heading up the dead leg might get reflected back to kick the gasses down the secondary tube again, I guess.

Yeah, Thorley's headers simulate a tri-y design by pairing cylinders (on the left side) that are 270/450 degrees apart, and (on the right side) that are 180/540 degrees apart, but as you say, they're really a 4-2-1 by your definition. However, if you believe in the dead end theory, shouldn't Thorley's headers work too? I mean, the combination of primaries would still make each primary look twice as long, wouldn't it? They wouldn't interfere/reinforce each other, but SOME aspects of their design is valid.

In short, a tri-y sounds very delicate, whatever explanation you believe. If you were to build a header, which one would you attempt? Have you designed, built, and been happy with a tri-y, Tom?

Again, I'm not trying to pick your explanation apart - I think something's getting lost in the translation (or transmission), and I don't understand what the authors of your reference were trying to explain. I know I'm biased, but I can "see" my expanation much more clearly...

Keep explaining, though! Remember, I'm a pessimist and I only trust myself, so you've got your work cut out for you.

Matt

[Anonymous]

That was me - I have no idea how I got logged in as Dale... Occasionally I borrow his login to read the off-topic or the for sale sections, but I haven't been logged in as him in weeks. Stuh-range!

Matt

[isaac]

I haven't read through this rehashed monster thread but have something to note.

Newest SCC has an SR20 engine putting out more torque throughout the curve with a 3" exhaust compared to a smaller one. Thats in a 2 liter engine only putting out about 154hp. It's all in the shape and flow.

[Anonymous]

Matt...I will have to recommend that you read "Chapter 5: Pressure Phenomena and their Application"...

...and since it is a scholarly tome, it will be rather heavy reading...I had to wade through that chapter at least three times...and apparently I need to do it a few more times: my attempt at a summary was obviously substandard.

Even though I cannot adequately explain it, how a true tri-y works is for real...and if you want a personal reference on that; ask slomo242...he has one in his ride...[discussed in the "NA v turbo' topic]

Would I try to build a tri-y? no...I can buy one...actually one of at least three configurations that are available through Unitek or KGTrimning and others...the price is reasonable; and truly cheap if factoring in the time to trial and error test one and develop it. I would not describe it as 'delicate'....'well tuned' would apply better I think....it is not a project for the average do-it-yourselfer; except for those DIYers with unlimited dyno time available....

The fact that three different configurations of the tri-y is offered for the SOHC redblock tells me that whoever did it did not try to do the one-size-fits-all BS approach.

*****

I've talked to two people who have used Thorley's TriY's...they were happy with the results. The interference action/reflected wave is probably a factor in the Thorley product. With the merge angle aspect alone [way too acute], most of the 4-2-1s I see out there don't appear to really try to be tri-y...[but I may err on that]...they are just 4-2-1s.

******

At this moment, I am not sure that I will have time to get part two done before I go to Bonneville for some salt flats speed demon watching. If not, I'll try to catch up when I get back. And hopefully, I'll do better in part two. After that, I'd like to get into the boosted side of it...partly cuz I expect to irritate some people then...heehee...[or shoud I have said 'irritate them more' ??]

*****

Isaac...A rehash?...perhaps
'shape and flow'?...of course: that is what it all boils down to...the sand in that vaseline is what shape and what flow...and I kinda thought that that is what we are discussing here: what shape; what flow; the hows, whys, and wherefores; and how to get them...

...I can understand a certain amount of a boredom factor so far for some; after all, we jus' been talkin' NA...the sand in that K-Y is that most of the principles re NA flow apply to boosted as well...a point many tend to discount, disparage, denigrate, and ignore...[their choice; I could care less]...boosted does not remove obstacles...it reduces some, compounds some, and adds others...if this discussion gets that far, perhaps some of those obstacles will be easier to identify, understand, utilize, or overcome......time will tell

[Anonymous]

Tom - thanks for indulging me. Even if this IS a rehash, it's good stuff to be reminded of. Perhaps it will bring some new understanding and some new insight to people and their beliefs.

Again, I didn't mean to make it sound like you weren't doing your jorb - I probably won't understand/believe the book when I read it, either. It'll have to sink into my subconscious and I'll have to "think of it" for myself before I truly believe it.

You suggest that a narrow convergence angle of the primaries is not beneficial to the tuning of a tri-y? That would make sense if a slug of air is needed to travel up the dead leg... As mentioned earlier, a pressure wave will change direction instantly and doesn't flow like air, so an acute angle between primaries would still allow the pressure pulse to travel to the dead leg, but wouldn't be condusive to gas flow in that direction. Hmm...

I guess I have nothing further to add at this time, so enjoy the salt and I eagerly await the next class, professor.

Matt

[CNGBrick]

Since the subject of educational material has come I though I'd mention these books again:

Winterbone and Pearson; Design Techniques for Engine Manifolds, Wave Action Methods for IC Engines; Society of Automotive Engineers, Inc; 1999; ISBN 0-7680-0482-9

Also the companion book by the same authors (more detailed mathematics for the above book): Theory of Engine Manifold Design, Wave Action Method for IC Engines.

The first book is actually fairly readable considering its an engineering text. Second book is only for hard core types who want to get into heavy calculations (partial differential equations, etc.) They cover intake and exhaust system design, sizing of runners and plenums, pulse converters, a bit of stuff on setting up a flow bench, etc.

Another classic read although no longer published is Watson and Janota; Turbocharging the Internal Combustion Engine; The Macmillan Press Ltd; 1982; ISBN 0 333 24290 4.

This book covers turbocharger matching, constant pressure exhaust manifolds, pulse type exhaust manifolds, pulse converters, etc. The main focus is diesel engines but specific sections cover spark ignition engines in automotive applications. Again, this is an engineering text but a good bit of information can be gained by the reader who puts in a bit of effort to read through it.

A good place to look for these books is at the library of any university with an engineering program. A big city main library could also have these. Universities will usually let people access their collections (at least in Canada) but you can't take out books unless you are registered or a member of the alumni.

RT

[Anonymous]

Bonneville was interesting; watched a streamliner hit 313mph at the traps. Did not see much on the turbocharged side; mostly NA or Supercharged.

I will endeavor to get back in the harness asap. Catching up with work is always a pain after playtime.

I have now the muffler on my turbo that is both quiet and flows well: a dynomax 17776...3inch inlet/dual 2.5in outlets....5.5in/11in oval; 23in long/29in OAL....big momma...and my universal mounting system is almost perfected for the 240s...more on that later; hopefully with some pics.

At the Flats, I saw 2 interesting turbo'd rigs:

...a streamliner with 2 inline V8s; a turbo on each side...I think that was the one that did the 313.

...a roadster with a Mirage V6...looked very much like the PRV V6...turbo'd with one large unit. The plumbing was not impressive. It looked like the guy pulled the motor from a F1 car and dropped it into his roadster, intercooler and all. My son may have some pics of these; if possible, I'll try to get some pics together and share.

It was interesting to watch and look. The overall approach was "no replacement for displacement". And most of the higher speed vehicles were old; some of them probably as old as myself....[yup: really old]...all of the bikes I saw that were running were NA; and a few were running around 200mph...talk about gutsy...

the exhaust systems ranged from very stubby pipes to some rather intricate fabrications...and yes, I saw no mufflers on anything there...it was beautiful to listen to, in the pits, and on the course...

I did spend some time reviewing some material for this topic; and will post it as soon as I get caught up with some customer projects.

[Anonymous]

As I spent some time reviewing some material, and looking over related info, and reviewing what has been posted so far here, I realized that a discussion of torque and the effects that the exhaust system has on torque output is definitely needed. Here goes.

Torque is force. Torque applied over a distance is work. Work applied for a period of time is power: horsepower. [the terms 'torque', 'force', 'work', and 'power' are important to understand, and to understand correctly...the relationships of the terms affects proper understanding of engine performance theories...esp torque vs HP]

Our engines make torque. And they make horsepower. The horsepower generated is calculated from the work done over a period of time...as in revs per minute. Torque is what we feel when we accelerate.

How does all this relate to exhaust? and to backpressure? Glad you asked.

The torque output from our engines is related to the VE of the engine. And three systems affect VE: induction, cylinder head, exhaust.

The torque curve we see on a dyno plot for an engine has a 'peak'. This 'peak torque' occurs at max VE.

Yeah, a lot of things affect the torque output...and there are volumes written on that. Feel free to research it further.

But the key thing is that the peak torque output of the engine occurs at the rpm of max VE...ie at the rpm when the three systems affecting VE are most in sync. Which brings us to the tie-in with the exhaust system and torque.

The exhaust system, if badly mismatched to the other systems affecting VE, can have bad effects on VE, and thusly bad effects on torque output. Too restrictive an exhaust sys lowers VE and torque output...affecting both overall torque output [lowering it] AND lowers the rpm of 'peak torque'. If the exhaust sys is mismatched to the other systems in that it flows too much--unrestricted; too free flowing--, that also has negative affects on torque output. The VE balance of the three systems is thrown off. If the balance is way out of wack, torque output is decimated. If the imbalance is not too excessive, the result is that the 'peak torque' rpm is raised to a higher rpm; but with penalties in torque output below that new peak torque rpm.

Another interesting thing is this: it seems that max VE can be obtained, and has been observed, when the exhaust gas flow is in the 240-300 fps velocity range...and some say the area is a bit tighter: 240-260 fps. [ain't gonna quibble the exact value: it is in the ballpark]

Ok...so what. Well, what that means is that when the three systems affecting flow and VE are in sync enough to give us a gas flow in the 240-300fps range at whatever rpm that that flow speed occurs, that is where we will have max VE...and have our 'peak torque'.

What this all means is this:

...the larger you go with diameters of induction components, intake runners, intake ports, valves, exhaust ports, exhaust piping [for the same displacement engine]: the higher you have to rev the engine to get the gas velocities into the max VE/peak torque range.

Think about it. I know that I have given a very simplified explanation; and that some of my postulations may seem off-base. But I believe them to be correct. And helps to demistify why things work how they do re larger diameter runners, ports, valves, pipes etc....as well as smaller diameter components.

*****

I wanted to get this info on the table, and then proceed from there further along the discussion of the topic at hand: BP and exhaust systems. Because what effects sizing etc has on performance is keyed to and based on the fundamentals I briefly outlined above....[things like torque curves and how sizing affects that...aka rockin' the curve]. And I do invite research and comments.

*****

Ian...I do like the layout of the exhaust sys you described: excellent, and of sound principles. Where I might disagree in minor things will be addressed as the discussion progresses. But any disagreements on my part would only be on minor points, not on the overall design...you pretty much have nailed it.

[Anonymous]

Car Craft magazine came out with a big discussion on headers this past month - they published several generalizations and possible errors, but they did note that the mean exhaust velocity that is associated with peak torque is 240-260 ft/sec.

However, one thing that was ignored in the article (and in Ian's earlier post) is that the exhaust valve timing events will fudge this value somewhat, as will the mass and temperature of the exhaust gas. Supercharged engines have more gas in the cylinder, which means more exhaust gas when the valve opens, and that will require a larger diameter primary. Turbocharged engines have more backpressure, so the flow velocity is reduced and smaller primaries are required. Bigger exhaust valve timing events require SMALLER dimeter primaries because they allow a fixed amount of gas to take longer to leave the engine... Stuff like that.

I think exhaust valve timing and overall VE (or VE * Boost Ratio) are important to consider when deciding on a header.

One interesting thing was that they (Car Craft) tested Thorley Tri-Y headers and came up with a huge midrange torque boost, with a subsequently huge loss at low and high RPMs, when compared with fairly normal, unequal length long tube headers. However, unequal length headers WILL give the broadest (though least dramatic) torque improvement, so from that it's pretty clear what a little tuning can achieve!

I've heard of higher numbers for intake flow velocity - up to 450 ft/sec - but that's to be expected, really - cold, dense, fuel-laden intake gas isn't going to want the same optimal gas speed as hot, dry, highly-motivated exhaust gas. Trying to lump them into the same category is dangerous, I think.

Just throwing this out for argument's sake.