Combustion Chamber Size Vs. Burnrate? Also Squish VS. Swirl

[Captain Bondo]

Anyone have any good info or thoughts on these 2 compromises? Or what appear to be compromises?

It's odd, some of the best flow improvements on the 8v head lie in combustion chamber valve unshrouding. But I've been thinking, does moving the chamber walls further away slow down the burn considerably? Basically, creating a longer distance for the flame front to travel?

Also, my chambers have been modified such that if you draw and imaginary line extending from the edge of the valve seat out to the chamber wall, the "cut" from the unshrouded material is in-;ine with this imaginary line (which is standard porting practice) and then the cut extends until it is essentially tangential with the bore.

This allows the gases to follow the most natural path into/out of the port.
This tends to leave a little material near the roof of the chamber which apparently helps swirl.

Swirl is also a major factor in burn rate, since even mixture and fuel being dispersed in an aerosol-like format evenly througout the chamber will create the most even burn.
How does an unshroud like I described above effect swirl?
Another thing that improves swirl is a large piston-to-deck value, but that destroys squish, which is obviously important. So how can we maximize swirl and burnrate yet still improve gas flow?

Anyone have nay thoughts on this? My assumption is that you just try to run a flat top, do your unshrouding as efficiently as possible and leave it at that. Just thought I'd see what others more knowledgable think.

Cappy, this will be a lengthy extended discussion: this is just part 1...... :D :D

CC size affects burn time duration...the larger the CC, the farther the flame front has to go to burn all the fuel: more time til done...you are right there; or I should say we are agreed.

you say unshrouding the valves improves flow.....okay....but at what cost, if any? also: is that the "best" way to do that? [(I am really getting to hate that word: "BEST".....it has the wrong connotation)]...

what I mean by that is this: if unshrouding the valves improves flow but creates or exacerbates other problems, is that the route to follow?

you state an obvious side effect: larger CC/longer burn time....another side effect is reduced squish zone--you've belled out the head part of the CC--so there is less head surface to come into close proximity to the piston crown to make good squish.....(presupposing that you set up the deck height to give a good squish height)

a side effect to that result is diminished squish-induced swirl that occurs just prior to spark ignition of the A/F mix....

another side effect of the belled-out head CC is a larger floor for the burn chamber: the less squish zone between the head and crown means that the size of the floor area for the burn is larger...also lengthening the burn time....and transmittimg more heat to more of the piston crown...and that makes its own problems, which we can discuss in another installment....

so what have you accomplished so far?--rhetorical/not accusatory--.....

well, you have "improved" flow past the valves.....

at what cost?....larger CC; reduced squish zone; larger CC floor...(to name a few)...

resulting in: longer burn time; a larger burn chamber; less squish-induced swirl; more area of the piston crown exposed to more heat....(also to name a few, but not necessarily all)...

that does not seem to me to be a win-win....

to get to the answer you are asking for, we will need to analyze what you are trying to accomplish and how to get there with fewer bad side effects.....

what is it that you are trying to accomplish/achieve/obtain/reach/and make happen?
and if you tell me that you just "want to improve flow", then I will get upset at your over-simplified answer....flow is only a part of the picture.....and only a part of the answer....

to assist your ruminations: why is it that closed chamber heads make more power than open chamber heads? YES. they do.....

answer that, and you will have a part of the answer to your question...

[Captain Bondo]

Well, the idea with unshrouding that i described, the floor itself is not enlarged, material at the floor is left there, partially because of swirl, and partially because it does not help flow to remove it.

Also, the actual quench pads are relatively untouched, especially opposite the sparkplug at the point where the flame front travels furthest.

Another though is that the increased burn time can be compensated for with more total timing, so if the chamber is increased for flow, but the mixture can be kept even, then even though the burn time is longer, it is as CONSISTENT as possible (which i think is a big key) and still fairly complete, so good pressure is developed.

My goal is not to improve flow per se, my goal is to develop the highest BMEP possible at a given octane level. I know better than to think port flow is the end-all be all.

The reason unshrouding works, as far as I can tell, is that the positive effects on combustion chamber pressure seen by the improvement in ve and cylinder filling more than offsets the negative effects on it due to the increase in burn time.

But the goal here is a combustion pressure/octane rating ratio that is as high as possible. A good goal I think? possibly THE goal?

reprise:

before proceeding further in this discussion, some clarifications need to be made...

my comments on the head work you have done were in part based on viewing the pics you posted on pbase......

I am not desirous of turning this into a critique of the reshaping and reworking of metal that you did on the 398 head....I would submit that to the discerning eye of one like Mike Aaro. I am confident that he could look at the head and rattle off at least a dozen things that you did well and otherwise. And I would listen very closely to his comments.

That said, I would submit that there are issues involved with improving flow through the ports of a cylinder head that are much different than just making the "pipe" a larger diameter...among them:

....the flow is not in a straight line; and because of that, the sizes and shapes of the radius' along those turns are critical...and often, just making the "hole" larger actually impedes flow....

....the shape of the bowl--on the intake side-- affects the flow volume/velocity and the swirl of the incoming air fuel mix.....AND affects the direction of the flow into the cylinder....if altered incorrectly, the flow will be directed at the cylinder wall--a barrier-- instead of being directed towards the middle of the bore to create a vortex swirl....

....to that end, I submit that flow testing a head that is not mounted on a fixture that recreates the conditions of the engine--the confines of the cylinder, for example--is self-delusional....I also expect severe disagreement with that assertion....

I will return to cylinder head improvements later in this discussion; there are some options that will deserve scrutiny, comment, and knock-down, drag-out fighting over...

preface to part 2:

a good discussion of the topics you have raised and the questions you have asked necessitate some further clarification on your part....

such as.....what other mods/changes have you done to your engine etc....and some indication of planned further enhancement....

.... a statement of usage or goal for usage of the engine, ie how do you drive it or intend to drive and use it.....

....and a statement of method.....NA? turbo'd? stock boost? high boost/high EBR? high boost/low EBR?

I request these so that we are all talking apples and apples....

the topics raised and the questions asked are, to myself at least, very important.....and there are aspects of engine performance involved therein that we have not yet broached.....hopefully, we will....

I look forward to your reply...

[700mopareater]

Hey, this is a good read

Keep this one going, i'm learning a lot on it

[G96nt]

guys...
after Reading Through.. and gettign What I could From it...


why Are We going through hell to get more air Through a 8-valve head?

I'm assuming the CFM potential of the 8-valve is less than The DOHC (unless the valves Are half the size of the 8-valve? or if The lift/duration is that little of the DOHC)

why Are you scratching, and pulling to get from the SOHC, when the DOHC is a bit easier?

I'm sorry if the principal Is too simple? maybe I'm missing a lot?

but, if you can keep a similar combustion Chamber shape, and volume, but allow for much free-er Flowing setup.. it just makes for higher efficiency. Plus, you now have an extra pair of Valves/cylinder to remove heat from the motor which is a big plus for the Turbos


I think what I'm getting at...

why not go DOHC?
I'm Assuming you can Use the timing belt from a DOHC, as Well as cam-gears to make the timing work (similar to putting DOHC on a SOHC mitsubishi 4g64)

this would yeild a MUCH more Efficient intake/exhaust path..(and possibly raise compression From 7.5:1 to something more performance-oriented like 8.5:1, or 9:1?

[Captain Bondo]

my comments on the head work you have done were in part based on viewing the pics you posted on pbase......

"I am not desirous of turning this into a critique of the reshaping and reworking of metal that you did on the 398 head....I would submit that to the discerning eye of one like Mike Aaro."

Would be cool for him to comment, although i think that might be getting pretty close to talking about trade secrets, and certainly would not expect him to comment in-depth. ;) Which i fully am ok with and understand.

"I am confident that he could look at the head and rattle off at least a dozen things that you did well and otherwise. And I would listen very closely to his comments."

Agreed. there are some things (primarily the chamber floor) that could be done better.

"....the flow is not in a straight line; and because of that, the sizes and shapes of the radius' along those turns are critical...and often, just making the "hole" larger actually impedes flow...."

Agreed. Especially short side radius are key at both ports, and the exhaust short side is terrible, it's almost sharp, like you could cut yourself on it almost. My head porter made several disdainful comments about this.

Interesting side note for those listening in (stealth knows): the bigger your valves get 9especially exhaust in this case) the tighter the short side gets... another interesting tradeoff...

"....the shape of the bowl--on the intake side-- affects the flow volume/velocity and the swirl of the incoming air fuel mix.....AND affects the direction of the flow into the cylinder....if altered incorrectly, the flow will be directed at the cylinder wall"

Agreed again, another point that my head porter discussed, this relates to what i mentioned about the chamber unshroud beginning in-line with the seat, and ending-in line with the bore.


"....to that end, I submit that flow testing a head that is not mounted on a fixture that recreates the conditions of the engine--the confines of the cylinder, for example--is self-delusional....I also expect severe disagreement with that assertion...."

None here. Especially on the intake side, the bore is an "extension" of the combustion chamber, which is in turn a extension of the bowl/seat.
The entire flow path needs to be accounted for IMO.


"a good discussion of the topics you have raised and the questions you have asked necessitate some further clarification on your part....

such as.....what other mods/changes have you done to your engine etc....and some indication of planned further enhancement...."

well, not a lot. major modification are the head, which is fitted with a K cam at this point, as well as the huge IC. I also have a header in the works- basically just large, short primaries.

".... a statement of usage or goal for usage of the engine, ie how do you drive it or intend to drive and use it....."

I plan to drive it like an animal. LOL. but really, it's going to be a street car, but I can put up with a lot "driveability" wise. Expect redline to be about 7000rpm, with main useable power around 4000-6500

"....and a statement of method.....NA? turbo'd? stock boost? high boost/high EBR? high boost/low EBR?"

Turbo. With the header will be fitted a Super 70 trim hybrid, 63 A/R exhaust side. Boost anywhere from 18-25psi on toluene. ;)

basic premise of my project is: better than stock breathing, expecially on the exhaust side so maximize the benfits of a higher lift cam, low backpressure, lots of efficient boost with a lowish CR (8.2:1), excellent intercooling, and correct engine control.


"the topics raised and the questions asked are, to myself at least, very important.....and there are aspects of engine performance involved therein that we have not yet broached.....hopefully, we will.... "

That's the hope. ;)

looking forward to your continued observations...

[SLO240]

On the topic or swirl. If swirl creates a more even and "better" A/F mixture, would it be benificial to add a vortex patern to the to the walls of the head, to "spin" the incoming air and fuel and direct it into the chamber, much like the rifling on a gun to spin the bullet for better accuracy. Or would that kind of modification create more hinderance and no benefits.

G96nt...contrary to popular misconception, I would suggest that a well-prepped 8v head can perform as well as a similarly prepped 16v head....ask Mike Aaro...

if I were doing a Street/Strip or Racer approach, I might consider the hassle of going 16v....but since I am normally a Daily Driver--and occasionally a 004 Bondo syndrome "animal" enjoying the turbine whine and exhaust roar-- I see no need to go through the effort of converting to 16v.....I can get what I want with the 8v.....and I will admit a personal antipathy towards the 16v.....and yes, it is a personal choice....if 16v turns you on, go for it....

further, I would not describe it as going through hell re the 8v.....this will be explained as the discussion continues.....no offence intended...

SLO240...

what you describe has been tried, with less than gratifying results....cuz when the air/fuel mix reaches the bowl, then turns and spreads around the intake valve, any swirl created in the runner is undone and totally redone....the bowl/valve seat area is one of the critical spots....

but you're thinking in the right direction.....swirl is very important....

Cappy, thanks for the info....will work on part 2....I have a few questions for you....

[Boris740]

I wish I had a camera. I just finished porting and polishing a 530 head. I was afraid to remove to much material as not to bugger the CR. Instead of unshrouding the intakes I have undercut a grove in the wall nearest to the intake valve and in the direction of the sparkplug. The profile of that part of the wall now resembles a semicircle. This was to improve the swirl by providing a easier path for the intake gasses in that direction. In order to compensate somewhat for the removed material I had those rectangular indents opposite to the sparkplugs fill welded. The exhaust valves were mildly unshrouded. The valves were polished to a chrome shine. The head is out for three angle valve grind and when it comes back I am putting in the Enem V15 cam.

[Bready]

Great thread

Nice to hear from the mighty stealth again.

...pardon the interruption.

JB
KTP

I was pointed to an excellent source of info on head flow work parameters and guidelines; which reminded me of some things that need to part of this discussion....and I also reviewed some material I had read a while back....

before proceeding further, some items related to porting heads for max flow need mention....

the one thing that limits the maximum flow obtainable is the size of the valve head......and the shape of the valve head can have an effect as well...

most sources I have consulted strongly caution against the tendency to enlarge the size of the ports, stating that velocity through the passage is the more important thing, and that making the cross-sectional area of the passage too large in relation the the valve area would actually reduce maximum power...

in other words, for a particular valve size, there is an optimal cross-sectional area for the runner (the port) in order to achieve the maximum flow....if you exceed that max size, you WILL reduce the maximum power output....why? because maximum power is obtained when you have the smallest port cross-section that will deliver the flow.....

let me rephrase that once more: the limiting factor for maximum flow through the port is the size of the valve head: it's area....if you make the port too large in diameter (it's cross-sectional area) for that valve head area, you WILL reduce the maximum power that that head can make....because you have affected negatively the velocity through the port....and the velocity of the flow has a serious impact on power making capability....yup, those ports are HUGE...they be BEEEYOUTEEFULL....and the motor makes less power......

OH NO!!! CAN'T BE!!! I would suggest visiting the superflow.com website....(they make flowbenches)....go to the support section and read the FAQs.....

and for further information on valve area and port size relationships and problems, I suggest going to www.g-speed.com/pbh and reading the article on cylinder head tech, written by a guy whose company does 700hp turbo porsches....INTERESTING STUFF!!!

after reading the info from those sources, I am definitely NOT going to get out my dremels and go after my 398 SCP......but the choice is yours....

besides cautioning against going nuts with the dremel, the one thing that really stood out was the statement that the VALVE is the limiter of max flow.....HMMMMMM....sounds like if I really want to increase the flow through my head, I had better evaluate the valve sizes and probably go with larger valves...or at least a larger exhaust valve.....

HMMMMMM....that coincides exactly with what I have read at and heard from UNITEK......

Cappy, I was pleasantly surprised when you said that you had gone over your head with a "head guy"....I apologize, because I had thought that you had just done the dremel thing.....my error....and I am glad I was mistaken; the head really looks sweet....

a sidenote: in the g-speed article, the author discusses multi-valve heads....his conclusions are worth the read.....

now that those loose ends are tidied up, I want to broach squish and swirl again.....

"the goal here is a compression pressure/octane rating ratio that is as high as possible".........yessir, I agree that that is a good goal...a VERY good goal......and in a way: THE goal....

how to achieve that goal is where the challenges arise....

In order to have us all on the same page, so to speak......and also because I am a lazy old fart....I would suggest some reading material for all interested in this area:

go to www.theoldone.com...(the ENDYN website)..go to the articles section and read the articles on "the soft head"....also go to the archives section and read the articles there.....several hours of very informative reading.....

THEN, we will proceed....at least I will.....but I have several things to get done over the next few days--will be in and out--before I can resume my side of this discussion....

Cappy, when you read about the soft head, you will see why I lean towards a tight squish, flat tops, small CC....

And then we get to argue about valve events, EBR, charge density......

as I can, I will check in and see if there are any posts here.....but I will be tied up til the weekend....(no, not that way, Cappy....)

[945ti]

OK, I don'y have a lot of experiance with the specifics, but I got a 530 head off of a mid 80's 240 and plan on using it on my car eventually. I will keep my post short and probably not that informative, so sorry in advance.
[quote:26e216d335]most sources I have consulted strongly caution against the tendency to enlarge the size of the ports, stating that velocity through the passage is the more important thing, and that making the cross-sectional area of the passage too large in relation the the valve area would actually reduce maximum power...
[/quote:26e216d335]
This is quite true, many volvo tuners make little change to the intake channel, some to the exhaust and put in much bigger valves. One of the best mods that alot of people seem to swear by are large valves and multi angle valve grinds with the guides ground down some. Stealth mentioned this, but I just wanted to add my little bit, since this is the route I plan to go. I don't think for an amituar like me that big valves are really a concern, but I would think multi angle valve grinds and a mild exhaust side rework would improve about all aspects of teh power band and thermal efficancy. Can't really say anymore, since I have lots of theory, but at this point the rest of my information would be better supplemented with numbers and drivability effects. The 530 head seems better suited to about 9:1 pistons since it seems that the squish is setup nicely for the small cc flat top combo that stealth mentioned. The dish on the B230FT pistons alteast seems to be preventing some of that important squish from occuring.
On a side note that maybe doesn't belong in this thread stealth mentioned about ignitons may be relevant or not. You mentioned liking the small cc flat top piston narrow gap long duration spark combo. Now I don't doubt you experiance, but because of the nature of the non centered spark plug on SOHC heads and how the squish works, wouldn't it make more sense to help center the spark plug more and promote the most vertical burn possible. THis all being done to promote maximum downward velocity. I would think a larger gap sort of deal would promote this, but alot depends, and how important is that silly centered spark plug you get on the silly 16 valve head?

[poulson01]

May I?
I think Kenny, at least, knows my position in the 8V vs 16V debate so I'll leave that one alone. :slap:
One thing I'll add is that the 16v squish areas (the 16v CC has two identical lands, one by the exhaust valves and one by the intake) are much smaller than the 8v and that's where (I think) it loses torque vs the 8v.

The dished turbo pistons might *help* squish a little because of the Bernoulli effect on the A/F mixture beig shot out of the squish area. You would think that it would cause a nice swirl.

I know an engineer who researched combustion chamber burn in Universitat Zurich for years. He told me that the intake stream has little to do with CC swirl compared to squish area. The best way to get a good swirl (according to this fella) is with direct injection under very high pressure. (Sounds a bit like a DI diesel, huh).
With boost, a "bad" intake port design or low gas velocity isn't going to hurt VE very much. Just ask Porsche.
The turbo creates lots of micro vortices that help the intake stream slip down the intake runners and through the ports with less turbulence/eddies than without the turbo.

I think the basic idea is to clean the head up and make your squish area as tight as you dare. The rest of it is trial and error (hypothesis). That's why guys like Mike A aint talkin. I've tried for years to wear him down but he won't budge.

[swede242]

Then I wonder where the secret of Unitek heads are? In the shape of the combustion chamber perhaps? BTW...anyone out there ever buy a Unitek head? Was it a noticeable difference? I asked this once as a post and never heard a word from anyone.

[SquareD]

[quote:6b5cf90d85]most sources I have consulted strongly caution against the tendency to enlarge the size of the ports, stating that velocity through the passage is the more important thing, and that making the cross-sectional area of the passage too large in relation the the valve area would actually reduce maximum power... [/quote:6b5cf90d85]

With a *pressurized* (ie turbo) application, wouldn't opening up the the intake port be beneficial? You are already "artificially" increasing velocity with the turbo. Anything that allows better flow/easier filling of the bowls/combustion chamber would be a good thing, while the exhaust side would be where you need to maximize velocity, so drastically increasing exhaust port size wouldn't be so productive.

[Captain Bondo]

Wow this exploded....

Replies to Stealth:
"the one thing that limits the maximum flow obtainable is the size of the valve head......and the shape of the valve head can have an effect as well... "

Especially the size, since backcutting represents very few cons, bugger valves is something i should have done, and will do eventually.
I put larger emphasis on the bowl, proper unshrouding, and gentle short-side radii. My head flows better than many of the people with "big valve" heads are reporting.

:most sources I have consulted strongly caution against the tendency to enlarge the size of the ports, stating that velocity through the passage is the more important thing, and that making the cross-sectional area of the passage too large in relation the the valve area would actually reduce maximum power..."

Also this needs to be in raltio to the expected operating power band of the engine. IE a higher revving engine flowing more dynamically will have decent gas velocity even with larger port volumes.
But in general yes the name of the game is flow vs. port volume.


"in other words, for a particular valve size, there is an optimal cross-sectional area for the runner (the port) in order to achieve the maximum flow....if you exceed that max size, you WILL reduce the maximum power output....why? because maximum power is obtained when you have the smallest port cross-section that will deliver the flow....."

Agreed.

"let me rephrase that once more: the limiting factor for maximum flow through the port is the size of the valve head: it's area...."

Agreed, and that is what limits the max power any particular head will support, maximun availible valve area.

"after reading the info from those sources, I am definitely NOT going to get out my dremels and go after my 398 SCP......but the choice is yours...."

Damn right. After i got my head back form the porter, my first thought was "wow, i could NEVER have done this myself".

"....sounds like if I really want to increase the flow through my head, I had better evaluate the valve sizes and probably go with larger valves...or at least a larger exhaust valve....."

yes and no, the exhaust side is major crap. there are decent gains to be had there before even enlarging valves. Unshrouding the existing valves makes them more "efficient", the head "uses" the availible valve area more effectively.

"Cappy, I was pleasantly surprised when you said that you had gone over your head with a "head guy"....I apologize, because I had thought that you had just done the dremel thing.....my error....and I am glad I was mistaken; the head really looks sweet...."

Wow thanks. Ya, no way I could have done that work myself. It's just rediculous when you feel inside the ports, as if you were feeling body work, they are fairly modified and they are unbelevably CONSISTENT. That is what shocks me, every chamber and every port is EXACTLY the same. it's almost scary.



"go to www.theoldone.com...(the ENDYN website).."

Agreed. those guys are amazing. having a set of their nutty pistons made up (and a suitable cam) would be wicked.

"Cappy, when you read about the soft head, you will see why I lean towards a tight squish, flat tops, small CC...."

i hear ya, those guys are big on building engines that are nearly miller cyle engines as well, the cam bleeding pressure is a major component as well. I don't think it'd work as well with and average cam.

"And then we get to argue about valve events, EBR, charge density......"

hehe.


Replies to 945ti

"This is quite true, many volvo tuners make little change to the intake channel, some to the exhaust and put in much bigger valves. One of the best mods that alot of people seem to swear by are large valves and multi angle valve grinds with the guides ground down some."

Yes and no. The chamber unshroud and short side are the big ones imo.

"I don't think for an amituar like me that big valves are really a concern, but I would think multi angle valve grinds and a mild exhaust side rework would improve about all aspects of teh power band and thermal efficancy."

Multi angle are overrated. the key is that the transition form port, to bow, to chamber, to bore are a continuous path. Yes I have 5 angle setas but my head guy said it was pretty minor.


*
Replies to Mike (poulson01)

"One thing I'll add is that the 16v squish areas (the 16v CC has two identical lands, one by the exhaust valves and one by the intake) are much smaller than the 8v and that's where (I think) it loses torque vs the 8v."

I think it is a matter of at low rpm, there isn't enough flow to keep the velocity up, the 16v has a ton of valve area which makes the gas speed too slow at low flows.

"The dished turbo pistons might *help* squish a little because of the Bernoulli effect on the A/F mixture beig shot out of the squish area. You would think that it would cause a nice swirl."

Now THAT is an interesting thought. I could see this...

"With boost, a "bad" intake port design or low gas velocity isn't going to hurt VE very much. Just ask Porsche."

Well, it's more that VE is moved to a differne range of flow values i think...



"I think the basic idea is to clean the head up and make your squish area as tight as you dare. The rest of it is trial and error (hypothesis). That's why guys like Mike A aint talkin. I've tried for years to wear him down but he won't budge."

probably a big part of it for sure.

Relies to swede242:

"Then I wonder where the secret of Unitek heads are? In the shape of the combustion chamber perhaps? BTW...anyone out there ever buy a Unitek head? Was it a noticeable difference? I asked this once as a post and never heard a word from anyone."

no secret imo. Mike works on the 1+1=3 theory. A head gives some power, a cam gives some power, but a head AND cam give even more, the whol is greater than the sum of the individual parts. The add a header with minimum pressure drops, and an intake, it's the sum of it all.


*
Replies to D Hulting:
"
With a *pressurized* (ie turbo) application, wouldn't opening up the the intake port be beneficial? You are already "artificially" increasing velocity with the turbo. Anything that allows better flow/easier filling of the bowls/combustion chamber would be a good thing"

Pressureized or not pressurized, the engine will breath in a certain volume of air per intake stroke (the volumetric efficiency of course being the ratio of that volume to the actual volume of the cylinder).
So an engine that breathes in %90 of its volume at 0psi of boost, will breathe in %90 of its volume when the air is pressurized to 1 bar. That volume of air happens to be much denser and contain more oxygen, but it is still the same volume. The issues of velocity, and the air having "interia" still aply. Hope that makes sense.

"while the exhaust side would be where you need to maximize velocity, so drastically increasing exhaust port size wouldn't be so productive."

Yes and no, velocity of of less consequence here since there is no real scavenging happening, these days the trend is to have as little restriction as possible on the exhaust side, since the gases are just entering the turbine scroll, which will actually dictate the velocity at which the exhaust hits the turbine. large, short header primaries are generally used and the goal is to lose as little energy due to pressure loss and heat soak as possible.
Again, hope that makes sense. These are rather advanced issues that take a little mind bending.

[Bready]

"I think it is a matter of at low rpm, there isn't enough flow to keep the velocity up, the 16v has a ton of valve area which makes the gas speed too slow at low flows. "


:kiss:

JB
KTP

[Oskari]

stealth wrote:"most sources I have consulted strongly caution against the tendency to enlarge the size of the ports, stating that velocity through the passage is the more important thing, and that making the cross-sectional area of the passage too large in relation the the valve area would actually reduce maximum power... "

I have some dynoed fact about this. with stock 160head+a cam and 1,7 bar of boost result: 272hp

same engine, only changes were group a head and cam. Result: 1 bar of boost and 270hp.

I have to take some pictures next time i´m pulling the head..

[Captain Bondo]

Here is an original, hi-res closeup of my combustion chamber, courtesy of Dana.

I think the unshroud work/overall chamber shapre is ace, but the roof could look a lot better and is kinda ratty.

[poulson01]

Originally Posted by CaptainBondo

Replies to Mike (poulson01)

I think it is a matter of at low rpm, there isn't enough flow to keep the velocity up, the 16v has a ton of valve area which makes the gas speed too slow at low flows.

"With boost, a "bad" intake port design or low gas velocity isn't going to hurt VE very much. Just ask Porsche."

Well, it's more that VE is moved to a differne range of flow values i think..

That's what Mike says. Those statements seem to apply when the boost is at zero. Wouldn't you agree? I keep hearing that same old thing too: "if it works for N/A it'll work the same under boost, only better".
It's true enough but there is some "vital information" missing. Like the part where a turbo motor doesn't run worth a fart without one. I don't have a dyno in my garage but... :booty: ...my cheek-ometer is dialed right in.

The 16v heads don't have more valve area than the 8v head. Those 8valvers have huge holes compared to the 16v. It's the big valves that slow down the intake charge as it's being blasted into the combustion chamber and again on the way out. The port design, being oval as opposed to being round like the 8v, would seem to have a negative effect on flow. But every 16valver that I've ever seen has oval ports. Most of them make pretty good power, especially with a turbo. I still think that its the skimpy squish area that kills throttle response compared to the 8v. And at least they're not square :beer2:

As for VE, it's either good or bad. If it's good everywhare, then you're making lots of power everywhare. If it isn't, you have flat spots. The best way to increase VE is to add a turbo, right? So 80% of the battle is already won. A little manifold and head work (including cam) and you're about there. (Except for the years of trial and error part. I forgot about that.)

[Volvo78GT]

here's some pics of a partially disassymbled 16V head...
(mainly parts)

http://www.pbase.com/kalazdar/16v

[780GT]

Just a minor correction here... 16V heads have significantly more valve area than 8V heads do. 23% more to be specific. This is not really important though, because air does not flow through valves. It flows around the circumference of the valve.... And the 16V head has almost 48% more of that.

Also, the ports on a 16V head are not oval. On the outside of the head they are, but this is really more like an extension of the manifold. The port itself starts inside the head where the runner splits. The ports after this split are round on the intake side, and D shaped on the exhaust side. D shaped exhaust ports flow better than round or square ports.

[iadr]

[quote:bfb00776be]Especially short side radius are key at both ports, and the exhaust short side is terrible, it's almost sharp, like you could cut yourself on it almost.[/quote:bfb00776be]

Touch your finger to the flow of a moderate running water tap. The contact provides a "pivot" and pulls the flow toward the finger. It *might be* that the edge pulls flow around and into the lower half of the port sufficeiently to do quite well. Sure it's production economy, but it might have been an examined compromise.
Only my opinion, (but it *is* tested on flow bench to provide decent gain of 15-20% with 10% less overall volume) is that if you round it off you should also fill in the lower 15% of the port

[quote:bfb00776be]I would suggest that a well-prepped 8v head can perform as well as a similarly prepped 16v head....ask Mike Aaro... [/quote:bfb00776be]

I don't ever recall him saying that, and I listened pretty closely. He said, in effect, " I can get closer than you'd think, without all the fabricating manifolds and so on." Hmmm, closer than you'd think- think about the evasion in that sort of comparison for a sec. anyway not quoting Mike A or especially crticizing him. He said the same as everyone else says when the issue of 8 vs 16 comes up, nothing definitive. My collected bench figure chart shows the comparison stock to look like: int/exh @ .100, through 400, 8v being average of 398 w/o bore fixture & 405 with bore fixture. 16v head was (I believe) w/o bore fixture. Bore fixtures are the correct way but influence the numeric results far less than you'd think.
8 v : 60/39 119/81 155/94 166/111
16v : 87/81 167/159 220/194 232/196

[quote:bfb00776be]I suggest going to www.g-speed.com/pbh and reading the article on cylinder head tech, written by a guy whose company does 700hp turbo porsches....[/quote:bfb00776be]

Fantastic article, very readable. There was another article apparently by the same guy at a now defunct rice perf page. Note that the guy you reference that does Porsches, carefully explains in the 1st few lines that he only retyped the article for distribution. He did not write it.

[quote:bfb00776be]a sidenote: in the g-speed article, the author discusses multi-valve heads....his conclusions are worth the read..... [/quote:bfb00776be]

He doesn't think 5 valve has much advantage over 4 valve in motorcyle use. Beyond that you'd have to have totally misread the article to think it supports 2v/cyl. There are no known recent Cosworth engines running 2v/cyl, for one very basic point.

[quote:bfb00776be] 16v squish areas [are] where (I think) it loses torque vs the 8v. [/quote:bfb00776be]

Given cams properly selected for the same purpose, and equal compression, I believe it would be extremely unlikely that the 16v would lose torque. Feel free to provide figures or even theories and impressions to back up a comparative loss. A failure to gain is not the same thing as a loss.

Two rules of thumb from a guy who built his own 1000+cfm bench, went to David Vizards classes, and used the bench a lot over a period of years- he now runs a vinyl graphics outfit. If that's not a reason to hate ricers I don't know what is. Anyway, rule of thumb in playing with various things including intoducing machinists blue into the airstream was that the short side of the intake is always the place to widen or otherwise remove material. He never ran across a head having highest flow on the intake port roof. Not once- Sounds like a pretty safe rule of thumb. On exhausts the opposite occurred- flow is always on the roof. You need something like a 4 inch radius, where there is usually a 1 inch. The flow hits the flat of the roof where to bends to horizontal. Articles on the latest NASCAR heads, like the McLaren Ford, seem to bear this out.

Originally Posted by swede242

Then I wonder where the secret of Unitek heads are? In the shape of the combustion chamber perhaps? BTW...anyone out there ever buy a Unitek head? Was it a noticeable difference? I asked this once as a post and never heard a word from anyone.

I have bought a Unitek head from Mike. It is in my opinion a work of art. The intake and exhaust ports are a straight smooth shot into the combustion chamber. All the ports are balanced as far as shape and size. I had the bigger valves added that fit nicely in the smooth combustion chamber. I am building an engine with this head and soon to have a short block. All to go into the 940t with a supra 5 sp tranny. I have not run this head as of yet but I will tell you that it is a class A1 job. It looks like it will run standing still. I hope to have the 940t up and running sometime this spring. I will keep you posted if you like. Mike has always been helpful to me.

bis später, jt