exhaust flow charateristics of single vs. dual pipes

[boostdemon]

interesting benchracing topic, i need some input on exhaust system design. in particular, due to lack of ground clearance im considering opting to do something more like dual 2" tubes from the cat back instead of a much larger 3" single pipe (possibly ovalized but not certain yet).

Thoughts?

[linuxman51]

well the simple Pi are squared reveals that you get more area out of a single 3 inch than two 2 inches; 78.8 vs 88.7 (for the 3in)
depends on your goals tho? 200whp? 300? 700?


edit.. volume is similarly less as well, depending on length.

[boostdemon]

well this doesnt have a HP goal in mind, this will be for the b230f+t in the g/f's 245. issue at hand is ground clearance even with the stock system it's a problem. My intentions are to maximize volume but reduce diameter... of course it will be all home-made and i will tuck it along the underbody rather closely.

so say I used 2 x 2.25" tube... that would be more area than a 3" but would that actually flow more w/o the same velocity and reversion?

[wolf_walker]

Not so much in the ground clearance department, but I've been tossing around the idea of a 3" dp with 3" back to a 3" magnaflow that has dual outlets, and having dual tips on the rear.
Call it old age, I like the symmetry. Haven't looked under the back to see about room to do that yet.

The dual smaller pipes is not unheard of though, there were a lot of euro cars that ran something like what your talking about from the factory, I imagine for the ground clearance reason. Takes some patience and care to make it look clean though, matching the bends and such.

[RvolvoR]

oval piping

drgas.com

[aibast]

BMW has dualpipes. X over in the cat and somewhere else. Keeps nice and tucked under the body

[miguels244]

what's on my mind...
3" DP with performance Cat integrated...
transition to ovalized tube at the lower bend...
3" equivalent area pipe to the back...
transition back to a muffler

[neptune]

The P2 V70/S60R cars do this. I'm not positive, but I think they split after the cat and rejoin before the rear muffler, the twin pipes are just for ground clearance as they run along the driveshaft toward the rear. Not sure on sizes either.

I was thinking about splitting my 3" in the back so it won't hit the axle every time I take off, but I can't see a good way to do so in my situation.

[towerymt]

Use taller tires and it will be fine with a 3" system tucked tight to the floor.

Unless you're not going to run a cat, you'd have to split the downpipe before the 2nd bend, since the bottom of the 2nd bend in the downpipe is the spot that is mostly likely to rub on a low 240.

[Lord Tentacle]

so say I used 2 x 2.25" tube... that would be more area than a 3" but would that actually flow more w/o the same velocity and reversion?
my understanding was that the turbo acted as a flow restricting device and the pressure in the system after the wheel was a constant flow that did not have any reversion

[Hagar17]

go one big pipe - 2 pipes = 2 X cost, weight, etc

Although 2 does give a more balanced look...

:P

[towerymt]

my understanding was that the turbo acted as a flow restricting device and the pressure in the system after the wheel was a constant flow that did not have any reversion
Still have a running turbo car? Put your hand in the exhaust stream coming out of the tailpipe and see if it feels like smooth, constant flow.

[gsellstr]

I know in the NA truck world the 3" is used for tow rigs, whereas the dual 2" is more for the speed freaks. Keep in mind though that's the non-turbo'd world.

If you can get ovalized 3" that'll work on your setup I'd go that route personally as it's got the higher volume capabilities. You'd keep the 3" abilities and simplicity, but the 2" clearance. Reversion shouldn't be any more of an issue than standard 3" pipe and you won't have the irregular flow characteristics between pipes that a dual-2 system would have.

[wolf_walker]

That oval stuff is slick, but pricy. Get what you pay for I guess..

[kiwibrick]

A 3" pipe ovalized does not equal a 3" round pipe area wise.

[Forg]

my understanding was that the turbo acted as a flow restricting device and the pressure in the system after the wheel was a constant flow that did not have any reversion
Not quite; fairly simple proof is the fact that on a modern car (say a recent turbo Nissan Skyline) with an ECU that can adapt, you get more power at the same level of boost when you fit a 3" exhaust system & the factory one was 2.5".
Turbo cars aren't quite as susceptible to performance degradation due to an oversized pipe - ie. all the swirling-around & gases slowing-down in an oversized exhaust doesn't affect them quite as badly. But they can definitely get "backed-up" if the exhaust isn't large enough.

[miguels244]

A 3" pipe ovalized does not equal a 3" round pipe area wise.

++++

[Lord Tentacle]

Still have a running turbo car? Put your hand in the exhaust stream coming out of the tailpipe and see if it feels like smooth, constant flow.

at idle it was lump lump lump, puff puff puff
as soon as it got above 1200rpm it smothed out and became a nice even flow out the tail pipe

[wolf_walker]

A 3" pipe ovalized does not equal a 3" round pipe area wise.

I'm not following, you take a 3" pipe, squish it down, the volume is still the same isn't it? You decrease one dimension and increase another? Or is that not what the oval pipes are, as in 3" wide by design and less than 3" across? The former was what I'd assumed.

[kiwibrick]

I'm not following, you take a 3" pipe, squish it down, the volume is still the same isn't it? You decrease one dimension and increase another? Or is that not what the oval pipes are, as in 3" wide by design and less than 3" across? The former was what I'd assumed.

A circle has the most cross sectional area per perimeter of any shape, move it away from a circle and the area gets smaller, think about it this way, if you squashed the pipe flat, it would still have the same perimeter/circumference length but the area would be basically zero

[wolf_walker]

A circle has the most cross sectional area per perimeter of any shape, move it away from a circle and the area gets smaller, think about it this way, if you squashed the pipe flat, it would still have the same perimeter/circumference length but the area would be basically zero

That makes sense.

Thinking though about a length of 3" pipe filled with water, isn't there an amount that one can oblong it that the volume remains constant? I can see it'd decrease at some point, but it seems like to a definable point the expansion of north/south (for lack of a better term) would account for the contraction of east/west.

[miguels244]

That makes sense.

Thinking though about a length of 3" pipe filled with water, isn't there an amount that one can oblong it that the volume remains constant? I can see it'd decrease at some point, but it seems like to a definable point the expansion of north/south (for lack of a better term) would account for the contraction of east/west.

no...
a bubble is round for a reason...
the sphere/circle is the lowest energy state...
that means that any other shape with the same
surface/perimeter has a reduced volume/area.

from above you post read about what happens at the limit.

dr gasses 4" ovalized tube has about the same area as 3" round.

this is why mandrel bending is important...it maintains the area

[wolf_walker]

It ain't quite clicking, but I'll take your word for it.

[towerymt]

I'm not following, you take a 3" pipe, squish it down, the volume is still the same isn't it? You decrease one dimension and increase another? Or is that not what the oval pipes are, as in 3" wide by design and less than 3" across? The former was what I'd assumed.
I thought about it too. 3" oval pipe tends to be flat on two sides. Instead of stretching a circle into an elipse, it's mashed flat on two sides which would reduce the area. But if the pipe starts out larger than 3" round, it could end up with the same area once made "oval." Burns Stainless says: "The nominal "size" is roughly equal in area to round tubing of that diameter."

http://www.burnsstainless.com/OvalTubing/ovaltubing.html

[miguels244]

take a water baloon...
now the pressure inside is pushing out with equal force in all directions right...
(we'll ignore gravity here)
if you push your finger in it displaces water...
this water pushes outward somewhere else and stretches the baloon...
if the new shape was the same size the water wouldn't need to push back...
because the baloon would have the same area.

in the extreme...
take a baloon and put it between two boards...
squish them together...
at some point the baloon breaks...
this happens because the water contained inside is not compressable...
it try's to flatten out and the surface of the baloon follows, stretching as it goes.
Eventually the baloon can't stretch anymore and it breaks.

any help

[Captain Bondo]

The oval stuff is neat, assuming the low point we are trying to eliminate is the bottom bend on the downpipe, it could be pretty handy I think.

Only consideration is might take some fiddling to transition the round pipe to the oval, and then back to round, but it shouldn't be too bad.

On my car, if the bottom of the downpipe cannot be made to sit tight enough to the car to allow acceptable ground clearance, offending sheetmetal in the vicinity will simply become the victim of gross and senseless violence via BFH. :rofl:

whether 4" or 3" is up for debate. Will probably just go 3" though...

[stealthfti]

well this doesnt have a HP goal in mind, this will be for the b230f+t in the g/f's 245. issue at hand is ground clearance even with the stock system it's a problem. My intentions are to maximize volume but reduce diameter... of course it will be all home-made and i will tuck it along the underbody rather closely.

so say I used 2 x 2.25" tube... that would be more area than a 3" but would that actually flow more w/o the same velocity and reversion?

Dana, that will be about 4 times the work and about 3 times the cost of doing a high and tight 3in system.

if this is a low rider, the only place that you have nearly no wiggle room is at the trans cross member: you need about 1/2in clearance at the crossmember arch. And that is where you fab the brace to locate the rearend of the DP.

behind the crossmember is where you S bend the pipe over towards the prop shaft, and install the flex coupling.

then follow the prop shaft on back past the carrier brg support, and S bend over to near the stock middle support...and fab a cradle support.

go under axle; and mod the R/S crash guard....do not eliminate it
...unless being a volvo version of the ford pinto crispy critter is something you'd like to be...

Get the DP sized right on the down leg so that the pipe is right at 1/2in from the trans crossmember, and follow the prop shaft on back, and you'll flatten the drip rails on the rockers before you smack the 3in pipe. [well.....probably do them at the same time]

You can get a 3in system up high and quite tight. just takes some time and effort; but a lot less than going beemer-style twice pipes.

TF

[socalsean]

if this is a low rider, the only place that you have nearly no wiggle room is at the trans cross member: you need about 1/2in clearance at the crossmember arch. And that is where you fab the brace to locate the rearend of the DP.

behind the crossmember is where you S bend the pipe over towards the prop shaft, and install the flex coupling.

TF

If you were to do it this way, wouldn't the down pipe be under tremendous stress from the engine moving under load while the rear of the down pipe is mounted to the immobile transmission cross member?

Wouldn't it be a better idea to have the down pipe bracket mount to where the transmission oil cooler line bracket is, or somewhere else off the transmission? Wouldn't this allow the down pipe to move along with the engine?

[stealthfti]

to clarify: the brace is to be like the one used on the 92+ 700/900s; attached to the trans side of the trans mount...

...so that the DP is stable to the motor/trans motions

...then the flex coupling acts as the flex point of the exhaust between the motor/trans and the chassis

but yeah, the brace could be at the bellhousing. But, the potential for pipe to crossmember contact is increased.

...which is why I suggest that the pivot point...the flex coupling....be behind the trans crossmember. Then, the DP can be located quite close up and tight to the crossmember arch....because it will be moving very little right there; and won't need to move more than a very little.

TF

[towerymt]

to clarify: the brace is to be like the one used on the 92+ 700/900s; attached to the trans side of the trans mount...
I'm not familiar with this hanger. Anyone have a picture? I was going to use the flex pipe in the downward section of the downpipe, but I think the downpipe will need a hanger after the second bend to prevent it from sagging.

[aibast]

But they can definitely get "backed-up" if the exhaust isn't large enough.
So true, that why I'm moding my cars exhaust from 2" to 2,5" and straight trough.