2.5" vs. 3" exhaust

[Marcus86GLHS]

i think what CSX400 and TIM wrote above is in general true, keep in mind that "size matters" and so does the "layout" (ie: smooth bends and transitions, etc).

i would really like to see a dyno test, where the only change was the exhaust size. anybody ever do this?

[91DSX]

Quote:

Originally Posted by Chris Faulk So I guess my car isn't fast?! You CAN go plenty fast with a 2.5-inch exhaust as I have shown. Will a 3-inch exhaust flow better...sure some, but I've still gone 112mph with a crimp-bent 2.5-inch exhaust with no cat. and a Mangaflow muffler.

Glad to hear, Chris! I have almost the same exhaust but with a custom 3in swingvalve and downpipe, I don't think I will hit 112MPH though

[moparzrule]

Quote:

Originally Posted by Marcus86GLHS i would really like to see a dyno test, where the only change was the exhaust size. anybody ever do this?

I plan on it! Going with TU 3'' SV and side exit exhaust no cat and maybe bullet muffler. Right now I have full 2.5'' crimp bent no cat and bullet muffler.
Here's my setup now-
87' daytona
87' T2 block
stock swirl head, T2 crossdrilled
T1 cast crank
T2 rods
Mahle pistons, .020 over
stock oil pump
88' roller cam
1 piece intake, stock
lightly ported exhaust manifold
super 70 turbo....48 turbine. 2.5'' SV and exhaust no cat, Bullet muffler
dual stock core intercooler
A520 trans with Chrome Moly Transplate
T2/T3 clutch
Custom cal from TU soon to be installed, 3 bar +40's....2 stage boost settings 16 and 22 PSI.
Running 17 PSI right now with 60 PSI base fuel pressure, hoping for 230 WHP with 2.5'' setup.

[turbovanman]

Quote:

Originally Posted by moparzrule I plan on it! Going with TU 3'' SV and side exit exhaust no cat and maybe bullet muffler. Right now I have full 2.5'' crimp bent no cat and bullet muffler. Here's my setup now- 87' daytona 87' T2 block stock swirl head, T2 crossdrilled T1 cast crank T2 rods Mahle pistons, .020 over stock oil pump 88' roller cam 1 piece intake, stock lightly ported exhaust manifold super 70 turbo....48 turbine. 2.5'' SV and exhaust no cat, Bullet muffler dual stock core intercooler A520 trans with Chrome Moly Transplate T2/T3 clutch Custom cal from TU soon to be installed, 3 bar +40's....2 stage boost settings 16 and 22 PSI. Running 17 PSI right now with 60 PSI base fuel pressure, hoping for 230 WHP with 2.5'' setup.

You better be able to make 230 WHP, look at my sig, lol!

[jrayk]

Bigger is always better with a forced induction engine.

Most people's views and figures are based in a natually aspirated world.

[Chris Faulk]

Bigger is not always better here...you should learn what those are.

[91DSX]

^^Should backpressure be the key phrase?. . .thats what appears to be what really matters, and it should be zero!

[jrayk]

Quote:

Originally Posted by 91DSX and it should be zero!

Indeed... I've seen this same discussion take place on a couple dozen websites over the last 10 years. The conclusion is always the same. On a forced induction engine, try for the least backpressure possible.

The ol' more backpressure for low end - less for high end comprimise that occurs in a naturally aspirated engine is non existent when dealing with forced induction.

[Stratman]

I have to chime in and give you results from my recent 1/8 mile test runs when I was trying to get my 2.2 auto to spool better using the .63 turbine housing. I went to the track with the 2.5" down pipe dropped since I was having spool problems to see if the spool was better at all with if disconnected from cat back. I raced a few passes with it dropped, and due to slow spooling on the low end, I was running an annoying 10.5-10.7 which was a full second and a half slower than I used to run from the factory turbo with the .48 housing.
When I hooked downpipe back up and ran a few more passes and immediately felt it spooling quicker and knocking .3-.5 sec off the time slips there after for the rest of the night.
A few days ago I was driving WOT down road set at 15 psi and the down pipe blew off from the cat. When it happened, it felt like someone came up and bump to car and gave me a nice little thrust into the seat.
The conclusion I came to is that the exhaust on my 2.2 was open too much, which when not under boost (or real low boost) is not so different than a N/A vehicle and will loose low end torque just the same. Of-coarse, as mentioned above, it definatly helps the higher RPM and boost HP.

In all, no matter what, the exhaust set-up will always also really depend on the engine set-up and at what RPM and Boost you will be running most of the time. Too many varibles on an engine to say what one particular thing will gain more HP or torque.

Quote:

Originally Posted by jrayk Indeed... I've seen this same discussion take place on a couple dozen websites over the last 10 years. The conclusion is always the same. On a forced induction engine, try for the least backpressure possible. The ol' more backpressure for low end - less for high end comprimise that occurs in a naturally aspirated engine is non existent when dealing with forced induction.

[Putter]

Stratman is correct in the fact that offboost performance is going to be effected.

[Chris Faulk]

I knew this...but what do you tell people that know already and have no interest in learning otherwise?!

Minimal/zero backpressure after the turbine is easy to attain, but a tuned exhaust of some sort is still better than nothing at all. A 4-inch dump-tube 1.5ft long is still something.

The real efforts should be spent with backpressure between the exhaust valve and the turbine section of the turbo. But that's for another thread...this thread is about exhaust sizing.

EDIT: These portions are still considered part of the exhaust though...exhaust runner, manifold, turbine wheel/housing selection...so I guess it is part of this section.

[Chris Faulk]

Quote:

Originally Posted by Stratman In all, no matter what, the exhaust set-up will always also really depend on the engine set-up and at what RPM and Boost you will be running most of the time.

Absolutely. But think about it for a sec...my car is built to the *nines* and still functions very very well with the crappy crimp-bent 2.5-inch exhaust. That is still the issue in this thread 2.5 vs 3-inch.

Quote:

Too many varibles on an engine to say what one particular thing will gain more HP or torque.

Actually the variables have been well-documented. Saying HOW MUCH will be gained exactly is the uncertainty. However, these cars have been studied well enough that you can know what effect changing a part should have and whether or not HP/torque is affected.

[turbovanman]

Funny you say that Stratman, someone had brought this up earlier, he put a cat in his turbo dodge, and said the turbo spooled up faster and seemed to pull harder. Guess you made him sane, lol!

[Marcus86GLHS]

well i'm confused for sure.....

[Chris Faulk]

It is highly likely that something else in the tune changed in that case...