3" or 2.5" full exhaust system??

[Romogst]

I"m getting new downpipe, high flow cat and the rest, which size should i go with, a 3" full exhaust or go with a 2.5 set up?? I have a 99 GST and want to reach 350HP in the future. Thanks.

 

[jbillakdsm]

Go with the 3". That way you don't run into flow issues later. It's much easier to spend a little extra and get the 3" right away than to swap later.

 

[98TsiAWD]

Anything over 300whp and I would get 3", it will gain you a few hp over the 2.5"

 

[OwN1GST]

No question you new a 3" exhaust fora turbo car.

 

[dunndilly]

The N1 systems start at 2.5" and expand to 3".

 

[UltraImports]

ugh........

 

[PoppaSmurf]

3", back pressure is the enemy

 

[4pistons]

Wow, how many times do we have to have this debate?

It would be interesting to see back to back dyno tests of a 2.5" and 3" exhaust on the same car. First on a lightly modded car, then on a highly modded one. I think you'd all be surprised. I would bet that it only has an effect on the highly modded car with a big turbo.

No question you new a 3" exhaust fora turbo car.

You don't NEED a 3" on a turbo car. It all depends on your setup. A 3" exhaust doesn't "give" you more HP. If your setup can outflow a 2.5" then 3" would "allow" you to produce more HP. There are some fast DSMs that still have the stock exhaust! Phil Beers runs 12.02 at 112 mph with a 2.5"DP and a 3" back.

The biggest difference between the two on a lightly modded car would be noise. Most 3" systems use a straight thru muffler because it's compact. Compared to a 2.5" Turbo style muffler, it's noticably louder. Use a 2.5" straight thru and it is probably about the same.

Rick - '91 GSX

 

[JayHass]

Originally posted by 4pistons
There are some fast DSMs that still have the stock exhaust! Phil Beers runs 12.02 at 112 mph with a 2.5"DP and a 3" back.

Phill is a very good friend of mine and will be the FIRST person to tell you that 2.5" downpipe is holding him back big time. FYI at the same boost (20psi) he is only 1mph stronger than my run and he has a ported o2 housing, ported 95 manifold, external dump mod and a few other small things done. Our weight is within about 50lbs of each other.

The best exhaust for a turbo is none at all, so go as big as you can...period. Don't argue with me, argue with Rob Cadle, the Garrett Engineer who I also call "friend".

 

[4pistons]

The best exhaust for a turbo is none at all, so go as big as you can...period. Don't argue with me

No argument at all, I agree. My point is that Phil HAS run that fast with JUST a 2.5" DP. You don't NEED a 3" exhaust to go fast.

Phill is a very good friend of mine and will be the FIRST person to tell you that 2.5" downpipe is holding him back big time.

I would love to see what gains he gets from ONLY the bigger DP but he isn't our research lab and probably isn't much interested.

so go as big as you can...period.

As big as you can afford and put up with the noise.

Rick - '91 GSX

 

[Romogst]

Thank you guys, that's what i thought, a 3" is what i'm getting.