Well my OEM/stock exhaust is on the outs and I have a question, 3" or 2.5"? I plan on running the ATP Gt28rs with software and injectors... What are my possible problems using the larger 3" exhaust? Or even the 2.5" exhaust with this setup?

 

The 3" will make more power and be louder. Both will flow enough for a gt28rs.

 

Due to the larger volume (and hence somewhat lower exhaust velocity) you may spool slightly slower with the 3".

 

Larger diam exhaust will spool the turbo faster, not slower.

 

I'd think that'd depend on where in the revs you are though - one of the factors in turbo spool is exhaust velocity, and a larger diameter will slow that velocity some. I guess it'd depend which side of the velocity-vs-backpressure comparison wins out.

 

2.5 for the 1/4 and 3 for over all

 

Why do you say that?

 

First, kemp...WHAT!?!?!

YOu car confusing restrictionwith velocity here. You want no restictions post turbo and a larger diam exhaust will always make more power and spool a turbo far faster. The reason people dont run bigger than 3" is because of the law of diminishing returns. Once the restriction isnt a restriction, you wont see any more in the way of gains.

Recap, a3" will out spool a 2.5" any day of the week. If you dont believe me, go find a stock evo vs a turbo back evo dyno

 

I can believe that to an extent, but some of it will also depend on the size of the turbo being run. On a stock k03, going by the law of diminishing returns as well (kind of a side note), I'd think that a 2.5" would be more than sufficient. I don't have a 1.8T so I don't have the first hand experience with it to say for sure, but it seems that on that little turbo, 2.5" should be enough. I know that on my Z, a 2.5" exhaust was enough to flow close to 700hp worth of air, and that's on turbos that were T28 or larger.

As for the Evo, I believe it, but that's also a car running higher boost pressure stock and making significantly more power right out of the box. On a lower boost, smaller turbo, lower power car like the A4, I don't think it's going to make so much of a difference. Yes, a 3" will outflow a 2.5" anytime, that's not up for debate, but the point I'm putting out there is that the higher velocity of the 2.5" system may well help a small turbo like a k03 spin up better. Once it's spooled up and cranking, a 3" would probably be better off overall.

 

Turbo is a turbo and larger is larger. Doesnt make a differnce on the size of the turbo or how much psi its running. As long as the exhaust isnt a restriction, it will spool faster and make more power. Even on a tiny ko3, a 3" will make more and spool it faster, sure it may be <100rpms, but its still going to be faster.

As for that Z comment, sure 2.5" worked, doesnt mean it was the best. A gti made over 500whp on a gt3076r and would of made aconsiderably more if he ran a 3" turboback instead. Its just like ebay ic setups, sure its better than stock, but its far from best. On a 28rs like in his future, it will make a more pronounced diff than a ko3