Bought a 2005 EVO8 a few weeks ago and love it.The clutch was slipping so have a act street/street flywheel set up going in as I type this.So the thing is the car has had a mandrel bent 2.5" pipe run from the cat into a magnaflow straight thru round polished muffler. I guess my question is what would I gain by going to a 3" catback? Or am I better spending my money on other mods?Currently have a K&N drop in filter but may be getting cold air intake.

 

It's a turbo'd car. All turbo cars benefit from 3" exhaust. If it was a N/A car, it would be a different story. But if you're going to just do cat back anyway, it wouldn't be as big a gain as doing a full TBE. There's a handful of threads about this on here, you should check them out. A lot of useful information. I'd get one myself; but according to the SCCA regulations, it would pull me out of the stock class and i don't really want to do that right now

 

any turbocharged car that the engine is not naturally aspirated should not run a 2.5" so if ur running a TURBO engine you should run 3" .

 

i just bolted on an ams 3" tbe w/ a magnaflow hfc and the car really came to life, compared to the 2.65" hks downpipe w/ stock exhaust i was running before. 3" highly recomended here.

 

That's not exactly true, even though the same notion is repeated over and over on the site. In the case of the highly tunable VW FSI 2.0T engine, a 2.5" exhaust dyno'd better than the 3":


Buschur recommends 2.5" exhausts for lightly modded Evos:

http://www.buschurracing.com/evo9-stage0.htm

 

you will gain

 

If a 2.5" exhaust outflowed a 3" exhaust then it was simply poorly designed.

There is a reason drag cars with turbos have a pipe that is the shortest possible and widest possible right out to near the front wheels. It is because the best solution for a turbo is the maximum amount of flow out of an exhaust.

Unlike NA cars turbo vehicles do not benefit from exhaust scavenging which happens due to the pulsing nature of exhaust gases. To explain exhaust scavenging it happens in NA cars where the pulse of exhaust gases pulls fumes out of the combustion chamber providing a cleaner environment for new air/fuel to burn in. A turbo car wont see this kind of benefit because the turbo itself acts like a large blockage in the exhaust stream thus eliminating the scavenging effects. So the best compromise for such a system is to reduce any further blockage to nothing.

As a turbo produces boost its going to produce back pressure in the exhaust manifold.

 

I undertsand that 3" moves more air.My point and question is 1.the stock pipe with resonators I have it, and is 2.25" with sevral bends,so 2.5"with less bends and the same magnaflow muffler that is on their catback I have to be moving more air. second do i spend money on adown pipe/high flow cat or test pipe and maye a boost control. Or leave evreything stock and get a 3" catback I have around $1000.00 to work with right now.So whats the most gain for the buck.

 

2.5? I'm waiting for someone to make 4 inch turbo back...that would be nice for the big turbo fanboys!

 

U could get a Full 3inch TBE plus a MBC and a boost guage for 1,000. Get the 3 inch TBE

 

ams makes a 4"

 

The biggest restrictions in the STOCK pieces are the muffler and the cat.

So if your on a budget perhaps a catback and a cat delete is the way for you to go.