i was told when i bought my gst that it had a 3inch downpipe and a 2.5 cat back exhaust. i put the car up on jacks to put the new rims on and i noticed the catback piping looks bigger than the downpipe. the downpipe is a greyish color but its not super dirty or rusted or anything. i still have the catalytic converter between the downpipe and the exhaust.

Should be just the opposite. You'd want a 2.5" dp with a 3" cat back. The gasses expand as they cool so you'd need it to be bigger furthur back.
If it has the stock cat, I'd assume that it has a stock DP since most 3" DP's either eliminate the cat or have to be mated to a high flow cat or testpipe.

ok im thinking it is the stock down pipe but what should the stock one look like? maybe he goofed and i have a 3inch catback with a 2.5 downpipe?

I can take a pic of mine tomorrow. I think the stock DP has a heat shield on it but I'm not sure.

ok its a good thing the previous owner was a memebr of a local dsm group. i found one of his posts selling the car and it says i have a archer racing 2.5" downpipe w/ stock cat

how much hp could i gain from a 3inch downpipe

None. You want a 2.5" dp, test pipe, and 3" catback. Hotter air travels faster, so you want to keep it from expanding too much right out of the turbo. Once it gets a little further away and cools off anyway, having a 3" pipe is better to flow more air.

Yup and if I am not mistaken I believe a 2.5" DP helps to fight Boost Creep as well.


Member # 333

whoa thats good to know. get me a test pipe asap :D