Originally Posted by endothermicbiped
I've got a question along this same line. If you're not pushing more air with a bigger turbo, etc is there any need to go any bigger than stock on the downpipe? I'd imagine most people (like me) are out for maybe just a little more kick but mainly want everyday economy so they've got some sort of programmer and a different intake, etc. So, would a huge downpipe be necessary? The reason I ask is that mine has a big downpipe (4 1/2" I believe) and now I have a crazy amount of turbo lag. I've got to do something about it so what would be the best size to go with? Go back to stock or just a shade bigger like 4"?
Huge downpipes are not needed, stock is 3" and most turbo back kits include a 3.5" downpipe and typically its sufficient for everyone, there isn't a need for anything any large, I do feel the 3.5" is better then the stock downpipe and you will see about a 50* EGT drop with that 1/2" larger DP.
Unless you have a custom made downpipe, the largest available is 4" so that's probably what you have. If you are having extreme turbo lag, its not because of the downpipe(or lack of back-pressure) as boost is made from the exhaust gases (pre turbo) not in the down pipe (post turbo) SO with that said, my guess would be that you have an exhaust leak and with you having a 4" downpipe, I would start looking at the EGR up pipe that has the flex section and the Y that connects to the EGR cooler, with the 4" downpipes if they are not positioned perfectly, the down pipe will rub the up pipe bellows and eventually it will rub a hole in it, when this happens you will typically hear it, but if you don't hear it, you can feel in, IE extreme turbo lag.