ive got a 13bturbo in a series 2 with a series 4 high flow with a to4 compressor wheel on the turbo. In trying to decide what to use as an exhaust system i decided a 3inch dump from my turbo and 3 inch all the way would be ideal. My friend however told me to do a 3inch downpipe from the turbo and run 2.5 inch all the way. He says with 3inch u'll overboost but i wasnt sure what he meant. So which setup is best do u think?

 

3" is what you want.

And your friend is half right. As far as "overboost" goes, if you have a properly matched wastegate, you should never run more boost then desired. If you can't get your boost down when the wastegate is fully open get a larger one. In your case you have an internal wastegate that can only be ported if it hasn't been already, and with a to4 compressor, you will definatly reach the stock units limit. I wouldn't build an exhaust to compinsate for boost problems. I would look into getting a aftermarket manifold with an external wastegate......and let the boost rein