ahh sorry i should have explained better, yeah upgrading from the stock dump is a good idea. but i rekon changing from a 2.5" dump to a 3" will do stuff all. there is no back pressure required on a turbo car, there is already enough there supplied by the turbo
if the pipe is TOO big, especially off the back of the turbo, then the air wont flow away from it, it will come out the back and 'billow' around and create flow problems. the air is still hot and expanded already, so velocity is required (more so) to move it away from the back of the turbo faster, as it cools, i guess it doesnt matter, its waste anyway........
any one know if there are tuned length headers available for a VQ25DET?? i got some wicked spool times when i changed my manifolds on SR20s