As we all know the 06 uppipe is indeed catless, but it is still the same diameter that it was before. We always increase our downpipe diameter, but isn't it only as high flowing as the worst bottleneck? Maybe I am wrong but wouldn't it still be worth it to replace the uppipe on the 06's? I probably wouldn't but just a thought.

 

if its catless and the same diameter, it should be fine. The up-pipe is not like the DP, in that you dont want it to be too large, your exhaust velocities will be too low, and your turbo will spool slower. The largest problem with the wrx up-pipe never was the diamter, it was always just that cat

FOr the DP, you want it free flow as possible so there is no restriction in airflow when the turbo is flowing

 

+1

Unless you upgrade to a twin scroll, keep your stocker.

 

theres a completely differnet reason why you want a larger diameter downpipe than up pipe, the diameter of the stock up pipe is fine. also keep in mind that once the gas enters the turbo it gets necked down to less than an inch before it hits the turbine wheel anyways, so do bother going to a larger diameter up pipe since all you will be doing is killing your eg velocity