no argument there about the piping sizes BEFORE the turbine, but there's some argument there about being bigger = better AFTER the turbine (means the exhaust piping), setup and cc of the engine needs different piping sizes, not all engines need large size exhaust pipes. If you are using SR20det with STANDARD turbine, 2.5" is enough, that's because the only function for the exhaust pipes are to push exhaust air fast enough out of the engine, if you use 3" or 3.5", hot exhaust air will become colder when travelling through the pipes, colder air means heavier air, heavier air means the engine will use more power to PUSH the air out of the exhaust pipes, meaning engine will loose power when doing this. So using the SUITABLE size of exhaust pipes IS VERY IMPORTANT ALSO!!! If you use 3" or 3.5" or even 4" of exhaust pipes in a 3 litre or 4 litre engine, it's suitable, because these engines produces denser and hotter air because of their large cubic capacities.