another stupid question from an ameture

[scoobiesport]

Piping size isn't everything. 3" isn't necessarily better then 2.5. Horsepower doesn't just increase with piping diameter or everyone would run 5" pipes. In some cases too large of a diameter can decrease power.
Moose

If you are refering to backpressure then arent you are only talking about N/A cars? Studies show that the best exhaust for a turbo'd car is one shaped like a short cone or an old recordplayers "megaphone" right off the turbine exhaust . this is not a choice for us street cars but the least amount of restriction is better...