i am planning out my exhaust for my na build. pretty much every exh pipe size calculator i find online says that roughly 2.5" piping is optimal for a 3.5L engine with a reasonable power output estimate at higher rpms. most aftermarket exhausts are 3". why is that?

 

most dual exhaust are not 3".

only single exhaust that Y together use a 3" pipe back. And thats way less volume than a dual.

 

i was referring to single, as i will be keeping some sort of a ypipe, every online calculator ive seen calls for about a 2.5" single given the 3.5L, hp range, etc.