My rule of thumb: use 2.25" dia. (or 2.5") up to 375 or so HP, use 2.5" exhaust up to 485 HP, over 485 use 3". There is not a HUGE difference in cfm flow between the 2.25" & 2.5", but there is a quantium leap in cfm flow when you go to 3" from 2.5".
You'll lose bottom-end & midrange if you go with too large of diameter for your engine's output. Maintaining proper flow velocity & backpressure achieves optimum exhaust performance. Bigger is not always better! Also, use mufflers that will flow adequately for the desired results. Running a 3" exhaust & then into
restrictive 3" mufflers defeats the purpose of running the larger system!
-Eric S.
www.classicchambered.com