i know i posted this in another thread but i thought it was an important enough topic to have its own thread....hes my post:
Quote Originally Posted by AVE0SAM
theres only one person i know of that has gone 2.25 to 2" and that was brian and he said that you can feel gains from switch from 2.25" to 2"....which makes sense since this is such a small engine.

http://www.k20a.org/forum/showthread.php?t=38783&page=2

a little bit down on this thread a person by the name of Froth uses a formula to help calculate around what size exhaust an n/a engine needs to be effiecent.

area=3.14159265 x 1 x 1 = 3.14159265 in sq

3.14159265"sq x 115cfm = 361.28315475cfm of capable flow

then you divide that by 2.2hp per cfm which equals 164.22 hp, therefore a 2" exhaust will theoretically flow enough to meet the needs of a 164.22hp engine before youstart fighting the exhaust restriction....hard bends also play a factor in this but considering our engine barely makes over 100hp to the crank i think 2" is plently
this should put an end to the whole 2 or 2.5" discussion so i can stop telling people 2" over 2.5" but that whole thread on k20a.org is very informative