i did not truly need the 3.5 i bought. and hindsight i should have kept the
3"atr.
on mine, the 3.5" is very tight, it is less than a 1/8" from frame and i think this was my knock issue.
so my bright arsh just went to a different turbo setup and now it rubs the frame so i beat it, beat it and now its cracked...
so i would say buy a 3"