how much less loud would be a 2.25"?

I would say that if you run single 2.25 into a single straight through muffler, it will be quite loud. At our local auto-x's though, they are not too strict about sound (at least at most sites). If you're worried about it, you will want to put a pre-silencer in there. It will make it sound much better and quiet it down some. Though, if you're only running a true single the whole way, you may want to step up to 2.5".



I recommend calling a rotary specific company (mazdatrix, racing beat, etc.) and talking to them. Don't buy their stuff necessarily, but at least listen to what they tell you. No one on here can say that a 2.25 will be at 76 dB under WOT versus a 2.5 at 82 dB. We can only tell you what we have and then say it's bearable.



Note: Baldy seems to have the setup you're looking for

^ I think the noise limit in my area (SoFL) is 95dB at WOT 50ft away.