Originally Posted by 98CoupeV6
I don't know of any stock air boxes or exhausts that Honda make that contribute to emissions reduction.
Taking out your cat will decrease your MPG and reduce performance. Honda makes catalytic converters that are so efficient and so unintrusive that it will actually hurt your horsepower to take it off, especially with the reduction in back pressure. They're not like the old cats that came on 80s cars that were basically high density filters.
I'm not too sure about that, I have a hi-flow cat converter in mine and I can tell you that it's made a difference in performance.
Also he didn't say that he had a stock air filter, some air filters aren't as thorough as others (stock, for instance), thus making the burnt emissions of the substances getting through more harmful to the atmosphere... i think... however just the muffler wont effect it for sure..