I'm looking for a high flow cat and wondering what size dia. do I need for my car?

 

the stock size is 2.25, i'd go for about a 2.5. summitracing has some good deals. and btw, the only wya i would get one is if you go for the larger diamaeter than stock, other than that it won't really help you. of course, if you're doing all of that you need to go for the downpipe and a good exhuast system. it depends on how much modding you plan to do.