No, this is not another which brand to get question. :P haha.

I have the HRC stage 2 (not on yet). The downpipe is 2 1/4". And I was thinking about getting a 3" or 2 1/2" exhaust, and also either a 3" or 2 1/2" hi-flow cat to go with it.

But if I have a 2 1/4" downpipe, I wouldnt think a 3" cat and exhaust would work. Or will it?

Basically.... What size cat and exhaust will work/fit on a 2 1/4" downpipe? Thanks.

I have the HRC stock downpipe mated to my stock down pipe and flex section, then I'm running a testpipe and 3" Thermal catback. I haven't had any problems. It bolted right up because the Thermal is 2.5" for the first inch or so, then opens to 3".

I got the HRC DP mated with the stock DP, then connected to a 2.5 in. testpipe to a 2.5 in. Borla cat-back.

I have the HRC 2.25 dp. Last week i took it to an exaust shop and had them put a fitting on the end of the dp and run all new 2.5 pipe with flex back to a new striaght through muffler. No cat. }(. It sounds sick, not ricey at all! BUT i have not driven it since, i just got back from vacation. So right now im waiting on someone to pick me up to get my car and give it a run. My vortech fpr just came in too. happy happy joy joy!

You don't have to worry about the different sizes between your dp, cat, and exhaust. You can easily obtain a 2.25" to 3" connector connecting your dp to your cat or any other size connector. However, if you don't like that idea, a muffler shop can weld them together. Good luck.