I was considering this subject myself. One question is: With the JDM ITR header, and modifying the exhuast to 2.25 or 2.5 in, what does this do to your emissions results?

 

As long as your cat is still in place then you will for the most part not change much about the emissions testing.

As for 2.25" versus 2.5" piping when going to a 2.5" collector, that's a matter for discussion. I guess a more proper choice of words on my part would have been that you won't take full advantage of it unless you have 2.5" piping all the way back.

If you replace a header that bolts up to the stock cat and the cat itself with a 2.5" header and a 2.5" cat flanged on the other end to mate up with a 2.25" aftermarket exhaust then you will realize a good increase in flow. However, the full potential of a 2.5" system--which is conceivably why one would step to a 2.5" header--doesn't quite happen.

As for HyTech and SMSP, sure they're the best headers out there but you're not going to see any more of a gain out of them than the basic bolt on stuff unless you have opened up the motor. With just bolt-ons we're talking variances of a couple hp here and there and it's kind of pointless to drop that much more money for 2 hp. On a built motor then you can see more of a gain over the bolt-on stuff but I'm pretty sure we're not talking about a built motor here.

Similarly, a Hytech or SMSP header should not be bolted up to some kind of bolt-on 2.25" catback. No way.