HFC fitment...2.5" or 3"?

bcfromfl

Well, generally speaking, HFCs are a direct bolt-on in the position directly behind the headers. There is just too much heat and pressure from forced induction for a cat here. Those who are going forced induction have a few choices of high-performance cat "blanks" that are welded into the exhaust further downstream. There are several out there that CLAIM to be robust enough for this, but when you take a closer look, there are really only a couple of choices. Interestingly enough, the amount of cells is not the deciding factor when considering how well the cat flows.

You are right...a 3" cat will be more efficient in burning remaining fuel than a 2.5" one, but there are potential issues welding the larger one into a 2.5" pipe. I won't know for certain until I get the parts and can examine inside and outside diameters. What will probably happen is the fabricator will "build up" the outside of the X-pipe after he cuts it somehow, to get a tight fit on each side.

There are two reasons I'm going with cats: first, I want a car that doesn't stink, and second, it will act as a secondary baffle of sorts to help cut down on excessive noise. I'm hoping to retain the factory Nismo muffler for this same reason.

In the U.S., cats are required on race vehicles, and a properly-designed high performance cat does not reduce horsepower by a significant margin.