Same question. I know some of you guys are rockn a Toda header...

 

Yes, you do need to either modify your stock cat or get a carsound cat.

 

I'm guessing the carsound cat goes from 2.5' to 2.25'? Isn't the stock piping 2.25'?

 

The Car Sound typically has both a 2.5" inlet and outlet. The flange is physically the same size as stock, it's just the internal opening that's larger, so there's no problem bolting it to a stock-sized exhaust system. That being said, the stock (and many aftermarket) B-pipe flange opening is quite small and restrictive, so bolting the high-flowing 2.5" cat to the much smaller opening of the stock flange isn't ideal. I'd suggest that you purchase a tapered 2.5" -> 2.25" replacement flange, cut off the restrictive stock B-pipe flange, and put the new one in its place. That is, of course, unless you plan on running a full 2.5" cat-back system...