I am looking to get a header for my B16... I would like a 4-2-1 and i am wondering if a header with a 2.5 inch collector will mate to an OE sized cat?

 

Type R cat maybe? but usually you need to get a custom length cat to work with a header that has a collector outlet that size. This is because those headers tend to be longer. A stock type cat is very restrictive though which would negate using a larger collector and experiencing any real power gains.

 

Right now i have a 4-1 DC sports thats cracked. The cat seems to be bolted up nicely and on the other is a ITR mid pipe and muffler so i am curious to know if the collector is a 2.5 inch one already

 

If you have the DC JDM 4-1, it has the 2.5" collector, easiest way to tell is to look for a C.A.R.B. plate welded to one of the exhaust primaries near the head. The JDM 4-1 doesn't have one but if it has a CARB plate, it's the smaller 2" or 2.25" collector header.