What size cat to go with my setup?

[BigBoy]

im gonna go with a carsound cat.

Im getting the rmf header which has a 2.5 in collector, and my exhaust is 60mm(2.36in). SO should i go with 2.25 or 2.5? I heard somewhere that the cats are measured according to outer diameter and not inner like exhausts and headers are.

 

[E_SolSi]

get a 2.5"

 

[phunky.buddha]

2.5

They're measured internally, just like all exhaust piping should be.

 

[B16CRX]

2.5". If you get the 2.25 you are going to restrict your flow more than you have with the 60mm exhaust.