I've done a good bit of searching for this clarification, but I seem to find contradicting answers. I know the Type r factory exhausts are around 2.25". The JDM type r's came with the 4-1 exhaust manifold with the 2.5" collector. I'm having a hard time finding where the step down/bottleneck is. Is it at the cat? After the cat? I'm thinking of ditching my 2.5" exhaust setup and going for the Type r oem pipe and muffler. I have the JDM ITR 4-1 manifold and a 2.5" cat now, so i'm hoping the bottle neck is after. If 2.25" exhaust pipe is good enough for a type r, its good enough for my GSR with bolt-ons and CTR cams. The canister style 2.5" exhaust and sub par resonator is getting old....real quick.