Acutally on the 6th gen, the header is different with the F23A4 and F23A1. The F23A4 header has a smaller down pipe and the F23A4 cat is different. The O2 sensors on the F23A4 is different where the one on the header is a Air/Fuel sensor where the one on the cat is an O2 sensor. Everything else like the bore and stroke, the cam lobes and the other mechanical stuff is the same. The crucial difference is the ecu and thats the difference between OBD2 and OBD2.2.