Closest possible. A 2.5" OD is 2.375" ID IIRC. I already have the CS downpipe and an MAM midpipe so anything that would match up with them would be nice. I just don't want the RB because I personally don't like how it looks.Kansei said:2.5" ID or OD? if OD, the racing beat is (2.375" ID).. (uhm)