I've got a helix DP mated to a Apexi N1 which starts at 2.5 and turns into 3'', no problem. It's not really the pipe diameter its the flange size on the end of it. I'd just call someone that sells the DP you are looking into and ask them if they know if it will bolt up well with your catback. I personally wouldn't get a 2.5 if i had the choice, i thought mine was going to be a solid 3'' mandrel bent but it wasn't. Just depends how much you plan on doing to the car if it's even going to make a difference.