[Plums]

My sidepipes step up from 3.5" at the collector to 4" for the main body of the sidepipe, then back down to 3.5" at the exit.

The cats are 3.5" OD (as are the hotdogs) and came from an exhaust wholesaler here in Perth. I had to get him to order them in though, but I think he did get a bunch of them at that time.