I have used the Bisson inverted and pitts mufflers on 2-2.4ci engines and one 3.2ci engine.
They are by far the quietiest of the mufflers that I have used on these engines.
The only problem that I had is that the mounting flange over time will break.
I assume it is due to repeated heating and cooling but I don't know.
This has happened on all the pitts (2) and one of the side mount (out of 3)