I have a Pitts muffler for an OS 61FX that fits the OS 91FX. Can this muffler be used on the 91FX?

No. It will rob power and cause the engine to overheat. Look at the difference between the stock mufflers on the 61 and the 91. A pitts for the 91 needs to be larger also.

Jim

A few months ago I decided to pull the .61FX out of my P-40 and install a .91FX. The Pitts muffler was a custom fit a it would have been a chore to use one designed for the .91. I tried it with the .61 Pitts and actually pick up some rpm over what the engine would turn with the .91 Pitts. I'd say give it a try and see what happens.