Hi Geoff,
I wanted to add that I am running an OS 65AX in a Sebart Mythos 50 (I don’t know what the ‘50’ means because the correct glow power is in the 65 to 80 range) set up on an old Hanno red pipe. I’m running a 14x6 APC with just about unlimited vertical and surprising horizontal speed. I originally planned to put a YS 80FZ in it, but it would have required significant modification to the cowl. I would say the 65AX in this set-up produces comparable power to the venerable Hanno Special. The Jett ‘muffler’ is really a mini pipe and will definitely provide a noticeable power boost. My only 2 ‘complaints’ with the Jett muffler are that it puts more weight in the nose of the airplane and you have to tune the engine to the mini pipe by finding the best prop combination of engine performance and airplane performance. Jett mufflers have, historically, been designed for high rpms, but since the AX series of engines are designed to produce peak power at a lower rpm range, perhaps the Jett muffler has been designed with this in mind.
Since this was a classic pattern question, what airplane have you built?
Regards,
will