RE: Jett SS/LX Muffler - Gold
Gettign up over 17,600 on the 8.8x8.75 is pretty darn good. That is a lot of prop. And it about where the muffler wants to stabilize at most of the time.
The Jett 60LX typically runs in the 17,500-18,000 rpm range.
Remember - that is ground rpm. The engine will unload more in flight. Find the ground peak rpm, back off 600-700 rpm from that for flight.
The 53 runs out of "breathing room" in the crankcase - no way you would expect to see 19,000 out of it unless it was underpropped a bit.
What plane are you going to fly with this? Prop selection has to be matched to the airplane.
Bob