I've got a new to me Moki 2.1 that can only swing a Xoar 20/8 at 6900/7000 RPM. It's got a Perry pump/reg that supplies enough fuel to run slobbery rich if needed. Large fuel tube and clunk. It is side mounted and has a Bison Pitts Muffler. OS F plug running on Omega 5% with a little extra Caster added. The engine runs great otherwise.

Is the Xoar 20/8 a high load prop for this engine? I honestly expected 7500+ RPM, maybe closer to 8K. The benchmark prop for the 2.1 is supposed to be the Moki 20/10 @ 7500. My altitude above sea level is 900'.

I've heard that the Bison mufflers can restrict the exhaust a bit without trimming 1/4" off the tubes. For those in the know, how much more RPM should I expect after trimming the tubes?
BB

I answered my question on the muffler tubes. Cutting off the knurled portions raised my peak rpm by about 200. I am now getting a solid 7200 out of my Xoar 20/8 and the Bisson Pitts. Nice, but still not enough.

An APC 20-8 (not W) perked the Moki up. Getting 8100 RPM now, but it feels like I doubled the horsepower. I guess the Xoar 20-8 was too much load.

I have two and neither one of them would turn a 20x10 at 7500 either. Could be our props today are more of a load than what was used back then.