Uh. .DesertRat. . I'm running tuned canister mufflers. . .they usually are good for 300-350 more rpm from the motor at least, sometimes more. . AND. . you have to remember. . the AM28-10 has a bit narrower of a blade than a Mejzlik or Menz, so it will RPM just about 100 rpm faster from the same motor usually. that adds up to about 400 rpm over what people are usually turning a comparable 28" prop with this motor, but using standard mufflers.