RE: What is the max prop RPM?
Hi!
The HP .21 isn't a "hot" engine by todays standards.
Back in the days when we raced Quarter Midget pylon, with 3,5cc engines, over here in Sweden(25 years ago the engine of choice was Rossi .21 and we used APC 7x5 and later 7x5 carbon props.
Both those props rewed 25000rpm with the magic mufflers and long pipes we used then.
An old, not so powerful HP .21 will not reach those rpm figures but still, a 7x5 prop is what I recommend.