ORIGINAL: 50+AirYears
...A high performance .91 might turn a 9-7 prop at 25000 rpm, but you wouldn't replace a milder .40 turning that prop at maybe 15000 if the plane were a 4 pound 500 sq" Quickie 500. The 91 would be too large and heavy to be practical.
Spinning a 9x7 APC prop at 25,000 RPM would, according to the PropPower calculator, require 4.26 HP.
Can you please name the high performance .91 engines that you were referring to, which can actually make this HP
AND be capable of spinning at this RPM?
Also, using telemetry some years ago, most engines, when appropriately propped, would unload around 1,500 RPM in high speed flight.
However, when you prop a muffler equipped, sport .61 engine with a puny 11x7 and it spins very close to its HP peak
in static conditions, it will be 'over the hill' when air-speed takes some of the load off the prop and it will unload significantly less... making less HP.
If anyone wants the PropPower (MS Excel file), E-mail me.