Hi,
All engines (glow or gas)develop maximum power(torque) at a certain rpm.
I try to prop my engines so they turn slightly above that rpm.
Some experimentation and/or measurement is required to find that out.
I've been aiming for mid to high 6000's for my 3.2.
Having said that, the best prop for my engine/airframe/flying style combo might turn at an even higher rpm.
Horsepower numbers should not be used to evaluate engines since they include the dimension of time in the calculations and is therefore an intangible figure.
Power, also known as torque, is really the only thing the engine is producing.
Thanks
John