4 stroke engines do indeed often have more torque if we compare HP to HP. Power is a function of torque times RPM. That relation is linear. Take two engines that both make 1HP, one is a 2S the other a 4S. Most likely the 2S will make it’s 1HP at a higher RPM than the 4S. Then is should be obvious by inspection that the 4S makes more torque. if it makes the same power, but at a lower RPM. In practice, this is what generally happens. It is this higher torque at lower RPM that can effect prop design. A 4S, because of the higher torque, lower RPM, can turn a high load prop. On the flip side, a 2S with lower torque, but high RPM, prefers a lighter loaded prop. Yet they both can produce the same power.

The comparison of 2S to 4S based solely on displacement is a good “text book” exercise, but really doesn’t apply much in real life. Displacement isn’t important when looking at selecting a prop… torque and RPM are important.

There is some confusion on the nonlinear relation between power and prop RPM. There is also some confusion I think on what power and work is vs torque and RPM. Torque is a force. Power is a force over time which can do work. Ultimately, we are interested in work. It is basically true that you need 4 times the power to spin said prop twice as fast, but this in no way negates the fact that 4S engines tend to produce higher torque at lower RPMs compared to 2S engine. The examples by B.L.E are correct, but I respectfully submit that they don’t prove his conclusion on 4S vs 2S.

Britbrat, a 12-5 APC @ 11,100 RPM I’m showing in Thrust HP to be over 1HP, not the 0.6HP you mention. How did you compute the 0.6HP?

Cheers.