While it is basically true that a larger prop is more efficient than a smaller one and turning a large prop slowly is more efficient than turning a small prop fast, the bottom line is horsepower = torque x rpms. My souped up Homelite 30cc for example, certainly provides more power turning at 9000 rpms than my brother's Ryobi 31cc at 7500, both with the same prop and/or prop load.

But a higher compression ratio, given that the engine can handle it without detonation, overheating, etc., provides more power all across the rpm band. It is one of the few things you can do to an engine that is "all good". It improves overall efficiency, and costs nothing in bottom end torque like most other modifications. Remember, these engines are supplied in a very mild state of tune in order to be "idiot proof", and run for years with very little maintenance. They are many times run with the cooling fins half clogged with trash, run on old gas, run for hours on end, etc., etc. There is almost always room for improvement for use in our applications, which are generally much more "engine friendly", even if they are modified to make more power.

AV8TOR