Originally posted by MikeL
HP figures are dubious at best. They're calculated differently by different manufacturers, and don't translate into actual performance when you put a prop on the motor and fly it.
I consider the RPM figures folks around here post, their collective impression of the engine, and my own personal experiences.
MikeL is shooting you the A answer here. HP ratings are USELESS on these engines. Consider:
Zenoah G38, 2.5 HP
OS 160FX, 3.7HP
Both turn a Zinger Pro 18x8 Prop right on 8000 RPM (seen many times and tached myself at the field).
So, what does the extra 1.2 HP of the 160 get you?