combatpigg was only joking when he said to use a 5x3 prop.

What you want is a prop that will load the engine typical to a airplane engine of the same size. Use the same prop on all three engines, to give you some idea of the differences between them. I suggest you use a 9x6 wood prop to start. If your engine is not getting enough rpm then step down to a 8x6, then maybe a 7x6. Be sure the prop is thick at the hub, so that it doesn't fly apart with high rpm. NEVER stand in front of the prop when the motor is at full power and don't stand inline with the prop arc either.

BTW, there was a 'motor break in' stand marketed not to long ago by Hudy/Serpent that was exactly what you are talking about here with the prop on a car engine.

http://www.twf8.ws/new/tech/engine/b...hudybench.html

Here is one like it:

http://rc-mushroom.com/product_info....oducts_id=4531