Yea 20X10 is a little much to expect out of a 31cc gas. The higher pitched 16 and 17 inch props will give you more airspeed and the lower pitched 18 inchers will give you more thrust. The airplane you choose and your taste/flying style will determine what balance of speed/thrust you prefer.

A fence post is great for a test stand, I clamp my stuff to a picnic table

BTW; beware, You start doing conversions and you will have more engines that you know what to do with :stupid:

Adam