I used 18x6 wood in a 13.5lbs biplane on this engine and the damn thing would pull you over! I'll be fitting one in a Funtana 90 soon and already have a supply of 18x6's ready to go. 18x6 is a running in prop on this motor, the manual says 18x8 to 18x10. I've always thought it's pull a 20x6 and I'll try it too when I get it done. Let us know your results.