Tried out this one that I picked up. Cool spring day. Used 15% Ritches brew, not my usual 10%. A 4 1/4" - 4" APC prop give me 18,300 rpm. A MA 6-4" prop gave me 14,400. Nothing special. Huge headspace and low compression head. I made up a turbo plug head and it gave me 24,000 and 17,500 respectively. Pretty happy with that. It is the control line version with a muffler which didn't even slow it down.