Yep, the YS 63 likes high RPM. Mine happily spins a Graupner 12x7 at 11,200 rpm on Powermaster YS 20/20 fuel. I tried a 12x8 and a 13x7 prop, but the engine seemed to be bogging down too much. You are asking for trouble if you run your YS 63 in "mid nines".