I've been hearing that a 13x4 is a great one for the YS 63. I haven't broken my 63 in yet, so I don't have any first hand experience, but from the "research" I've been doing, 13x4 and 14x4 with nitro of 20% or more (30% optimally) have been what most people like. Dave Shadel on the YS Support forum says he likes his props to turn in the 10-11k range. He says the throttle response is best at that RPM.

Reid