Hercmate (I think that I am the Robert that Steve is referring to... ), I am using an APC 15x6 on my YS 91FZ. With Powermaster 20/20 the engine is a real wildcat-- I am getting almost 10K (still rich after almost two gallons... I could probably do better). The plane has very decent vertical even at that setting (and we are at sea-level here in the Seattle area).

I do have a 16x4W but have yet to try it. The 15x6 has been fine so far.