Actually, Webra has replaced that .91 with a new compact light weight one on which they recommend a 14x10. Webra has recently been lowering their rpm ratings and increasing prop pitch recommendations. They are really stump pullers now. Call Paul at Paul's model supply to find out more. I just ran my older converted Webra .91 today, it turned a 14x11 Graupner at 7,550 on Davis Diesel fuel.