I like to let mine turn up a little, so I'm using an APC 10x7. An 11x6 would be good too, but I think an 11x7 is a bit much. These engines are happier when they rev, and if you are an experienced flyer, I suggest an "Aquacraft" or Tower muffler to free-up some power. What type of airplane are you putting it in, because that helps determine prop choice.