ORIGINAL: smithcreek
Not sure what prop the ASP will like, but I have an Extra with a OS 91 FX and use a 16 x 6 prop. Some people say that's too much prop for a .91 and recommend ones as small as a 13 x 8, but they're confusing a screaming rpm with useful thrust. I've tried lots of different props and nothing compares on that plane. I'm using a 17 x 8 on the YS 140.
Prop testing is a must do thing until you find the prop that will fly the plane the way you like the plane to fly. The engine does have a working RPM range that it should stay in. The 16 on a .91 may be lugging it down but as long as you are happy and the engine keeps running who cares what other people say.