RE: OS 91FSR
13x8...14x6...14x7...15x6
The .91 FSR is a good, powerful engine...a bit heavy, but excellent quality. Don't lug it down too much...let 'er spin up some...prop it for around 11 to 12K.
It will turn a large prop, but mine seemed happier if I let it rev. I used 10% nitro and an O.S. #8 plug...14x6 prop.