YS 63 vs OS 91

The OS should turn that prop greater than 9500 easily. Don't know the RPM of the YS. Assuming it is about the same then the speed will be about the same too. The pulling power of the 13" prop vs the 12" prop will be considerably greater. I like the biggest diameter prop that will fit the plane, 6" - 8" pitch at

9500 - 10k RPM is fast enough for me. I like the price of 10% fuel better than 30% too. Not really endorsing either engine. The OS 91 I have ran great right out of the box. Very easy engine to operate and the fuel economy is excellent.

Charlie

If he's looking for speed he should reduce the diameter of the prop and increase the pitch. The bigger diameter will give him pulling power but it will also act as a giant air brake.

Depends on the plane. If it is a 60 size WW I bipe, the OS may fly it faster due to the larger prop diameter. If it is a slick sport plane, the YS will rule because it will be turning the prop a good 1-2000 RPM faster at speed.

if you are attempting to correct me, you would have done better to to provide some rpm data for the ys engine turning the prop he asked about ch