Well that’s what I don’t get. A 12X14 tuning at 8700 is producing 1.365 Hp, a 12X14 tuning at 9700 is producing 1.893 Hp. My OS91FX with standard muffler turn a13X8 at 10800 RPM, that ‘s 2Hp with a tune pipe it should turn at11800 that 2.68 Hp I know a slow spinning prop is more efficient than one spinning at hi revs, but I think my 13X8 is producing more pulling power at a lower top speed than your 12X14.