i run a 15/6 on the os 91 fx i look for the best prop for power not rpm some time a engine likes to be loaded more than a jet or nelson turning 22 grand plus the apc 16/6 works good on the 91 anything over 10500 on a satio is wasted pull[8D]

ok and hope you never have to use the engines warnty on the os your under proped it tecks a 13/8[]

the 15/4 was just a thought it mite not work any better than under proping a engine but that engine is not for rpm it more for tork they run 14/6 props on them i dont see how a 15/4 would be so unreasnable


unless your at really high altitude that would be the reason your engines are not proped right[X(]