ORIGINAL: drumbum
a friend of mine just bought an O.S 91fx 2 stroke. i will have to do some comparison between the two. ( same brand same displacement) motors and see what i come up with,, it says it is 2.8 HP at 15,000 rpm. and it also says with a 16x6 prop it will swing only 8900 rpm. that would be nowhere near 2.8 hp at that point.. I will have to see. let you know..
I think the best prop for the .91FX is a 15x6 apc. Mine was on a 10 pound Spacewalker. Although I did not time my flights I noticed a big difference in fuel economy and thrust when I went from the 13x8apc to the 15x6apc. I plan to transer the motor to a 6.5 pound Ucan-do type plane next and will most likely run a 13x6 apc and just let it rev up. I like smaller dia props on lighter planes, but that is another story.
Another consideration when choosing between 2 and 4 stroke is the survivability of a crash. I had a 1.08 on a plane that I recently put in. The motor was fine. In this case I think the YS110 would have been going back for service.