Spiff, don't get me wrong, if your 91FX is turning a 16x6 at 10K, just check for overheating issues. Don't ditch it right away. My take on the OS "verbage" was that it was an excuse for changing the prop recommendation. For pattern or sport type flying, I'd go with a 13x8 to 14x6. For 3D I'd try a 15x6 or 15x4. Maybe with your motor, you'd get better than 10,500 on those. I had one that would not. I had to go to a 14x6 to break 10,500 on the ground. Four other pilots in my club did the same. Like I said, maybe you got a good one. I've also wondered about their mentioning of fuel as being different. Most overseas fuel is 0% nitro. Maybe the 91FX likes lower nitro. Some high compression motors try to detonate with high nitro and big props. Anybody try a 91FX on 5% or 0%?