I used the OS91 in a Dick Hanson Elite with good success (about an 8 lb plane), using 10 and 15% nitro. I used an APC 14-10 prop and standard muffler. Made for a nice, no messing around, light set up. I never had it noise checked, so don't know if it would pass, but it was fairly quiet. I then came into a used YS91 cheap which I retrofitted and promptly trashed a tru turn spinner when it back fired! The YS throttled better, but I had to use 25% nitro to get the same kind of power (same prop size).