Iuse the SK .91s and love them. Ed Moorman in RC Report got me into them after he wrote an artical about them. Istill have his report and how to break them in. There was also a mistake in the instructions reguarding the prop size for this engine, anything smaller then about a 15X6 tends to make them run lean. Ihave used both the Brison Pitts muffler and now I'm using the stock muffler and there were no pressure problems at all with either one. To date Ihave never had a dead stick with mine!!
So far Ihave run them in 60 size pattern planes and a heavy 60 size Extra 260 and they pull like a tractor. Whenever Ineed another engine for a 60 size plane I just give Kangke SuperKraft a call, as much as I like OS Ilike SK better!!