Thanks everyone for the feed back. Firstly Dar, if I understand you correct the “blow down” time determine how much gain the engine can get from a tune pipe and not just the exhaust timing. Yes Harry I have been looking at the pattern pages because this 91 will end up in a 91 size pattern plane, our limit for sportsmen is a 91 two-stroke and a 120 four-stroke. From what I read it seems that most people get about 1000 rpm more with a tune pipe if you let it rev between 11000 and 12000 rpm. Most pattern boys, it seems, is trying to turn their 91 into a 140 with a very long tune pipe and a big prop of about 14X10. I think “jet style” pattern is a thing of the past and you will never get maximum power from a 91 at 9500 rpm no matter how long the pipe is that you put on it. But remember I do not have the experience and I am only thinking about the matter.