Thanks 1Qwksport2.5r, In looking at the design of the jett stream mufflers, I noticed that the sport mufflers are a bit longer than the racing mufflers. So I figure if I start a bit longer than the sport size and slowly cut shorter, I should come up with a design that will give me the best RPM for the prop I plan to use. My can diameter is a bit larger than the jett mufflers so the dimensions might be just a bit different. I am going to make this first muffler for my ASP 91 and if it works well might make one for the 75. My biggest concern is if the welds and thin aluminum mousse can will hold up to the vibrations of the engine.