I ran the echo 65cc conversion today. I polished the ports but made no change in timing. The timing was pretty aggressive already. Running gave interesting results. Bambula 22x8 for both runs. The first run was with a straight pipe of about 0.65" interior diameter. It is resting on top of the head in the picture. I got an impressive 7600 rpm for about 5.5 hp. It was PAINFULLY loud. It is not an option for real use. The second run was with the muffler attached to the engine in the picture. The box was gutted and a pipe of 0.65" ID was brazed on. I lost 900 RPM! I got in the area of 6700 RPM. The muffler box has a cylindrical coarse mesh screeon over the exhaust port. It is in there good and resisted all reasonable measure to remove it.

I know this engine will run good with a large volume can and a larger , about 0.75" pipe. I made a couple of such mufflers but the aluminum solder kept on melting in flight.

I want to make a steel body muffler. I was thinking of a propane cylinder. Those fat squat ones or the skinny ones for the cylindrical part of the body. The reason is so I can braze on the pipe or pipes.

Is there some formula or guide line on the expansion chamber volume or shape? I am guessing I was getting some inertia and laminar flow effect from the straight pipe. I bet that some expansion chamber volume might have a simmilar effect or even a resonate effect like a tuned pipe.

Any opinion on this?