Two stroke engines without valves are dying fast. Two strokes with exhaust valves sell very well for ships. It is great fun for an old man to see if exhaust-valved two strokes can be useful in aircrafts. I made a PPL forty years ago and cancelled it when money went low 36 years ago. It was always a feeling of there must be a better way of turning propellers without making so much noise and pollution. Let me try to argue using two plant sizes: 80 horsepower at 2400 rpm/1,7meter prop.Direct coupled 40 horsepower at 3000 rpm/1,4meter prop.Ditto The difference in vibrations beween a six- and a four-cylindered C-172 was real. A friend sold aircrafts with two cylinder ,boxer HKS engines and that was unpleasant. Mass balance is almost perfect so the difference comes from the unsteady torque between engine and propeller. A two cylinder boxer two-stroke is a beast as well. A 90 degree V2 two stroke can be shown to be between a four and a six cylindered fourstroke in torque variation so that is my dream concept. Pistons are not really suited to control exhaust and normal valves has been formidable for this job.