What is the main reason of air-cooled radials producing much less HP/L compared to liquid cooled Vs?
I read somewhere once, maybe I can find it again, but it is because of the inherent limitations of cooling the cylinder walls. Air cooling is not as controllable, because the temperature of the air varies with altitude and season. Hence, the temperature of the cylinders in an air cooled engine are not as tightly controlled as a liquid cooled engine. So, the cylinders need to be able to operate over a greater temperature range. Since metal expands with temperature, that means the gap between the cylinders and the bore is typically greater in an air cooled engine than a liquid cooled one. So, this greater gap, even with piston rings means some of the cylinder head pressure after combustion escapes rather than act on the piston. Makes sense to me...