Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Thought about this thread when I was watching a documentary about engines.  They mentioned hemispherical combustion chambers as being important in the development of high revving, high output multi cylinder engines, in order to get more efficient air/fuel mixing by swirling the charge gasses and reducing the average distance between the gas particles and the ignition spark, and insuring a more even mix, so the charge wasn't stratified, with some leftover exhaust gasses layered like a cake with the incoming charge. 

The upshot I get from that is if anything, the hemi head will be more efficient at higher revs, where scavenging and getting a full charge into the chamber is important, but at lower revs in an engine with a deeply buried spark plug won’t likely make a helluva lot of difference, and if anything, improved mixing will somewhat offset the reduced compression. That would be under continuous firing. In a hit and miss at idle, it gets ample swishing around of air post ignition before the next hit, as even with only the exhaust valve open, it’s pumping and tending to clear exhaust gasses even before sucking the next charge.