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My gut feeling says that a flat head, with the combustion chamber in the piston, would be the best. I've seen this design in highly efficient diesel engines. Drawback could be a somewhat heavier piston, on the other hand modern pistons are so much lighter than the old...



One of reasons that works on a diesel is because they don't as a rule turn any RPM. Compared to a 6-7000 RPM engine the flame front on a diesel has all day to get its job done. I'm pretty sure the fact that the fuel is injected directly into the chamber enters into it too.

Kevin