I understand one of the advantages of the hemi CC shape is the ratio of volume to surface area, which cuts amount of heat lost to head. If we can ever achieve the adiabatic engine, I would assume that advantage would go away. Is this true?
Heat being absorbed from the combustion chamber does hurt power, and in this a Hemi is worse than a wedge style chamber , look at the surface area of half a ball compared to a flat circle. Hemi is just a come-on based on the past. Hemi isn't all that when it comes to breathing, which is where the power is made. Racing engines rarely if ever use that old design anymore.
Hemispherical heads were actually once quite common. Most of the old air cooled radial aeroengines were hemi's and so were many motorcycle engines and many car engines including the lovable but slothful Citroens, many Japanese fours, the Jaguar XJ six, and several others. There was nothing particularly great about the Chrysler Hemis-the old ones in Chryslers, Dodges and DeSotos were unbelievably heavy, and the vaunted Race Hemis were poor street engines that inevitably died young in marine or Autobahn applications.
What really killed the Hemi design was multiple valve heads, any serious engine today has 4 or more valves per cylinder if performance is the design goal.
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