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OK so what is the deal with "hemi" engines? is a good intro. shows the old Mopar hemi combustion chamber shape and compares it to period Chevy and Ford heads. Hemispherical combustion chambers had been used in radial aircraft engines for decades before Chrysler designed an automotive engine that used them.

Ideally, the combustion process would burn all of the fuel-air mixture instantaneously throughout the combustion volume so as to minimize combustion pressure loss due to heat transfer into the cylinder head, piston, and cylinder. This is never achieved in practice, so some efficiency is lost. Combustion pressure is what pushes the piston down in the cylinder to generate power, so minimizing pressure loss is important to improve efficiency. Various features are designed into all engines to get as close as practical to the optimum combustion event.

The theory behind the hemi head design is that the head and valves are shaped approximately like half a sphere (hemi-spherical) because a sphere has the largest possible ratio of volume to surface area. A beneficial side effect is that the valves can be very large, which is why all top fuel drag racing engines are based on the old Mopar. There are two major flaws with the theory, though. First, in order to produce an acceptable compression ratio, the piston shape has to fill most of the volume, so the surface area of the piston is much greater than it would otherwise be. The thermal efficiency gained in the head is lost in the piston. Second, it is difficult to design an effective turbulence-inducing squish region in a hemi head, so the combustion efficiency improvement available from squish is hard to get in a hemi engine.

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