Hemi advantages

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Don Stauffer - 28 Oct 2005 14:36 GMT

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?

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SCOBO - 29 Oct 2005 20:13 GMT

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.

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Bret Ludwig - 29 Oct 2005 22:20 GMT

> 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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