Re: First hemi??

Posted by Bob M on September 12, 2011 at 17:23:38 from (69.207.87.100):

In Reply to: Re: First hemi?? posted by John T on September 12, 2011 at 14:16:28:

Hey John T...

Beyond marketing hype - which from an engineering perspective is worthless(!) - the advantages of a hemispherical head are:

1 - Spark plug is centrally located in the combustion chamber. This promotes faster and more thorough combustion with less chance of detonation - especially at higher compression ratios.

2 - Has space for larger diameter valves, hence better engine breathing than other head designs.

Combined these permit slightly more power output per cubic inch - especially at wide open throttle - than other head designs.

The downside is a more complicated (expensive) valve mechanism, and a physically larger rocker box to enclose it. Thus racing hemis were often referred to as the "elephant motors" on account of their large envelope.

Incidentally virtually all large aircraft piston engines after about 1920 employed hemi heads.

Bob M - retired mechanical engineer

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