what _IS_ a good cruise A:F ??

[Gregory R. Travis]

On Wed, 26 Mar 1997, Tom Cloud wrote:

> I've read that closed chamber designs and flat-topped or dished
> pistons lessen the likelihood of detonation whereas open
> chambers and/or domed pistons tend to aggravate the problem.

One of the big benefits of the "Hemi" head, apart from the fact that
it allows the use of larger valves for a given bore, is the observation
that the hemi head allows much higher compression ratios than a flat
head.  That is, it allows much higher compression ratios WITHOUT having
to go to higher octane gasoline to preclude detonation.

Something I was reading recently said that this was true for "clean" heads
only.  After some time in service, the hemi head accumulates carbon, etc.
(just like any other head) and once that happens, things go downhill very
rapidly.

Back to detonation.  As everyone knows, an early signature of detonation
is an extremely clean combustion chamber and/or a slightly mottled/rough
appearance to the piston tops (but the piston tops are otherwise very
clean and free of carbon).

In normal combustion there is always a "shield" of unburned gas between
the piston/combustion chamber wall and the burning mixture.  This is what
prevents the piston, with its 500-600F melting point, from melting during
the 2000+F combustion event.

Detonation disrupts this shield of unburned gas and allows the full heat of
combustion to impinge on the piston.  In early or mild cases of detonation
this simply melts and removes very small parts of the piston top.  In
serious cases it melts the piston clear through.

greg		greg at indiana.edu	http://gtravis.ucs.indiana.edu/