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Date: Sat, 02 Jun 2007 22:34:30 -0400
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Subject: Detonation & pre-ignition
From: "bob mackey" <n103md@yahoo.com>
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Paul Lipps quoted....

> "An engine that is making O.5 HP/in^3 or less can sustain moderate levels
> of of detonation without any damage; but an engine that is making
> 1.5HP/in^3, if it detonates, it will probably be damaged fairly
> quickly...within minutes."*

In my racing engine experience, at 5.7 HP/in^3, pinging (which I think
was initially detonation) takes between 1 and 2 seconds to blow the piston
crown into the crankcase. That was on a 2-stroke engine turning about
12K rpm, so each second is 200 power cycles. When I killed the engine
after just one or two pings, there was no apparent damage. If it pinged
for a second, the piston crown was cratered. Any longer than that and
a significant portion of the piston crown was in the crankcase. Probably
what happened is that detonation created hotspots that triggered
pre-ignition. In a 2-stroke, especially at high RPM, there is much
less time to cool any hotspots left from the power stroke. Also, these
engines were never operated lean of peak. The specific fuel consumption
was about 1.0 lb/hp-hr. TBO was about 2-3 hours. The spark timing
was in the 25 deg range, but the latency was much shorter than in a
straight gasoline engine thanks to fuel additives like propylene oxide
and nitromethane.

The specifics were very different from our aircraft engines, but
the principles are still the same.

-bob