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I'm having a little problem here in how the words
"detonation" and "pre-ignition" are sometimes used. I am of the understanding
that detonation refers to a spark-triggered event in which the air-fuel had a
more-than-even or explosive combustion take place resulting in the "ping"
we hear in our cars. Pre-ignition, as the name implies, is where combustion is
triggered by some source earlier than the normal spark event. As I have read,
"detonation can cause mechanical damage such as broken ring lands, abrasive
pitting of the piston crown, and overheating.* "Detonation is not necessarily
destructive"*, and "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."* "Detonation will actually cause EGTs to drop."*
"...most engines will live with a fairly high level of detonation for some
period of time. It is not an instantaneous type of failure."*
"There are no engines that will live for any period of
time when pre-ignition occurs. When people see broken ring lands they mistakenly
blame it on on pre-ignition and overlook the hammering from detonation that
caused the problem. A hole in the middle of the piston, particularly a melted
hole in the middle of the piston, is due to the extreme heat and pressure of
pre-ignition."* "...the most likely point for pre-ignition to occur is 180 BTDC,
some 160 degrees before the spark plug would have fired because that's the point
(if there is a glowing ember in the chamber) when it's most likely ignited. We
are talking some 160-180 degrees of burn being compressed that would normally be
relatively cool. A piston will only take a few revolutions of that distress
before it fails. As for detonation, it can get hammered-on for seconds, minutes,
or hours depending on the output of the engine and load, before any damage
occurs. Pre-ignition damage is almost instantaneous."*
*These quotes are taken from an article by GM Staff
Engineer Allen W. Cline appearing in Contact! magazine issue #54. Those who
would care to read the whole article with much more on the subject than what I
have conveyed here should go to www.contactmagazine.com and
order reprints of this issue.
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