Plug Heat range

[C M Smith <cmsmith@domain.elided>]

Hot plugs can hole pistons due to detonation in the end gas resulting from 
pre ignition induced by the hot plug nose.

Cold plugs will never cause pre ignition. They cause misfiring due to oil 
and carbon fouling.

Detonation is different from but related to pre ignition and is what puts 
the  holes in the  pistons.

Pre ignition is the ignition of the mixture prematurely due to over advance 
of the ignition or a plug so hot it ignites the mixture before the spark is 
generated, or glowing carbon residue in the combustion chamber causing 
ignition before the spark.

Detonation is the spontaneous ignition of the mixture due to compression 
heat, usually resulting from ignition of the mixture too early in the 
piston stroke. The expanding combustion gases after the spark ignites the 
mixture cause further compression of the unburnt portion of the mixture 
which then detonates with a bang. Frequently, detonation occurs at low rpm 
and disappears as rpm rises as flame front speed across the combustion 
chamber is pretty constant but the time the mixture remains compressed gets 
shorter and shorter as rpm increase. This is the primary reason that 
ignition timing needs to be advanced as rpm rise, and retarded as vacuum in 
the intake drops. Detonation is pretty much a BTDC event.

Pre ignition often causes detonation which is what holes the piston eventually.

That rattling sound you hear at low rpm under heavy load and wide throttle 
openings is detonation. You generally don't hear preignition nor do you 
hear the kind of detonation that holes pistons.

Cheers


Michael Smith
White 1991 164L
Original owner
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