So, if the plugs are too dark despite your EGTs and/or air-fuel meter showing
the mixture is good, would you move up a heat range?
If not, what would indicate a need to change the heat range of your plugs?
Scott (B.)
-----Original Message-----
From: Randall [mailto:TR3driver@ca.rr.com]
Sent: Friday, April 24, 2015 8:31 AM
To: Barr, Scott
Subject: RE: [Fot] Spark Plugs
> OK, so here's the stupid question of the day - what is it that makes a
> hot plug "hotter" or a cold plug "colder". Is a "colder" plug more
> resistant to absorbing heat? And if so, why?
Basically, hotter plugs have more (thermal) insulation between the tip and the
shell. Do a quick Google and you'll find lots of diagrams showing the
longer/shorter path for the heat to follow.
The reason for the differences in heat range (hotter or colder) is that the
plug tip needs to get hot during operation, to burn away and prevent combustion
deposits from fouling the plug; but not too hot. Stock engines tend to have
relatively low temperatures in the combustion chamber, so use a hotter plug to
avoid fouling. As the engine is modified for more power, the temperatures go
up, and a colder plug is sometimes needed to keep the plug from overheating.
Too high a temperature at the plug tip can damage the plug, but more
importantly also lead to detonation. The plug tip can get so hot that it
lights the mixture early (before the spark happens), causing loss of power and
even engine damage.
I also found that, with the MSD 6 ignition, colder plugs would last longer on
the street, but of course that's not an issue with a race car.
-- Randall
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