[Fot] Spark Plugs

[Barr, Scott]

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 at 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