Damon wrote: 

>>I've seen a few mentions of plug heat ranges. What
>>does this effect? What are the consequences of
>>running "too hot" or "too cold" of a plug?

Damon: 

I am no expert but if I understand this concept correctly the heat range of a plug affects the temperature of the tip when the motor is running. The engineer that designs the plug can change the heat range by making the electrode protrude more or less into the combustion chamber (I'm sure other factors come into play such as electrode alloys and insulator materials.) 

If the plug is to cold for your motor and your driving style then the plug will tend to carbon up and cause misfires (a fouled plug), if the plug is to hot the tip will start to glow or even melt and cause pre-ignition and damage your motor. As usual this is all a big compromise. An engine that is lightly loaded and never works hard will need a hotter plug than the same engine that is used to produce full output all the time. 

Experts please expand or correct if necessary. 

George, 

'85 Westfalia