Heat range key to right spark plug

Q: Can you explain the importance of the heat range of a spark plug? — Bob K., Pennsylvania

A: A spark plug’s heat range is critical to correct operation. The different characteristics of automotive engines in regard to compression, rpm, fuel octane rating and cooling ability make it impossible to run all engines with the same spark plug. A “standard” spark plug may get very hot in one engine, yet may reach a low temperature in another. A wrong heat-range plug could cause either pre-ignition due to too much heat, or misfires due to not enough heat.

In the first case (too hot), the air-fuel mixture would ignite on the glowing parts of the spark plug projecting into the combustion chamber (pre-ignition), and in the second case (too cold), the insulator tip would soon become so badly fouled by combustion deposits that misfiring would occur. If you make an error in heat range, let’s hope you err on the cold side instead of hot; pre-ignition is very bad for an engine.

Selecting the proper heat range ensures the plug will operate within its designed operating range of 500-900 degrees Celsius. In this range, the spark plug will be self-cleaning, yet will not be hot enough to pre-ignite the air/fuel mixture.

I know this is somewhat confusing and perhaps a bit technical for our average reader, but make sure you always follow the manufacturer’s recommendation on the correct heat range for your engine. Never change heat ranges on someone’s so-called professional opinion.