Spark Plug Heat Range
The heat range of a spark plug is the temperature range in which it performs well thermally. A proper heat rating for your vehicle's engine is critical, regardless of the kind of spark plug you select. If a spark plug becomes too hot, the heat it maintains throughout the compression stroke may produce premature ignition of the air/fuel mixture (known as engine-damaging knock) before the ignition system fires the plug. When a plug is driven too cold, carbon may build up on the tip, resulting in incomplete combustion and misfires.
What "Hot" vs. "Cold" Actually Means When It Comes To Spark Plugs
Longer insulator noses with a greater surface area are more exposed to heat from the combustion chamber in "hot" plugs. As a consequence of the longer nose, there is a longer path before the heat can be transferred away from the plug, causing the firing tip to heat up rapidly.
On the other hand, Cold spark plugs have a short insulator tip that removes heat from the combustion region much faster. When a result, even as cylinder temperatures rise, they remain cool under pressure.
Manufacturers of spark plugs usually give a numeric heat rating to their products. There is no standard scale for these ratings since some vendors use a higher number for "colder" plugs and lower numbers for "hot" ones.
When Are Cold or Hot Spark Plugs Best?
It's recommended to utilize factory-specified spark plugs in most common driving situations. If the engine has been upgraded in any way, or if the operating circumstances have changed, you should consider switching to a hotter or cooler spark plug.
The temperature at the spark plug tip should reach a point where it is high enough to burn off deposits but not so hot that it gets too hot or causes pre-ignition. If the plugs get fouled with deposits (a problem known as "carbon fouling"), a hotter plug may be a better option. A rich fuel-air combination, an oil-burning engine, or a large proportion of local and low-speed driving are all causes to convert to a hotter plug. A hotter plug will assist prevent the spark plug tip from getting fouled in any of these situations.
The engine may suffer from pre-ignition, which may produce pinging or, in the worst-case scenario, engine damage if the plugs run too hot, a condition known as "overheating." Plug overheating may be caused by poor-quality or low-octane fuel, a too-lean fuel-air combination, or continuous high-speed driving. Switching to a cooler spark plug in these situations will assist avoid overheating.
Going to a cooler spark plug may be suggested if you are altering the engine for performance, such as raising the compression ratio, advancing the ignition timing, or adding a turbocharger or supercharger.