The heat range of a spark plug primarily depends on the length of the insulator nose. The heat from the combustion is transferred from the insulator nose, through the centre electrode, to the plug housing and then to the cylinder head and the engine cooling system.

The challenge of spark plug design is to find the proper balance between the engine speed and the plug temperature. This is required to ensure oil or carbon fouling doesn’t occur at low engine RPMs, or pre-ignition at high engine RPMs or Heavy Duty applications.

Choosing the correct heat range is very important. Standard vehicles should use the heat range specified by the manufacturer. If your vehicle is not running at standard specifications you may need to change to a spark plug with a different heat range.

Spark plug temperatures must remain between 450°C - 900°C. Cooler temperatures won’t allow the plug to ‘self-clean’ by burning off carbon or oil deposits. This will lead to misfiring and poor acceleration. Hotter temperatures will cause overheating of the plug resulting in power loss, detonation, pre-ignition and possible engine damage.

Hot plugs have a long insulator nose which exposes more surface area to the combustion gases and takes longer for the heat to be passed out to the cylinder head. This keeps the internal plug temperature high overall which is ideal for applications where the engine is routinely kept at low RPMs.

Cold plugs have a shorter insulator nose which minimises the amount of surface area exposed to the combustion gases. This means that the heat has less distance to travel out of the plug to the cylinder head. The colder the plug, the more heat it can channel out of the combustion chamber. A colder plug is necessary to handle the extreme temperatures in engines operating at higher RPMs (eg race cars) or heavy duty applications.