What heat range means
A spark plug’s heat range describes how quickly the plug moves heat away from its tip and into the cylinder head. It does not describe how hot the spark is, and it does not make the spark stronger or weaker.
The tip of the plug needs to stay within a fairly narrow temperature window. If it is too cool, deposits of carbon and oil can build up on it. If it is too hot, it can overheat and cause problems such as ignition at the wrong moment.
Hot vs cold spark plugs
The difference mostly comes down to the insulator nose, the ceramic part around the center electrode.
- Hot plug: has a longer insulator nose. Heat has a longer path to the cylinder head, so the tip stays warmer. This helps burn off deposits, and these plugs suit engines that make less heat or see a lot of short trips.
- Cold plug: has a shorter insulator nose. Heat leaves the tip faster, so the plug runs cooler. This is typically used in engines that make more heat, such as high-compression, turbocharged or modified engines.
“Hot” and “cold” describe how the plug handles heat, not whether it is a better or worse plug.
What happens if the heat range is wrong
- Too cold for the engine: the tip may not get hot enough to burn off deposits. Fouling can lead to misfires and hard starting.
- Too hot for the engine: the tip can overheat. In severe cases this can lead to pre-ignition, which can damage the engine.
The engine’s manufacturer picks a heat range to suit the engine under typical use. This is one reason to follow the specification in your owner’s manual, as covered in how to choose the right spark plugs for your car.
What do the numbers on spark plugs mean?
Plug makers print a code on the plug that includes a heat range number. There is no universal scale, and the direction differs by brand. For some brands a higher number means a hotter plug, and for others it means a colder one.
That is why the question of whether a 7 or an 8 is hotter has no single answer. It depends on the brand and series. Check the manufacturer’s catalog or the part finder on its website, and do not compare numbers across brands.
What temperature do spark plugs reach?
The figure varies with the engine, load and plug design, and sources quote different numbers. It is more useful to know that the tip is meant to run hot enough to burn off deposits but not so hot that it overheats. For specifics on your engine, the plug manufacturer’s technical information is the better source.
Should you change the heat range?
For a normal street car, usually not. The factory heat range is chosen for the engine as built. Engines with major changes, such as added boost or higher compression, may call for a different heat range, and that is a decision best made with advice from the plug manufacturer or an experienced tuner.
If you are unsure, use the plug the manufacturer specifies. You can read more about plug materials and lifespan in how long spark plugs last and when to replace them, and about why replacement can be worthwhile in our spark plug and wire replacement guide.