Heat range is one of the most important — and least understood — spark plug specifications. Installing a plug with the wrong heat range can cause engine damage. Here's what you need to know.
What Is Heat Range?
Heat range refers to a spark plug's ability to dissipate heat from the firing tip into the cylinder head. It's controlled by the length of the insulator nose — longer nose = hotter plug (retains more heat), shorter nose = colder plug (dissipates heat faster).
Hot Plugs vs Cold Plugs
Hot plugs retain more heat, which helps burn off carbon deposits. They're suited for low-load, low-RPM applications and cold climates. Engines that run too cool with a cold plug can experience fouling.
Cold plugs dissipate heat faster, keeping the tip temperature lower. They're designed for high-performance, high-RPM, and forced-induction engines where excessive heat causes pre-ignition (engine knock).
Using the Right Heat Range
Every engine is designed for a specific plug heat range, and the OEM recommendation is the starting point. For a stock engine, always use the manufacturer-specified heat range. Deviating without cause can cause fouling (too hot) or pre-ignition and piston damage (too cold).
Modified Engines
If you add a supercharger, turbo, or increase compression, you generally need to go one or two ranges colder to prevent detonation. This should always be done in consultation with a tuner who can verify combustion temperatures.
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