The heat range of a motorbike spark plug refers the speed at which it can transfer heat, starting from the firing tip, and into the cooling system. The majority of spark plug creators suggest that the heat range should be between 500-850 degrees Celsius. Each manufacturer will display its heat range in a number. But bear in mind that different brands have different methods of assigning heat ranges. These can be broadly divided into what are commonly known as ‘hot plugs’ and ‘cold plugs’. The former have long insulator nose lengths, which slow down the rate at which heat is transferred from the firing tip to the cooling system. They are best for applications which run at low rpms, and they have the advantages of allowing the plug to self-clean and prevent fouling. Alternatively, you have the cold plugs, which are better for high rpm engines as they have a short insulator nose head, which transfers heat more rapidly from the firing tip to the cylinder head water jacket. If you are not certain, it is always worth asking questions of the manufacturer.
For high performance engines, choosing the right heat range is essential. If you choose one with a heat range which is too cold, the spark plug will not be able to self-clean in the same way, and it will not be able to burn off carbon deposits. On the flip side, if you choose a range which is too hot, this could lead to negative effects such as detonation, pre-ignition, and power loss. Look for a special performance spark plug.