Spark Plug Heat Ranges
A Spark Plug's heat range is a measure of its ability to conduct heat away from its firing end. A COLDER or 'harder' spark plug is more suited to a hotter running engine (i.e. high performance, high revving, tuned or driven very hard), the shorter heat path will conduct heat more quickly, ensuring that the tip does not overheat. Conversely, a HOTTER or 'softer' spark plug has a longer heat path and will maintain a higher tip temperature, carbon deposits are more likely to 'burn off' and the plug tip will stay cleaner.
This is a *basic* table of approximate spark plug heat range equivalents from manufacturers NGK, Denso and Champion. Note that Champion have further additional heat ranges for different application types (e.g. Marine engines).
*Note that the Champion spark plug heat ranges marked with an asterisk are extracted from an alternative heat range scale but are still considered roughly equivalent to the NGK and Denso heat range numbers shown.