An unaltered engine will run within the optimum operating range straight from the manufacturer, but if you make modifications such as a turbo, supercharger, increase compression,
timing changes, use of alternate racing fuels, or sustained use of
nitrous oxide, these can alter the plug tip temperature and may necessitate a colder plug. A rule of thumb is,
one heat range colder per modification or one heat range colder for every 75–100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.
Do not make spark plug changes at the same time as another engine modification such as injection, carburation or
timing changes as in the event of poor results, it can lead to misleading and inaccurate conclusions (an exception would be when the alternate plugs came as part of a single precalibrated upgrade kit).
When making spark plug heat range changes, it is better to err on the side of too cold a plug. The worst thing that can happen from too cold a plug is a fouled spark plug,
too hot a spark plug can cause severe engine damage