High Compression Ratio ----> Detonation

[jondee86]

The heat range used should be chosen to suit the type of driving
you do. For everyday round town running the stock plug is most suitable
to avoid plug fouling. But for prolonged high speed/high load operation,
a colder plug will be better.

I would put in one heat range colder plugs when racing my car (stock
engine). For modified engines with increased power output, a colder plug
may be required for "spirited driving". But there will be an increased
risk of fouling if the same car gets caught up in city traffic.

Cheers... jondee86