What Everett says is true. In addition, there
are "extended reach" plugs in both hot & cold ranges. The extended
reach design was developed as an assist to maintain good flame fronts in higher
compression engines, and to help keep the tips cleaner. It should be
noted, as well, that the heat range is not a specific designator, it is only a
comparitive value based on relationship to other, similar plugs made by the same
manufacturer. The general idea is to use a cooler plug when using the car
in racing, highway, or other sustained high load use. The hotter range is
for more around town and lower speed uses.
My 2¢ worth
Mike Higgins
1955 Belvedere Sport
Coupe
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