Voltage 1.2 – 2.0 Volts
CHR = COMPARATIVE HEAT RATING – This denotes how `hot the plug is within ‘the Fire Power range but it does not directly relate to any other range.
SHR = STANDARD HEAT RATING – This denotes how hot the plug is on the `Standard Heat Rating scale. Any plug can be rated on this scale and it allows plugs to be compared from different ranges. The scale runs from 0 to 100 :-
0 = Excessively Cold 100 = Excessively Hot
DEFINITION OF HOT AND COLD – If an engine is adjusted to run well on plug A: And when plug B is substituted it is noted the needle valve must be leaned to get equally good running: Then plug A is said to be `hotter than plug B. Plug B is `colder’ because the element needs the extra heat, generated by the leaner mixture, to reach the optimum operating temperature in the required time.