Spark Plug Negative Range - BOMBARDIER 1996 Ski-doo MX Z440 Handbook

SPARK PLUG HEAT RANGE
Spark
plug heat ranges are selected by measuring actual combustion chamber temperatures. A
colder spark plug, one that dissipates heat more rapidly, is often required when engines are
modified to produce more horsepower.
The proper operating temperature or heat range of the spark plugs is determined by the spark
plug's ability to dissipate the heat generated by combustion.
The longer the heat path between the electrode tip to the plug shell, the higher the spark plug
operating temperature will be—and inversely, the shorter the heat path, the lower the operating
temperature will be.
AOOE09S
A "cold" type plug has a relatively short insulator nose and transfers heat very rapidly into the
cylinder head.
Such a plug is used in heavy duty or continuous high speed operation to avoid overheating.
The "hot" type plug has a longer insulator nose and transfers heat more slowly away from its firing
end. It runs hotter and burns off combustion deposits which might tend to foul the plug during
prolonged idle or low speed operation.
Generally speaking, if you have increased horsepower by 10-15%, you will have to change to the
next colder heat range spark plug.
The Formulas are equipped stock with NGK BR-9ES spark plugs.These are resistor-type plugs
which help reduce radio frequency interference. In racing applications, the resistor feature is not
required. The typical spark plug used in a modified Formula engine is an NGK BIOES or BIOEV.
Section 04 ENGINE PREPARATION
04-97