Spark Plugs heat range

[C M Smith <cmsmith@domain.elided>]

Basically, the wrong heat range will misfire. Too hot causes preignition or 
pinking (pinging to NA drivers) and too cold causes fouling, usually with 
oil or carbon. A hot plug will show white on the electrode and especially 
the ceramic insulator nose which may also blister. A cold plug looks black. 
the just right plug will show reddish brown if unleaded fuel is used, tan 
if leaded fuel is used.

My limited experience with plugs that are too cold is that it shows up as a 
heavy misfire under load especially at higher rpm. Fortunately, I have 
never had plugs too hot, this is by far the more dangerous for the engine. 
Extreme conditions can cause a hot plug to disintegrate or cause detonation 
which can destroy an engine. Generally, you run the coldest plug that will 
pull smoothly under all conditions. Years ago Alfa used to spec a colder 
plug for extended high speed driving and you were expected to swap plugs if 
you intended to drive especially hard. Modern plug designs obviate this.

As I say, I suspect the new Bosch Platinum Plus 4 have a wide heat range 
and that is why they run so nice and clean in the V6.

Cheers

Michael


Michael Smith
Calgary, Alberta,Canada
91 Alfa 164L
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