View Full Version : Spark Plug Temperature Answers (info for hot/cold plugs)

08-09-2007, 02:17 AM
A couple people asked me about changing their spark plugs to colder plugs so I will just post my e-mail response here for anyone who is thinking about it.

Typically, engines which are stock or are only slightly
modified, can use the original equipment (OE)
recommended spark plug.
Racing Modified Engines Generally, any engine
modification that alters the overall compression ratio
of the engine will involve spark plug selection issues.
One thing to consider is heat range.
The heat range of the spark plug determines how
much heat the spark plug is capable of removing
from the combustion chamber. The heat range of the
spark plug does not increase or decrease
horsepower. It changes the temperature of the tip of
the spark plug. If the heat range is too cold, the
deposits will not burn off the tip and the spark plug
will foul. If the spark plug heat range is too hot,
pre-ignition can occur.
A switch in heat range of the spark plug will change
the temperature of the spark plug tip by 70-120
If you are unsure of the correct heat range to use,
always start with a colder spark plug. If the plug
is too cold, the plug will eventually foul and a
misfire will occur. This will not hurt the engine.
Unfortunately, though, if a spark plug that is too
hot is inadvertently chosen, pre-ignition and
detonation can severely damage your engine.

08-09-2007, 07:33 AM
heres a thread to show the ranges of different plugs

its an old one from George D.


08-10-2007, 01:08 PM
Hence the reason for me running a slightly colder plug in a stock motor, as I can only get 91 octane out here, and I don't want to take chances. I know the knock sensor was kicking in occasionally with the stock plugs, but now I haven't felt it in awhile with the colder plug.