More spark plugs -- what does heat range mean?
 

What does "heat range" really mean, in simple non-engineer technical terms, when discussing spark plugs?

I know the common rule is to use a "colder" plug for modified engines or high-speed driving/racing, and a "hotter" plug for around town driving and/or tired stock engines (did I get that right?) but what really is the difference between say an NGK BP-6ES and a BP-7ES? Why can't they make one plug model that can satisfy a broader range of applications -- or are there such things now?

Just wondering,
Chris C.


[This message has been edited by campbellcj (edited 01-23-2001).]

Chris,

Well, it is all about thermal conductivity, or resistance to dissipating the heat gathered by the ceramic insulator nose around the center electrode. If you use an NGK BP-7ES as a 'standard' for comparison ... it has a cetain thickness and depth in the path to the outer metal shell that is threaded. A BP-6ES, being slightly 'hotter,' has a slightly thinner or longer path to the shell with higher thermal resistance, or lower conduvtivity. A BP-8 or BP-9, on the other hand, being colder, has a thicker and shorter path in the ceramic insulator, so it conducts the heat absorbed from the combustion chamber better to the shell of the plug. The operating range of the electrode tip is between 400 and 850 degrees C, and the plugs thermal conductivity must be adjusted to keep the tip in that 'self-cleaning' range. Mildly-tuned engines require a hotter plug, and higher performance engines need 'colder' plugs to dissipate the heat buildup on that tip faster so it won't overheat.
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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 01-23-2001).]

Thanks Warren - I think I understand now!
Chris C.

Chris,

Take a look at the first section of this FAQ:

http://www.extrememotorsports.com/plugfaq.htm


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Greg
72 911S

From http://industryclick.com/:

"The heat range refers to a plug's relative ability to dissipate heat. A hot plug has a long insulator nose, which exposes more of the nose--the center electrode wrapped in a ceramic insulator--to the heat of combustion. A cold plug has a shorter nose and smaller surface area exposed to combustion heat. Plus, the cold-plug architecture provides a shorter path to conduct combustion heat from the cylinder, through the plug to the engine block and, if it has them, to the engine's cooling jacket and radiator.

The goal in heat-range selection is to find a spark plug that will retain enough heat in the tip to minimize deposit buildup and promote ignition of the mixture, yet stay cool enough to avoid engine-damaging pre-ignition. In most cases you can't go wrong using the heat range specified by the manufacturer, which knows its engines best. Even so, the engine manufacturer deals with typical uses and cannot predict all the types of service a product may see in the field.

In general, an engine operating for long periods at maximum power in high-heat situations may require a colder spark plug than one running for short periods at, or near, idle in cold weather. Manufacturers typically recommend heat ranges that are cold enough to keep engines safe in the worst conditions. The reason is simple: * If the plug is too hot, the tip can become hot enough to ignite the mix prematurely (pre-ignition). The result of this condition is often a hole in the piston--and a hefty repair bill. * If the plug is too cold, the result is premature plug fouling, an inexpensive fix requiring only a new spark plug."

Also, plug appearance and other things:
http://www.clubplug.net/plug_app.html

Jw