I had this article saved when I was changing plugs....
The Heat Range is the function of the spark plug to regulate the temperature of the combustion chamber by its ability to maintain or to remove the heat produced by the combustion process and to take the excess of heat toward the cylinder head that will then transfer it to the cooling system. There is no direct relationship between the spark plug heat range and the spark voltage.
Each spark plug is manufactured with its own predetermined and specific heat range. They exists different spark plugs brands and each one of them manufactures its different series or models with a large variety of heat ranges.
Due to that all the engines are different; they require spark plugs with different Heat Range. A family car and/or a racecar, should operate at an optimal temperature in the combustion chamber, which must be between 840ºF. to 1560ºF. (450ºC. and 850ºC), what means that the heat range of the spark plug to be installed shall must be cool enough to avoid pre-ignition, but hot enough to prevent fouling.
This is why the vehicle manufacturers define a spark plug with a specific heat range for the power level and condition operations that supposedly is going to demand the end user of each model of vehicle. This selection must be precise:
Today, Professional Mechanics, Do-It-Yourselfers, and Spark Plug Vendors have Application Charts available in Spark Plug or Auto Part Catalogues as a guideline to identify the adequate spark plugs for a vehicle, whether as printed material or modern computerized version. Such application charts only indicate the adequate spark plugs for vehicles under standard operating conditions.
Users are therefore quite familiarized to the idea that is enough to follow the catalogues suggestions, however, during the useful life of a vehicle many different conditions that affect engine performance may occur, as for example:
o Different driving habits,
o Different traffic conditions,
o Different load conditions,
o Different engine's maintenance,
o Different fuel quality,
o Different weather conditions,
o Use of LPG or of Propane gas as fuel.
Moreover, these conditions may combine in endless ways.
And when such different conditions do occur, vehicles no longer operate under standard conditions, and the heat range of the stock spark plugs stop being the adequate for this particular conditions, being done necessary to install spark plugs with a Heat Range different from the original stock suggested in the catalogues by the manufacturers, and therefore catalogue Application Charts can not continue indicating which one is the spark plug that must be installed in a vehicle to best match each one of these conditions or the different combinations thereof.
If the stock spark plug heat range results colder than necessary due to the real operating conditions, the air/fuel mixture will not burn completely creating carbon deposits that will not burn and will stick to the spark plug, fouling it; and since carbon is an conductor of electricity, it would deflect the electricity to the ground instead of creating the spark, resulting in engine misfiring and malfunctioning. When this problem exists, the installed spark plug shows unburned fuel. Then the spark plug appearance looks similarly to a reach mixture. But it is not the real problem.
And if the stock spark plug heat range results hotter than necessary due to the real operating conditions, the engine would overheat and the installed spark plugs looks with overheating, similarly to a lean mixture.
When an engine is modified for high performance and its power is increased, the stock spark plugs very often result hottest than necessary due to the new power reached at higher RPM and the gasoline would end up pre-igniting or detonating without the need of an electric spark, creating an uncontrolled explosion that would severely damage the engine.
Be aware that never! Never a high performance modified engine, can continue using the stock spark plug, because more power is more energy, and more energy always is more heat.
Unfortunately, even very experienced mechanics have difficulty in detecting the differences in these situations that are very often misunderstood, misleading them to search the origin of the troubles that the spark plugs are showing, in the supposedly wrong air/fuel mixture, cooling systems troubles or other mechanical failures.
It is absolutely clear that is doing lack more information about the true one and transcendental importance of the heat range of the sparkplugs. This exact selection is critical!
Only with the proper Heat Range will spark plugs sustain the perfect combustion process that keeps your engine's top efficiency and functional reliability. Which benefits?
o On High Performance modified or racing engines the higgest power you never reach before.
o On standard or family cars, more performance, more mileage, more reliability, extending engine's life and, decreasing emissions.
But only a few of the most selected and experienced engine tuners know how to select the spark plug with the proper heat range best suited to match these different conditions, or the specific engine's level of modification, but them reach it only after long hours of expensive and difficult testing by trial and error with different spark plugs sets of different heat ranges.
What you need to find is the spark plug with the heat range, best suited to the combination of your own specific, individual and particular driving habits, traffic, weather, and engine operating conditions, fuel quality, that can be affecting your engine performance; tailoring this selection exactly for you driving habits or life style.
This is the only way that guarantees to achieve the peak power, performance, endurance, better mileage and the lowest emissions.
You don't have to take my words. Spark plug manufacturers have never failed to explicitly acknowledge that:
"Owing to different driving conditions and/or engine conditions a hotter or colder spark plug may be required" - NGK
"Bad plugs cause overheating, rough running, power loss and even engine failure" - AUTOLITE
"Wrong choice of spark plugs can cause an engine to go on strike" - BERU
"This is reflected in the increased importance assigned to precisely adapting the spark plug to engine. Customized solutions are the order of the day" - BOSCH
"This application list is to be used only as a guide, owing to different driving conditions and/or engine conditions a hotter or colder spark plug may be required". - NGK
"The spark plug application for racing engines is very individual and requires good experience in identifying appropriate heat value. Failure to meet these requirements may cause extensive engine damages". - BRISK
"For racing, the spark plug recommendations listed should be considered only as the initial guide. Final heat range selection must rest with the racing participant.". - AC DELCO
So, if the manufacturers are advising you how much is it necessary to customize the selection of your sparkplugs when is replacement time, take care on it.
You have the decision power of choosing the right parkplug to reach the best results on performance, mileage, reliability and lowering the emissions.
And, to do that is easiest and cheap than you may imagine. Now you know all what you need is precisely selecting the heat range of your replacement sparkplugs.
