Spark Plug Heat Ranges
The spark plug's heat range is an index of its capacity to withstand thermal loads based on engine characteristics. Variables such as operating load, compression ratio, engine speed, cooling, and fuel make it impossible to run all engines with one standard spark plug. The same spark plug that gets hot in one engine can have a relatively low average temperature in another. When hot, the air/fuel ratio ignites on the glowing parts of the spark plug projecting into the combustion chamber, and when relatively cold, the insulator tip becomes so badly fouled by combustion deposits that misfiring occurs due to the formation of shunts (additional ground paths).
To ensure the plug doesn't run too hot or cold in a given engine, plugs have different load capacities. The so-called heat range assigned to each spark plug characterizes these loading capacities, so the heat range is a yardstick for selecting the correct spark plug. There is no industry standard for heat range; it is determined by the spark plug manufacturer. A Champion spark plug for your particular engine may have a different heat range than an ACDelco for the same application. For this reason, we suggest you reference the sidebar on how to measure heat range.
When using unleaded fuel, the part of the insulator tip extending into the combustion chamber should not drop below 500 degrees C to ensure self-cleaning of the spark plug. Additionally, it should not exceed approximately 850 degrees C to prevent auto-ignition.
Particulate deposits (soot) are produced by the incomplete combustion processes of cold starts. Most of these deposits leave the engine with the exhaust gases, but some remain to form a coating on both the combustion chamber and the spark plug. As these deposits form on the insulator nose, they can produce a conductive path between the center electrode and the shell. This path absorbs a portion of the ignition energy, forming a shunt path and reducing the energy available for the ignition spark. The result is a multiple-cylinder misfire. Excessive contamination can prevent a spark from forming since it will follow the shunt path instead of jumping the electrode gap.
These deposits are dependent on the temperature, occurring mainly below about 500 degrees C. At higher temperatures, the carbon-containing residues on the insulator nose burn off, and as a result, shunts cannot form: The plug cleans itself. The goal is an insulator-nose operating temperature that is higher than the self-cleaning limit of about 500 degrees C. Additionally, the self-cleaning temperature should be reached as quickly as possible after start up.
About 900 degrees C is the upper temperature limit. Any higher and the air/fuel mixture can ignite prematurely on red-hot parts of the spark plug. Uncontrolled ignition of this kind (ping) is highly detrimental to the engine and may cause severe damage in a short amount of time, so the operating temperature of the spark plug must be within a certain range.
Thermal Loading Capacity
Operating temperature is the equilibrium point between heat absorption and heat dissipation. The spark plug is heated by the engine's combustion event, and the spark-plug shell reaches approximately the same temperature as the cylinder head, while the temperatures of the insulator is considerably higher. Nearly twenty percent of the heat absorbed by the spark plug is dissipated by the flow of fresh mixture during the induction stroke. Most of the heat is transferred through the center electrode and insulator to the spark-plug shell and from there to the cylinder head.
The supply of heat to the spark plug is dependent on the engine. Engines with a high specific output usually have higher combustion-chamber temperatures than those with a low specific power output. The heat-absorbing properties of the spark plug must match the engine type and application. Thus, the heat range is a characteristic of the thermal-loading capacity of the spark plug.
As already stated, each spark plug must remain within a specific temperature range during operation. To remain within that range, the spark plug for a hot-running engine must efficiently dissipate heat, while the plug for a cold-running engine must retain heat. Various design factors, with an emphasis on the configuration of the insulator nose, are used to adjust the heat range for specific engines and applications.
Heat absorption is determined by the surface area of the insulator nose. If a large area is exposed to the combustion gases (i.e., a long insulator nose), it gets very hot. With a short insulator nose, the area is small and the nose remains cooler.