How do I choose the correct spark plug heat range? The purpose of selecting the proper heat range is to ensure that the plug tip temperature stays within the correct operating range of between approximately 500°C and 900°C. A spark plug that operates for an extended period of time below this range will not be able to burn off desposits and will quickly become fouled. If the plug operates above this temperature it will be hot enough to pre-ignite the air-fuel mixture potentially resulting in catastrophic engine failure.

There are many factors that will influence plug operating temperatures, some are design and tuning factors while others are operating conditions. Obviously an engine that is run at sustained heavy loads and high rpms such as on the track will require the plugs to dissipate more heat and the plugs will run hotter. The opposite is true for an engine run at low loads and rpms. Tuning factors such as ignition advance, air-fuel ratios, and boost levels will all affect plug temps. A 10 degree increase in ignition advance will raise plug temperatures by 100°C. Lean air fuel ratios will also raise plug temperatures. Fuel selection also has an effect. In terms of design factors probably the most important is compression ratio with higher ratios requiring a colder plug.

As the heat range number is decreased the plug will run hotter while as the heat range number increases the plug will run colder. One step change in heat range will change the operating temperature of the plug by approximately 100°C. From a performance standpoint it is best to run the hottest plug possible that will not cause pre-ignition or detonation issues under any normal circumstances. However, when you are unsure about the proper heat range to use it is better to err on the cold side as the only potential damage is fouled plugs. A good guideline is to increase the spark plug heat range by one number for each 200hp increase in power output. A race engine will probably want a plug that is a further one to two steps colder. However, it will be difficult to keep a high power race engine from fouling plugs when run at part throttle and idle.

What is the advantage of an iridium plug? Spark plug electrodes will erode away over time and the rate of erosion is partly a factor of the material used for the electrodes. Iridium is a superior metal for plug electrodes in terms of its resistance to erosion. It is so superior that is allows the electrodes to be reduced in size which helps to improve the combustion process.

Are there differences between the various types of iridium plugs? The common thread between the various iridium plug series being marketed is that the center electrode is a fine wire iridium tip. However, the ground electrode design and material will often vary depending on the specific application the plug is being optimized for. NGK Laser Iridium plugs use a platinum disc welded to the ground electrode. NGK IX plugs use a conventional ground electrode but with a tapered design to reduce the quenching effect the ground electrode has on the flame front development. NGK Racing plugs use a variety of configurations depending on the application.

How often should the plugs be changed? Performance will drop off as the plug wears so the replacement frequency will really come down to how critical a small performance loss may be. In a racing application frequent plug replacement is essential for maintaining maximum performance and reliability. In a high performance street application a 30K mile change interval is reasonable. Trackday type applications should consider a more frequent interval of at least 15K. A serious motorsport team will likely want to change plugs after every event or every run and will save the old plugs, if they still look good, for practice and perhaps testing and other purposes.