Ignition Performance


The following factors affect the ignition performance of a spark plug:

1. Electrode shape and discharge performance

Rounded electrodes make discharging difficult, while squared-off or pointed electrodes facilitate discharging. As electrodes are rounded off through long use, it becomes difficult for the spark plug to discharge sparks. Therefore, the spark plugs must be replaced regularly. It is easier for a spark plug with thin and pointed electrodes to discharge sparks. However, those electrodes wear faster and shorten the service life of the spark plug. For this reason, some spark plugs have platinum or iridium, which resist wear, welded to their electrodes. They are usually called platinum-tipped or iridium-tipped spark plugs.


Spark plug replacement intervals Conventional typeEvery 10,00to 60,00km Platinum- or iridium-tipped typeEvery 100,00to 240,00km The replacement intervals may vary by vehicle model, engine specifications, and country of use.

2. Spark plug gap and required voltage

As the spark plug becomes worn and the gap between its electrodes widens, the engine can misfire. When the distance between the center electrode and the ground electrode increases, it is more difficult for the spark to jump across the electrodes. Thus, a greater voltage will be required to generate a spark. For this reason, the gap must be adjusted or the spark plug must be replaced at regular intervals.


If the required voltage can be provided despite a wide gap, the spark plug will be able to produce a strong spark and facilitate ignition. For this reason, there are many spark plugs on the market with a gap as wide as 1.1 mm. The platinum- and iridium-tipped spark plugs do not require gap adjustments because they are not susceptible to wear (they only need to be replaced).

Heat Range of Spark Plug

The amount of heat radiated by a spark plug varies by the shape and the material of the spark plug. The amount of radiated heat is called a heat range. A spark plug that radiates more heat is called a cold type, because the plug itself stays cooler. One that radiates less heat is called a hot type, because its heat is retained. Spark plugs are printed (inscribed) with an alphanumeric code, which describes their structure and characteristics. Codes differ somewhat depending on the manufacturer. Usually, the larger the number of the heat range, the cold plug, because it radiates heat well. The smaller the number, the hot plug, because it does not radiate heat easily. Spark plugs perform best when the minimum center electrode temperature is between the self-cleaning temperature of 450C (842 F) and the pre-ignition temperature of 95C (1,742 F).


The most appropriate spark plug heat range for a particular vehicle is determined by the model. Installing a spark plug of a different heat range will upset the selfcleaning and pre-ignition temperatures. To prevent these problems, always use the specified type of spark plugs for replacement. Using a cold spark plug when the engine is operating under low-speed and low-load conditions will reduce the electrode temperature and cause the engine to run poorly. Using a hot spark plug when the engine is operating under high-speed and heavy-load conditions will excessively increase the electrode temperature, causing the electrode to melt.

1. Self-cleaning temperature

When a spark plug reaches a certain temperature, it burns off the carbon that has accumulated in the ignition area during ignition, in order to maintain the cleanliness of the ignition area of the plug. This temperature is called the self-cleaning temperature. The self-cleaning effect of the spark plug takes place when the temperature of the electrodes exceeds 45C (842 F). If the self-cleaning temperature has not been reached, meaning the temperature of the electrodes is below 45C (842 F), carbon accumulates in the ignition area of the spark plug. This can cause the spark plug to misfire.

2. Pre-ignition temperature

If the spark plug itself becomes a heat source and ignites the air-fuel mixture without sparking, this is called the pre-ignition temperature. Pre-ignition takes place when the temperature of the electrodes is above 95C (1,742 F). If it occurs, the engine output will drop due to incorrect ignition timing, and the electrodes or pistons may partially melt.

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