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There is a lot that you can learn from a spark plug because a sparkplug is the bridge in the gap between all the vital systems operating your car. The spark plugs have a front row seat to the fireworks show that is combustion. They are located in the splash zone where they are subject to fuels, oils, and other chemicals as well as extreme temperature variations and pressures from the cam and piston timing so they get to see everything that happens in your motor up close and personal. By paying careful attention to a plugs color and gap as well as noting any chemical deposits that remain after a plug has been removed from the cylinder, you can assess the efficiencies and deficiencies of your motor. It is important that you check your plugs at least once a year, and perhaps more often when dealing with high performance engines.

How it works

Every spark plug has two jobs. The first job is to ignite the Air/fuel mixture and the second lesser known job is to help remove heat from the combustion chamber. In order to ignite the air/fuel mixture, a spark plug transmits electrical energy with enough voltage to cause the spark to jump the spark plug gap and turn the fuel into working energy. During combustion, the firing end of the spark plug must maintain a temperature balance. If the temperature drops too low, the plug will become fouled, but if it is too high, pre-ignition can occur. This temperature balance or thermal performance is determined by the heat range of the plug.

Heat Range

A common misconception is that spark plugs create heat, but it is important to understand that they can only remove heat. The heat range has no actual relationship to the voltage used to ignite the mixture. The spark plug, instead, acts like a heat exchanger as it will pull heat away from the combustion chamber and transfer it to the engines cooling system. The heat range of a plug is determined by the length of the insulator nose, the materials used, and the construction of the center electrode and the porcelain insulator. The longer the insulator nose, the hotter the plug, while a shorter insulator nose will make for a colder plug. The colder plug removes more heat from the combustion chamber and operates at a lower internal temperature. A colder heat range spark plug may be necessary in higher performance or modified engines that operate at a high load or rpm for extended periods of time. Since the colder spark plug removes heat more quickly, it can reduce the chance for pre-ignition and detonation.

Air Fuel Mixture effects on the spark plug

Due to the proximity of the spark plug to combustion, the plug must be able to deal with many different conditions that can affect its performance. Air fuel mixture is one such condition that can affect the performance of the spark plug. An overly rich mixture will lower the plug temps below what the plug requires for self-cleaning. This will cause the plug to foul out. Conversely, an overly lean condition will cause the plug to become too hot resulting in pre-ignition and detonation.

Plug Gapping

Proper plug gapping is important to get maximum spark energy. It is best to operate with the biggest gap possible, however, when you increase cylinder pressures by advancing timing or raising boost a smaller gap may be necessary to prevent blow out. Because there is a fixed amount of energy required to create the spark, changing the gap will effect spark duration, but not change the amount of energy in the spark. A wider gap requires more energy for the spark to “jump the gap”, so it will burn more quickly. This is better for solid and more consistent ignition. A smaller gap will not require as much energy to set off the spark and will therefore burn longer. The longer duration in spark time is beneficial as air/fuel mixtures become increasingly dense requiring a longer/fatter spark to set off ignition. This is why you need a smaller gap as boost and ignition timing are increased.

Here are some examples of what plugs can tell you.


A normal looking spark plug will be grayish/tan in color. This is telling us that the cylinder this plug came out of is operating within its normal temperature range. There is no pitting to indicate detonation, or build up to indicate a rich condition.


On this plug, you can see that the insulator nose is cracked and there is pitting on the ground electrode. This is a sign of detonation. The extreme heat and pressure situations created in the cylinder during detonation are what cause the pitting and cracking of the spark plug.

Carbon Fouled:

A carbon fouled plug will have a black sooty coating pretty much all over. This indicates that the mixture is running too rich, the ignition is too week, or your plug is the wrong heat range.

And here is a little picture guide I found that can be helpful when trying to read your plugs.

And remember, it is important to check your plugs regularly to ensure proper operating conditions in your cylinders.
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