From 2-wheels to 4-wheels, ‘modifications’ have become a mandatory part when you are planning to stand out of the crowd. You see enthusiasts so determined that either they are limited by their wallets or the availability of aftermarket parts for their type of vehicle. Following this trend, the biggest dream of ever car-guy with a hot pocket is to pump out extra ponies under his hood by whatever he can get his hands on and the first thing he tries to grab, is a spark plug.
When the air/fuel mixture is sucked in by the engine block, it has to ignite inside the cylinder in a timely manner to sustain output power, this is done by the spark plug. It has many entities which need to be taken account of before replacing one in your car. Most of us know about the spark gap and how it is associated with burn efficiency but what we don’t care for, is the heat range. It is the coefficient of thermal conductivity of the spark plug which determines how fast the spark plug conducts heat from the tip, through the insulator and into the surroundings. This sounds straight, however, a spark plug with a non-compatible heat range can simply decrease the precious power you are trying to forge out of your engine. You see the spark plug tip has to maintain a proper temperature in order to generate an ample spark. If the tip remains too hot (the spark plug is unable to dissipate enough heat), the grounding pin attains enough temperature that it may act itself as an ignition source, causing the mix to burn before the spark can occur, resulting in engine knock. Similarly if the tip remains cold (the spark plug is dissipating heat much more rapidly than the tip is able to maintain for proper ignition temperature), carbon fouling can occur, resulting in excessive carbon deposits on the spark plug. Prolonged conditions can result in poor or no spark at all.
The general rule of thumb is to stick with the OEM heat range provided by the manufacturer of your vehicle. But modifications on your engine (especially in forms of forced induction) can make your OEM heat range go haywire and you have to switch to another one. Heat ranges are generally referred in comparison as hotter or colder than one. As you go towards the colder side the spark plug increases its rate of heat conductivity (and vice versa). You have to go through the spark plug manufacture’s table of reference to know how they are numbered according to their heat range (For example on NGK spark plugs, a higher number represents a colder spark plug).
To self-determine that the spark plug installed in your car is of the right heat range, there is an experimental way by visually reading the spark plug condition. After installing a clean set of spark plugs, drive around almost 20-25kms under varying load conditions(using the whole throttle range) to make the spark plug go under stress. Drive back and remove the spark plugs to examine. You have to look at the threads from the top ring down for burn marks (brown glazed threads).
With reference to photo, it can be observed that the burned marks are persistent till thread number 2. If a spark plug with the right heat range is installed, the burned marks will persist till 1 to 2 threads. If they are reaching 3 or 4, you need a colder spark plug as a lot of heat is trying to escape through the threads meaning the insulator is not able to conduct enough heat out of the system. While if the burn marks are fading before reaching thread 1, a hotter spark plug is required as the insulator is conducting excessive heat out of the system and no heat is escaping through the threads. Using this principle a very near-to-exact heat range can be obtained for your engine.
More methods are available to determine heat range for a spark plugs recommended by tuners (for example from the grounding pin), but they are more reliable when running under very high stress with wide open throttle.