• P1. What is the function of a spark plug?

The spark plug is located on the engine head. Its main function is to conduct the electric current generated on the transformer to the combustion chamber, and turn it into high-voltage electric spark, which will start the combustion, i.e., the burning of the air/fuel mixture. Although its simple appearance, the spark plug is a highly complex part, consisting of a series of internal components that requires for its conception the application of highly sophisticated technology, therefore its perfect performance is directly connect to the engine performance, the levels of fuel consumption, the greater or the lesser load of pollutants in the gases expelled for the escape.

P2. Why are NGK spark plugs superior?
The NGK spark plugs meet the strictest requirements of automakers both in performance, technology and durability. Offering to the market the most modern technology developed for spark plugs.

1. It has a “wide heat range”.
The NGK spark plugs have copper nucleus in the central electrode. This characteristic ensures an excellent heat exchange expanding the operation heat range of NGK spark plugs. Resisting in a superior way to heating as well as carbonization.

2. It incorporates an isolator produced with high-tech highly pure alumina ceramics.
– It has superior isolation to high temperatures and guarantees spark production.

– It offers superior thermic conductivity and avoids over-heating.

– It resists thermic shocks (sudden heating and cooling) and its mechanical resistance is superior.
3. Its robust construction guarantees excellent combustion gas tightness.
The special powders that connect the isolator with the metal bonnet ensure a perfect seal to prevent the leakage of gas internal pressure of the combustion chamber.
4. The electrodes made from a special alloy of nickel guarantee high durability.

P3. What is the heat rating of a spark plug?

During the operation of the engine occurs the process of combustion of the air/fuel mixture, this process generates heat, part of it is dissipated by engine components as the block, head, pistons and valves, and another part is transferred to the spark plug. The spark plug dissipates heat through its isolator and spark plug isolator to the thread, passing from spark plug thread to the head, and from the head to the cooling system, i.e., the engine cooling.
The heat rating is the capacity that the spark plug has to dissipate heat. In a hot spark plug the heat dissipation will be slower. In a cold spark plug the heat dissipation will be faster.

Due to the fact that engines generate different thermal loads, each type of engine has a spark plug with specific heat rating.

The determination of the heat rating is performed through measuring instruments that are installed in an engine, which in turn is coupled to an engine dynamometer.

With the use of a thermowell spark plug, the temperature of the spark plug firing end inside the combustion chamber of the engine is checked. This measurement is performed to identify the most critical condition of the engine, and so in this condition is performed the pre-ignition testing which will determine the heat rating appropriate for the respective engine.

Due to the existence of several types of spark plugs with different heat ratings, it is very important that the applicator check out in the NGK recommendations catalog or in the manual of your vehicle, the correct spark plug code, in this way you will be ensuring that is using a product that has been developed, tested and approved for your vehicle.

– Temperature chart X rotation;

P4. What is spark plug carbonization?

The spark plug carbonization occurs when there is accumulation of coal in the spark plug firing end. The coal is an electric power conductor, therefore, a carbonized spark plug will suffer loss of insulation causing misfires.

As we increase the rotation and engine load, the temperature at the firing end is increased. When the temperature of the firing end exceeds 450°, it reaches a temperature range that we call of spark plug auto cleaning. Where the spark plug itself burn all the coal waste of the firing end.

It should be noted that fuels like ethanol (alcohol) and CNG (Compressed Natural Gas) do not generate waste of carbonization. This is due to a characteristic of the fuel that has smaller carbon chains.

Main factors that facilitate the spark plug carbonization:

– Application of spark plug colder than indicated;

– Very rich fuel mixture;

– Engine with oil burning;

– Lack of thermostatic valve, engine working cold;

– Use of vehicles on low rotations or low speeds for a long time;

– Delayed ignition point;

– Too closed clearance between electrodes;

– Low cylinder compression;

– Poor quality fuel;

P5. What is spark plug overheating?

The spark plug overheating occurs when the temperature of the firing end exceeds 850° for gasoline powered vehicles or 750° for ethanol powered vehicles.

We can recognize a spark plug that suffered overheating through spark plug firing end examinations. When firing end is whitish, vitrified with granules or black points on the surface is a sign that occurred the overheating. There are extreme cases where can occur smelting of the central and side electrode, reaching even to disappear completely the electrodes.

Overheating can generate the pre-ignition. That is a condition of abnormal burning. Where we have a hot point in the combustion chamber, when the fuel/air mixture comes into contact with this point the mixture ignites before the spark plug sparkle time. This burning condition can generate several damage to the vehicle’s engine.

Main factors that can generate the spark plug overheating:
– Spark plug warmer than indicated for the vehicle;

– Advanced ignition point;

– Lack of torque on spark plug;

– Installation of objects between the spark plug and the engine head (mechanical tappets; helical threads, etc.);

– Very high compression rate;

– Excess pressure in the combustion chamber (turbocharged engines);

– Cooling system problems;

P6. What do the codes of NGK spark plugs mean?

The NGK spark plug encoding is very important, as well as to demonstrate the main characteristics of the spark plug, it identifies the NGK spark plug worldwide.

Click on the image below to enlarge.

P7. What is the best way to select the correct spark plug?
The spark plugs are designed specifically for each type of engine. So for each vehicle we have a specific spark plug, we should not change the specification of the spark plug that was developed jointly with the vehicle’s manufacturer.

The customer can check the spark plug application in the vehicle’s manual or through the NGK recommendations catalog.

To the correct use of the NGK recommendations catalog we should have some basic vehicle information such as:

* The appropriate spark plug for the above mentionned vehicle

P8. Why is the firing end appearance of a spark plug important?

Through the appearance of the spark plug firing end we can assess the conditions of wear and spark plug burning.

Very worn spark plugs indicate the need for exchange.

Spark plugs with excess of residue may indicate an abnormal condition of engine operation or use of poor quality fuel.

The spark plug is also an important element for the diagnosis of engine condition. Through the analysis of the spark plug firing end we can also have a good reference of the combustion chamber condition. As the spark plug is the only part inside the combustion chamber that is easy to remove and install.

NGK offers in its recommendations catalog an engine failures diagnosis newsletter, where through the spark plug firing end analysis we can diagnose several malfunctions that affect the spark plug.

P9. What is the function of the grooves on the insulator?

The function of the grooves in the spark plug insulator is to increase distance between the spark plug terminal pin and the metal bonnet, without increasing the overall length of the spark plug, thus making it difficult the occurrence of flash over between the spark plug and ignition cable.

We note that some spark plugs projects do not have grooves, in these cases the spark plug insulator is longer, or we use noble materials such as Platinum and Iridium in the spark plug electrodes to reduce the voltage required for the sparking.

P10. Why should we use a resistive type spark plug?
With the entry of onboard electronics in vehicles, NGK has developed in the late 1970s, resistive spark plugs that have a ceramic resistor which aims to reduce the electromagnetic interferences caused by ignition system.


This interference is extremely detrimental to vehicle electronic systems such as:

– Electronic fuel injection;
– Sound system;
– Antitheft system (alarms, blockers and trackers);
– Automatic gear shift with electronic control;
– Traction and stability electronic control;
– Air Bag and ABS Systems.
– CAN Network;
– Air conditioning with electronic control;
Most resistive spark plugs have a resistance that can range from 3 to 7.5 kΩ or 1 to 2 kΩ for some spark plugs of Volkswagen and Audi.

We must be careful when perform transformations in old cars. These vehicles have ignition systems with low capacity and if we install spark plugs and resistive cables due to the use of electronic equipment, we must upgrade the ignition system.

The use of resistive spark plug is not related to the useful life of the spark plug, i.e., it does not affect durability.


P11. Which is the Green type spark plug and how identify it?
Developed in 1980s to meet a need of automakers to comply with the laws of pollutant emissions which are increasingly strict over the years.

Due to its unique configuration (central electrode with a shaped “V” cutting and concave side electrode).  This spark plug has a great advantage, because its spark occurs on the sides of the electrodes by increasing the energy of the flame front, providing a better fuel burning and also reducing the emission of polluting gases.


The electrode configuration provides a greater effect of the tips, thus reducing the voltage required for the spark plug sparking. Another great advantage of this spark plug is the use of fuels such as CNG, where due to the use of mixtures too lean require higher voltages to generate the sparking. The Green spark plug present excellent results in the use of this fuel.
Other advantages of using Green Spark Plug are:

– Resistive type spark plug avoiding interference in electronic systems, in some markets there are Green spark plugs of the conventional type, without internal resistance;
– Improved engine performance;
– Faster starting, even on cold days;
– Enables the use of air/fuel mixtures too lean;
– Excellent performance in gas powered vehicles (CNG – Compressed Natural Gas);
– Provides more stable idling speed;

We can identify the Green spark plugs through its shaped “V” cutting in the central electrode and concave side electrode, which are the main characteristics of this spark plug.

– Image of the Green spark plug


P12. What is flash over?
The flash over is the passage of electric current between the spark plug terminal pin and the metal bonnet passing through the spark plug insulator and the ignition cable. Normally flash over occurs when the sparking voltage between the electrodes is very high, making it easier the occurrence of sparking between the terminal pin and the metal bonnet.

When the flash over occurs we can observe a characteristic marking in the spark plug insulator in the ignition cable.

Once identified the occurrence of flash over, we should replace the set of spark plugs and ignition cables simultaneously, if only one of the parts is replaced, the new part will be damaged by the old part and the defect will return soon.


Some factors that favor the occurrence of flash over:

– Excessive wear of the spark plugs;

– Air/fuel mixture too lean;

– Dirt between the spark plug insulator and the ignition cable;

– Ignition cables loose in the spark plug insulator;

– Image of the spark plug and flash over cable;


P13. Which spark plug must be used in a conversion from gasoline to alcohol?

In addition to the materials used (surface treatment due to corrosion caused by alcohol). The main difference between the alcohol and gasoline engines is the compression ratio. Alcohol engines typically are more rated, thus using a spark plug colder than the gasoline engine.
In most of the conversions performed by the market, the changes performed consist to advance the ignition point and enrich the air/fuel mixture, that is, increase of the volume of fuel injected into the combustion chamber. In these circumstances we should not change the spark plug heat rating, and we can keep the same spark plug that was used when the engine was powered by gasoline.
If the engine has suffered any mechanical change, such as the compression ratio, it will be necessary to perform a thermowell testing to determine which spark plug is more suitable for the engine.


P14. The spot that appears between the insulator and the metal bonnet is caused by gas leakage?
It is common to observe in some spark plugs the occurrence of a spot on the insulator of the spark plug. Many repairers think this spot is due to the gas exhaust of the engine. In fact, if there is a gas exhaust of the engine, we have an extremely strong and characteristic noise of escaping gases.

The spot in the insulator is caused by the corona effect, when high voltage goes through the spark plug occurs the generation of an electromagnetic field wrapped to the wires of the spark plugs and to spark plug itself. Oil and fuel vapors that are in the safe of the engine are attracted to the spark plug insulator forming this spot effect.

Remember that the corona spot does not affect the operation of the spark plug.

P15. What precautions should be taken during spark plug installation?
All NGK spark plugs packaging have recommendations for installation of spark plugs on the vehicle. We recommend that the spark plugs are installed by a qualified technician.

The first item that we should check out is whether the application is correct for the vehicle, to do so just consult the NGK recommendations catalog or the automaker manual.

Before installing the spark plugs we should examine the clearance between electrodes, all NGK spark plugs already leave the factory calibrated, however from the shipping and handling of the spark plugs to the installation there may be some change in the clearance.

We should install the spark plug by hand to ensure the perfect alignment between the threads of the spark plug and the engine head.

We have to take special care with the tightening torque, NGK offers on their packaging the torque table. Below we can see the torque table, to apply the torque we should have a calibrated torque wrench.

We can also apply the angular torque, where the spark plug is screwed by hand until it touches the head and then the torque is applied:

The spark plug tightness is very important, because the spark plug dissipates the heat of the firing end through the spark plug insulator and from insulator to metal bonnet and from the bonnet to the engine head. The lack of this torque affects this heat exchange and can generate severe damages to the engine.

Another failure that is related to the spark plug tightness is the rupture of the spark plug thread or channel by excessive torque. When we apply excess torque the thread can break in the tightness or during the process of release. In both cases we can generate engines damage.

The slope of the wrench during the tightening process may cause cracks in the ceramic insulator and cause misfires

P16. What spark plug should I use in a CNG conversion?
The spark plug is developed for each type of engine to the original condition of the vehicle. With the adaptation of the CNG in a vehicle, we are changing its original characteristics.

CNG powered vehicles is characterized by work with a mixture more lean than the original and request a higher voltage to the sparking, there is an increase in the order of 5 kV on the voltage.
If the CNG kit is well-dimensioned and well-installed the original spark plug should work normally. Many installers recommend changing the original spark plug for a spark plug with better ignition as the Green, Platinum and Iridium spark plugs. This option is valid when there is recommendation of optional spark plug in the NGK recommendations catalog.

The use of CNG is considered a severe application, so the recommendation to Exchange spark plugs should be reduced by half of the original recommendation.


P17. How long will a spark plug last?


The spark plug is a component of natural wear, that is, as the engine runs the spark plug wear.

Who establishes the mileage of a spark plug exchange is the vehicle manufacturer within the specifications of each project. During the development of the vehicle several durability tests are carried out, on the basis of these tests is determined the mileage of exchange.

The automakers guide in their manuals that in severe usage conditions of the vehicle the interval between maintenances should be reduced by half, that is, if the automaker establishes a spark plug exchanging period every 20,000 km, on condition of severe usage the exchange should be performed every 10,000 km.
Below are some severe usage conditions:
– Use of the vehicle in heavy traffic;

– Use in short paths and with frequent stops;

– Long periods of vehicle downtime;

– Use of the vehicle as a taxi, ambulance, delivery vehicles, driving school, patrols and alike;
For cases where the last replacement of spark plugs is unknown, NGK recommends that the repairer removes the spark plug for inspection.

The use of worn spark plugs can cause damage to the ignition system, increased consumption, increased levels of vehicle emissions and starting difficulty.


P18. Should I diminish the clearance of the electrodes in cars converted for CNG?


The vehicles converted to CNG require greater tension of the ignition system for the formation of the spark in the spark plug, this occur in function of the characteristics of fuel CNG, therefore is very important the use of products of high quality. Using jointly the NGK’s ignition cable and spark plugs, it is not necessary to diminish the clearance between the spark plug electrodes because they have a high capacity of insulation.

There are cases where the kit installed is not suitable to the vehicle or the kit is incomplete, without all components. In such cases can occur Back Fire that is the burst in the intake collector or flash-back in the intake collector.

Many mechanics try to solve the problem of Back Fire by reducing the clearance between spark plug electrodes. This practice is not recommended, because it can change the operation of the engine in the original fuel as well as the levels of emissions of the vehicle.


P19. When installing a turbo compressor (supercharging) should I replace the original spark plug?


The spark plugs are designed specifically for each type of engine. When a supercharging system (turbo or mechanical compressors) is installed the original conditions of engine operation is changed, so the original spark plug will not meet the new operating conditions.

When installing a turbo compressor we are increasing the volumetric efficiency of the engine, therefore generating more heat in the combustion chamber and making it difficult the spark plug sparking due to the increase of internal pressure of the combustion chamber.

Thus, it is necessary the application of a spark plug with better ignition and heat rating suitable to the engine. In order to determine the heat rating of a spark plug is necessary to measure the temperature of the spark plug firing end, in some situations of engine operation, however these tests and assays can only be carried through appropriate equipment.

Therefore, there is no way to indicate a spark plug for an engine that has been modified.