Spark plug heat number table: decoding and selection

Correct selection of ignition system components is a critical condition for stable operation of an internal combustion engine. Among all the parameters, the most mysterious for the average car enthusiast is often heat number, which manufacturers indicate on the product body. The wrong choice of this characteristic can lead to serious consequences, including burnout of the pistons or, conversely, fouling of the electrodes and the engine tripping.

In this article we will analyze in detail the physical meaning of this parameter, consider international labeling standards and provide an up-to-date compliance table for major brands. Understanding the principles of heat dissipation will help you independently select high-quality analogues without fear for the life of your car’s power unit.

The spark plug has a dual function: it ignites the air/fuel mixture with a spark and conducts excess heat from the center electrode and insulator into the cylinder head. It is the ability to remove heat that characterizes heat number, which is a key parameter of the thermal characteristics of the part. If the heat removal is insufficient, the insulator heats up to temperatures that cause premature ignition of the mixture (glow ignition).

In turn, excessive heat removal leads to the fact that the working part of the candle does not have time to self-clean itself from combustion products. The operating temperature of the insulator must be within a strictly defined range, usually from 400 to 850 degrees Celsius. Going beyond this “thermal window” inevitably leads to unstable engine operation and a decrease in its service life.

The physical essence of the heat number

The heat number is a conventional value that characterizes the thermal operating conditions of a spark plug. It is determined in laboratory conditions on a special stand, where the time during which the spark plug insulator heats up to a temperature of 900-950 degrees Celsius is measured, causing glow ignition. The higher the numerical value, the more efficiently the candle removes heat from its working part.

There is a direct relationship between the design of the insulator and its ability to transfer heat. "Hot" plugs have a long thermal insulator cone that is deeply immersed in the combustion chamber. The large contact area with hot gases and the smaller contact area with the metal casing result in heat being dissipated slowly. Such products are designed for engines with low compression ratios and low loads.

On the contrary, “cold” spark plugs have a short insulator that fits tightly to the metal body. This ensures that heat is quickly dissipated into the cylinder head through the threads and O-ring. High heat rating necessary for forced engines, turbocharged units and high-speed operating modes where thermal loads are maximum.

⚠️ Attention: Installing a spark plug with an inappropriate glow number is not diagnosed by the vehicle electronics as a standard error. The engine can work, but with critical overheating or, conversely, with constant contamination, which will gradually damage the engine.

It is important to understand that there is no universal meaning that suits everyone. Each engine is designed for a specific thermal regime. Usage NGK or Bosch with an incorrect number may require a complete replacement of the piston group if the problem is ignored for a long time.

Manufacturer markings and designation systems

Global manufacturers use different coding systems, which often causes confusion when selecting analogues. The main difference lies in the direction of the scale: for some brands, an increase in the number means an increase in the “warmth” of the candle, for others it means its “coldness”. This fundamental difference must be taken into account when studying catalogs and lookup tables.

European and American manufacturers such as Bosch and Denso (in some series), they use a scale where a higher numerical value corresponds to a “colder” spark plug with better heat dissipation. For example, a spark plug with the number 9 will be cooler than a spark plug with the number 6. This is logical for engineers: the higher the load, the higher the heat removal number.

However, the company NGK and Brisk they use reverse logic inherited from old Soviet standards. For them, a smaller number indicates a “hotter” candle, and a larger number indicates a “colder” one. That is, a candle with the number 6 NGK will be colder than a candle with the number 9. An error in interpreting this difference can lead to the installation of a part with radically opposite properties.

• 🔥 Hot candles: Have a low heat rating in the system NGK (for example, 8-9) and high in the system Bosch (for example, 2-4), designed for a quiet ride.
• ❄️ Cold candles: Characterized by a high number of NGK (10-12 and above) and low Bosch (7-10), designed for sports and high loads.
• ⚙️ Mid Range: Most civilian cars use spark plugs with average values that ensure self-cleaning during the urban cycle.

When making your own selection, always pay attention to the first number in the marking or a special index indicating the thermal conditions. Ignoring this parameter when purchasing analogues instead of the original is a common mistake leading to unstable operation of the internal combustion engine.

Heat number correspondence table

To simplify the selection of analogues, below is a summary table showing the correspondence of the heat numbers of the world's main manufacturers. These values ​​are averaged, since within one numerical range there may be design nuances, but for the initial selection they are completely relevant.

Candle type	NGK / Brisk	Bosch	Denso	Champion
Very hot	8	2	16	N4
hot	9	3	20	N5
Average	10	4	22	N6
Cold	11	5	24	N7
Very cold	12	6	27	N8

Using this table, you can easily find a replacement if your garage contains a spark plug from another manufacturer, but with a known index. For example, if a standard spark plug Bosch has the index FR7DC+, then the number 7 indicates a fairly cold candle. In terms of NGK this would correspond to the number 6 or 7 (very cold/cold) rather than the standard 9 or 10.

It should be remembered that the table provides only a guide. Accurate selection must be made based on the vehicle catalog number or VIN code, since manufacturers can make changes to the engine design that require correction of the thermal regime even within the same model.

Consequences of the wrong choice

Ignoring the manufacturer's requirements for the thermal regime of the spark plug leads to two main scenarios for the development of events, both of which are detrimental to the engine. The first scenario is installing a spark plug that is too “hot” (with a low heat sink number for a given motor). In this case, the insulator does not have time to cool down and heats up to temperatures above 900 degrees.

At such temperatures, the mixture can ignite not from a spark, but from contact with a hot insulator. This phenomenon is called glow ignition. It causes detonation, overheating of the pistons and can lead to their burnout or destruction of the partitions between the rings. The engine continues to run even after the ignition is turned off, jerking and making metallic sounds.

The second scenario is installing a spark plug that is too “cold”. The working part of such a candle does not reach the self-cleaning temperature (about 400 degrees). As a result, carbon begins to actively deposit on the insulator and electrodes, which blocks the spark gap. The spark plug stops sparking, the cylinder stops working, and the engine starts to stall.

⚠️ Attention: Long-term driving with “cold” spark plugs in city mode leads to oil deposits, which are almost impossible to remove. The candle will have to be thrown away, even if it is new.

Diagnosing the condition of a removed spark plug can tell a lot about the thermal conditions. White or light gray insulator with melted marks indicates overheating. A black, velvety coating indicates that the spark plug is too cold for current operating conditions.

Influence of operating mode on choice

The choice of heat rating also depends on how and where the car is used. Factory recommendations are usually given for a mixed cycle, but adjustments may be required if operating conditions change. For example, when constantly driving along a highway at high speeds, the thermal load on the engine increases.

If you frequently load your car, tow a trailer, or use natural gas equipment (LPG), the temperature in the combustion chamber rises. Gas burns at higher temperatures than gasoline. In such cases, standard spark plugs may turn out to be too “hot”, and it is advisable to replace them with analogues with a higher heat rating (one unit cooler).

On the other hand, cars that are used exclusively for short trips around the city (“taxi mode” or delivery) are characterized by underheating of the engine. In this case, on the contrary, slightly “hotter” spark plugs may be required to provide sufficient temperature for self-cleaning of carbon deposits.

Owners of cars with LPG should be especially careful. Switching to gas requires a mandatory revision of the ignition system. It is often recommended to reduce the gap in the spark plugs and use products with higher resistance and suitable thermal conditions to avoid misfires and burnouts.

Diagnostics and visual inspection

Regular visual inspection of spark plugs is the best way to monitor the condition of the engine and the correct thermal conditions. It is recommended to carry out this procedure every 15-20 thousand kilometers, even if the engine runs flawlessly. The color of the soot will tell you about the processes occurring inside the cylinder.

The normal color of the insulator is from light brown to coffee. This indicates the correct ratio of the fuel-air mixture and the correct operation of the ignition system. The presence of small deposits is acceptable, but they should not cover the spark gap.

If you notice the following signs, you need to take action:

• 🔴 Red plaque: Indicates the presence of manganese-based additives in the fuel, which can lead to a conductive film and misfire.
• ⚫ Oily black coating: Indicates that oil has entered the combustion chamber through worn valve stem seals or rings.
• ⚪ White reflow insulator: Critical overheating, indicating a too “hot” spark plug or a lean mixture.

If white deposits or melting are detected, you should immediately check the ignition timing and replace the spark plugs with “colder” ones. Ignoring these symptoms as soon as possible will lead to costly repairs to the piston group.

Practical recommendations for replacement

The process of replacing spark plugs requires care and adherence to certain rules in order not to damage the threads in the cylinder head and to ensure proper heat dissipation. Before starting work, the engine must cool completely, since unscrewing spark plugs from hot aluminum can damage the threads due to the different expansion coefficients of the metals.

When installing new spark plugs, it is important to select the correct tightening torque. For plugs with a conical seal, the torque is usually less than for plugs with a flat bearing surface and a pressure washer. Insufficient tightening will lead to disruption of heat dissipation and possible breakdown of gases, and over-tightening will lead to deformation of the case and difficulty unscrewing in the future.

Use only quality tools. Cheap spark plug wrenches can lick off the hexagon edges or damage the ceramic insulator. After installation, it is recommended to drive several kilometers in a quiet mode so that the new spark plugs “break in” and the carbon deposits settle evenly.

⚠️ Warning: Never use graphite lubricants or copper pastes on the threads of modern nickel plated spark plugs. This can break the electrical ground contact through the housing and cause the ignition coil to malfunction.

Following these simple rules will extend the life not only of the spark plugs themselves, but also of the entire ignition system, and will also ensure stable engine operation in any conditions.

Frequently asked questions (FAQ)

Is it possible to install candles with a higher heat rating than the factory recommends?

It is possible, but with caution. Installing a “colder” spark plug (with a higher number for NGK) is acceptable if you have boosted the engine, installed a turbine, or constantly operate the car at high speeds. However, in normal urban conditions, this will lead to rapid fouling and triple formation.

Why do NGK and Bosch spark plugs have different numbers?

The difference is due to historically established standards. In the Bosch and Denso system, the number increases with the ability to remove heat (9 is cooler than 6). In the NGK and Brisk system, the number increases as the ability to remove heat decreases (6 is colder than 9). This requires care when recalculating.

How often should you change spark plugs?

The resource depends on the material of the electrodes. Nickel spark plugs last 20-30 thousand km, platinum ones - up to 60 thousand km, and iridium spark plugs can last 80-100 thousand km. However, in practice, in conditions of poor fuel and traffic jams, it is better to reduce the resource by 20-30% of the declared one.

Does the octane number of gasoline affect the choice of heat rating?

Indirectly affects. High octane gasoline burns at higher temperatures. When switching to higher octane fuel (for example, from 92 to 95 or 98), the heat load in the combustion chamber may increase, requiring the use of “cooler” plugs.

What happens if you mix up hot and cold candles?

If you put “hot” instead of “cold”, the engine will detonate and may burn out. If they are “cold” instead of “hot”, the engine will stall, have difficulty starting and lose power due to coking of the electrodes with carbon deposits.