Spark plug heat number table: how not to get confused by the numbers

Selecting the correct ignition system components is not just a formality, but a critical part of engine maintenance. An error in choice, even a seemingly insignificant one, can lead to serious consequences, including piston burnout or coil failure. Exactly heat number is the parameter that determines the thermal operating conditions of the spark plug in the cylinder.

Many car owners are faced with a situation where one marking is indicated in the manufacturer’s catalog, but the store offers an analogue with completely different numbers. A natural question arises: will they fit? Understanding of how a heat cone works and ability to read correspondence tables allow you to avoid fatal mistakes when purchasing consumables.

In this article, we will analyze the physical meaning of the heat number, learn how to translate values between different manufacturers, and understand why “cold” spark plugs are better than “hot” spark plugs for high-speed engines. You will no longer be afraid of complex tables and will be able to independently select the optimal kit for your car.

Physical meaning of heat number

Heat number is an indicator characterizing the thermal load that a spark plug can withstand in the cylinder of an internal combustion engine without the occurrence of glow ignition. In simple words, this is the ability of the product to remove heat from the working part (heat cone) to the cylinder head. The more efficient the removal, the lower the temperature at the electrodes.

There is a direct relationship between the design of the insulator and its thermal performance. If the central electrode and insulator have a large contact area with the body metal and a short thermal path, heat is dissipated quickly. Such candles are called “cold”. Conversely, a long insulator with a small contact area retains heat, creating a “hot” plug.

⚠️ Attention: Using spark plugs with the wrong heat rating can cause irreversible damage. A spark plug that is too “hot” will cause detonation and melting of the piston, while a spark plug that is too “cold” will accumulate carbon deposits and stop sparking.

It is important to distinguish between different manufacturers' numbering systems. While for most global brands (NGK, Denso) an increase in the number means an increase in heat transfer ability (the candle becomes “cooler”), for the company Bosch Historically, the opposite logic has developed for some series, although in modern catalogs they also strive for unification. That's why heat number correspondence table is a mandatory tool when selecting analogues.

Labeling systems from leading manufacturers

The variety of labeling standards often confuses even experienced auto mechanics. Each manufacturer uses its own coding system, where the digital value can be radically different for candles with the same thermal characteristics. Let's look at the main systems on the market.

Japanese companies NGK and Denso use a straight scale. Their heat number indicates the time during which the insulator heats up to the self-cleaning temperature (about 400-500°C). The number 8 means a “cooler” spark plug than the number 6. That is, the higher the number, the better the heat dissipation and the more powerful the engine for which it is intended should be.

• 🔥 NGK: Standard scale where the step is 1 unit (for example, BPR6ES, BPR7ES). Number 6 is standard, 7 and 8 are for sports modes.
• ❄️ Denso: Similar to NGK system, where increasing the number means moving to a colder type (for example, K20PR-U, K22PR-U).
• 🇩🇪 Bosch: It uses a two-digit system, where, for example, WR7DC+ corresponds to NGK 7, but other designations were also found in old catalogs.
• 🇺🇸 Champion: It has its own unique encoding, which requires mandatory cross-checking using cross-tabs (for example, RN9YC).

When selecting analogues, you cannot simply change one number to another without taking into account the brand. Replacing an NGK number 7 spark plug with a Bosch number 7 spark plug may not be correct as their physical heat cone dimensions may be different. Always use official catalogs or verified correspondence tables.

Heat number correspondence table

To make it easier to select analogues, a summary table has been compiled that allows you to quickly find an equivalent among products of different brands. The data in the table is valid for most standard internal combustion engines.

NGK	Denso	Bosch	Champion	Characteristics
5	16	9	9	Hot (standard)
6	20	8	10	Medium (standard)
7	22	7	11	Cold (sport)
8	24	6	12	Very cold
9	27	5	13	Extreme

Using this table, you can see that if the manufacturer of your car recommends NGK spark plugs with a heat number of 6, then the equivalent from Denso will be a product with a number of 20, and from Bosch - with a number of 8. This allows you to flexibly choose a spare parts supplier without worrying about the thermal conditions of the engine.

It is worth noting that these values are averaged. In some specific series of spark plugs (for example, iridium or platinum spark plugs with modified electrode geometry), the thermal characteristics may be slightly adjusted by the manufacturer. Therefore, when switching from ordinary nickel candles to premium series Iridium or Platinum It is recommended to check the specific manufacturer's catalogue.

Consequences of choosing the wrong candles

Ignoring heat rating recommendations is playing Russian roulette with the engine. The consequences can appear either instantly or as a cumulative effect over several thousand kilometers. Let's look at two main scenarios for the development of events.

If you install too “hot” spark plugs (with a low heat transfer number) on a forced engine, the temperature in the combustion chamber will become critical. The insulator becomes red-hot, and the air-fuel mixture begins to ignite not from a spark, but from contact with a hot cone. This phenomenon is called glow ignition.

⚠️ Attention: Glow ignition is often confused with detonation, but they are different processes. Detonation is explosive combustion, and glow ignition is ignition from a hot surface. Both processes destroy the motor.

If you install too “cold” spark plugs on a civilian engine, the problem will be different. The candle will not reach the self-cleaning temperature (minimum 400°C). Carbon deposits (oil or carbon) will begin to actively form on the electrodes and insulator. Over time, the carbon layer will become conductive, a spark will travel across the surface of the insulator ("shunting"), and the cylinder will stop working. The engine will start to stall and fuel consumption will increase.

How to visually determine overheating of a spark plug?

If the central electrode and insulator have a whitish, almost worn-out appearance, and melting is visible on the side electrode, the spark plug is overheating. This is a sign of a too high heat rating (too “cold” plug) or problems with the cooling system.

How to choose spark plugs for a tuned engine

Owners of cars with chip tuning or forced engines should pay special attention to the selection of spark plugs. Any increase in power, be it turbocharging, changing valve timing or simply aggressive ECU firmware, leads to an increase in cylinder temperatures.

The golden rule of tuning says: the higher the power and speed, the “colder” the spark plugs should be. If the standard engine ran on “sixes” (according to NGK), then after Stage 1 it is often necessary to switch to “sevens”, and for serious sports - to “eights” or “nines”.

• 🚀 Boost increase: Each additional bar of turbine pressure requires more efficient heat removal.
• ⏱️ High speed: Engines operating in the red zone of the tachometer generate more heat per unit of time.
• ⛽ Enriched mixture: Sometimes a rich mixture is used to cool the combustion chamber, but it is better to choose spark plugs with a lower heat rating.

When selecting for tuning, the clearance is also important. On turbocharged engines, the gap is often reduced to make it easier for the spark to break through the compressed mixture, but this requires a more powerful ignition coil. An integrated approach is important here.

Diagnostics of engine condition by carbon deposit color

The spark plug is not only a source of spark, but also an excellent diagnostic tool. The color and structure of carbon deposits on the insulator can tell about the processes occurring in the cylinder much more eloquently than any sensors.

The normal color of the insulator is from light brown to coffee. This indicates the correct ratio of fuel and air, as well as the correctly selected thermal mode of the spark plug. If you see deviations, you need to take action.

Black velvety carbon deposits indicate a rich mixture or problems with the ignition system (missing spark). Oily black soot is a sign of oil getting into the combustion chamber (wear of rings or valve stem seals). A white or light gray color, especially with splashes, indicates a lean mixture or, more dangerously, working on spark plugs with an inappropriate (too high) heat rating.

What to do if the candles are constantly sooted?

If you change spark plugs regularly, but they are constantly covered with black soot, simply changing the brand will not help. It is necessary to check: injectors (for overflow), oxygen sensor (can lie and enrich the mixture), compression in the cylinders and the condition of the crankcase ventilation system (PCV). It is also worth rechecking the heat rating - perhaps the spark plugs are too “cold” for your driving style in the city.

Is it possible to clean old candles and use them again?

Mechanical cleaning (sandblasting or brushing) is permissible only as a temporary measure. Chemical cleaning in an ultrasonic bath followed by calcination can restore performance, but the service life of such a candle will be significantly lower than a new one. For modern engines with high sparking requirements, a complete replacement of the kit is recommended.

Does the octane number of gasoline affect the choice of spark plugs?

Indirectly - yes. Low octane gasoline is more prone to detonation, which increases the temperature in the cylinder. When using fuel with an octane rating lower than that recommended by the engine, it is advisable to use plugs with a slightly lower heat rating (cooler) to compensate for the increase in temperatures, although it is better to simply pour the correct gasoline.

How often should you change spark plugs?

The resource depends on the material of the electrodes. Conventional nickel spark plugs last 20-30 thousand km. Platinum and iridium - up to 60-100 thousand km. However, in conditions of frequent traffic jams and short trips, the resource is reduced by 30-40%. Regular condition checks (visual inspection) every 15 thousand km will help avoid problems.