Spark plug heat rating table: reference book and explanation

Incorrectly selected heat number spark plugs instantly leads to critical consequences: from fouling of electrodes with carbon deposits to complete burnout of the piston and valves. When the engine runs on a lean mixture or with early ignition, the temperature in the combustion chamber rises sharply, and if the thermal regime of the spark plug does not match the load, the central electrode begins to melt, causing uncontrolled ignition of the mixture - glow ignition. That is why the table of spark plug heat ratings is a mandatory tool when selecting analogues for Bosch, Denso or NGK.

Failure to ensure that the vehicle's thermal conditions are within the factory specifications often results in difficult diagnostics, as symptoms may resemble problems with injectors or oxygen sensors. In this material, we will look at how to read the markings, compare the meanings of different brands and choose the optimal components for your engine to avoid costly repairs.

Physical meaning of heat number and thermal regime

Heat number is a numerical parameter that characterizes the thermal load that a spark plug in an internal combustion engine can withstand without glow ignition occurring. In fact, this is a measure of the spark plug's ability to transfer heat from the insulator and electrodes to the cylinder head. Thermal mode is determined by the design of the insulator: the larger its contact area with the body and the shorter the heat removal path, the more heat is removed and the higher the glow number.

There is a direct relationship: the higher the number in the marking (for most European and Asian manufacturers), the “colder” the candle. Cold candles Designed for highly accelerated engines with a high compression ratio and temperature in the combustion chamber. They quickly release heat, preventing overheating. On the contrary, “hot” spark plugs with a low number retain heat for a long time, which is necessary for self-cleaning of carbon deposits in quiet driving conditions, but is dangerous under high loads.

⚠️ Attention: Installing a spark plug that is too hot (with a low number) in a powerful engine is guaranteed to lead to melting of the electrodes and burnout of the piston due to detonation.

Selecting the correct heat setting is critical, as the temperature in the spark plug's working area must be maintained between 400 and 900 degrees Celsius. Below 400 degrees, carbon deposits do not burn, causing misfires, and above 900 degrees, the process of spontaneous ignition of the mixture begins. Golden mean Ensures optimal engine performance and long component life.

Labeling systems from leading manufacturers

Different manufacturers use opposing coding systems, which often causes confusion among car enthusiasts. While for NGK and Denso an increase in the number means an increase in heat transfer capacity (colder plug), Bosch and Brisk the logic may differ depending on the series, although in the mass segment Bosch also switched to direct dependence in new catalogs, but historically used the reverse one. Understanding these differences is key to making the right choice.

Let's consider the main marking systems:

• 🔥 NGK: The number at the end or middle of the code indicates the heat value. The higher the number, the “colder” the candle. For example, BPR6ES (6) is warmer than BPR8ES (8).
• ❄️ Denso: Similar to NGK, an increase in number indicates more efficient heat dissipation. Plug K20PR-U (20) is cooler than K16PR-U (16).
• ⚙️ Bosch: In classic markings (for example, WR7DC+), the number 7 indicates the thermal mode. The rule also applies here: the larger the number, the colder the candle (in modern catalogs), however, in the old Soviet and some specific series, the logic could be inverted, so cross-tabulations are required.

When replacing spark plugs, it is important to pay attention not only to the numbers, but also to the letter designations, which indicate the thread size, skirt length and type of sparking. Error in selection geometric parameters with the correct heat number, it can lead to the fact that the spark plug simply does not screw in or, on the contrary, rests against the piston.

Heat number correspondence table

To simplify the selection of analogues, below is a table of correspondence between the heat numbers of the main manufacturers. This data allows you to find a replacement for the standard spark plug if the original is not available. Remember that the table is for reference only, and tuned engines require individual adjustment of the thermal regime.

Using this table, you can easily determine what a standard spark plug NGK BKR6E (number 6) correspond to analogues Denso K20PR-U (number 20) or Bosch FR8DC+ (number 8). However, if you operate the car in extreme conditions (racing, long-term driving at high speeds), it makes sense to choose an analogue that is one step cooler.

Diagnostics by the appearance of the spark plug

Visual inspection of unscrewed spark plugs is the most reliable way to determine whether the heat rating is correct. The color of soot and the condition of the electrodes will tell about the processes occurring inside the cylinder more accurately than many diagnostic devices. If a candle operates in abnormal thermal conditions, this will immediately affect its appearance.

The main signs of incorrect selection:

• 🔴 White or light gray insulator: indicates overheating. The plug is too hot for the engine or the mixture is too lean. Requires replacement with a cooler analogue.
• ⚫ Black velvety soot: indicates that the plug is too “cold” and does not reach self-cleaning temperature. It may also be a sign of a rich mixture. It is necessary to increase the heat rating (in the NGK/Denso system) or check the power system.
• 🟤 Brick brown shade: the ideal color, indicating the correct thermal conditions and the optimal composition of the mixture.

Particular attention should be paid to the presence of melting on the central electrode or side contact. This is a direct sign that glow ignition has already taken place. In such a situation, operation of the engine must be stopped immediately until the spark plugs are replaced with cooler ones, since further operation will lead to destruction of the piston group.

The influence of operating mode on the choice of spark plugs

Factory recommendations for heat rating are designed for average operating conditions. However, your driving style and vehicle use conditions may require adjustments. If you often make short trips around the city at low revs, the standard “cold” spark plugs may become overgrown with soot, and it makes sense to consider a slightly “hotter” option within the permissible range.

On the contrary, sporty driving, towing a trailer or driving on the motorway at high speeds require the installation of cooler spark plugs. Under such conditions, the heat flow into the combustion chamber increases significantly, and a standard spark plug may not be able to cope with heat removal. The use of spark plugs with a copper central electrode in such conditions is less effective than nickel or iridium analogues with improved heat dissipation.

⚠️ Attention: When switching to gas equipment (GBO), the combustion temperature of the propane-butane mixture is higher than that of gasoline. It is recommended to install spark plugs 1-2 units colder (on the NGK/Denso scale) than standard gasoline ones.

Checking the gap between the electrodes should also be carried out regularly, especially when the thermal regime changes. For colder spark plugs, the gap may require correction, since sparking conditions at high temperatures and pressure are different. Use a feeler gauge to fine tune to engine specifications.

Common mistakes during selection and installation

One of the common mistakes is trying to “improve” engine performance by installing spark plugs with an extremely low or high heat rating without changing the ECU settings. An engine is a balanced system, and a sudden change in thermal conditions can throw off the readings of the lambda probe, as unburned particles or overheating products change the composition of the exhaust gases.

Tightening torque is also often ignored. An under-tightened spark plug impairs heat dissipation through the threads, which actually makes it “hotter” and can lead to overheating even with the correct heat rating. An overtightened spark plug can damage the threads in the cylinder head or deform the O-ring, breaking the seal.

You should also not mix spark plugs from different manufacturers or with different service life in one engine. Differences in spark generation and heat transfer characteristics will lead to uneven cylinder operation, vibration and increased fuel consumption. Always change the complete set using products from the same batch and brand.

Final recommendations for choosing

Selecting spark plugs is a balance between engine requirements, driving style and fuel quality. The heat rating table serves as an excellent guide, but the final decision should be based on a comprehensive assessment of the condition of the motor. Do not chase exotic models if the standard options completely cope with the task in your operating mode.

Regular replacement of spark plugs (every 20-30 thousand km for regular ones and up to 100 thousand for iridium ones) will eliminate problems with starting and fuel consumption. Keep an eye on the color of the soot, and it will become your best diagnostician, signaling problems long before serious breakdowns occur.