The heat rating index indicates the degree of heat load of the spark plug. The lower the heat rating index, the higher the heat load. Therefore, the spark plug can better dissipate heat, thereby preventing harmful pre-ignition (engine detonation). True, a spark plug with a high thermal load has the disadvantage that its self-cleaning temperature is also higher. Therefore, it becomes covered with soot faster, especially if the engine does not reach its operating temperature while driving (city traffic, short distance driving in winter). As a rule, "cold" spark plugs (heat rating index starting from 06) are used in "hot" engines, from the power units of which high power is required.
When using copper cores (Cu) in the central electrodes of spark plugs, and especially when using silver central electrodes, the thermal conductivity and, therefore, the thermal load increases.
The heat range index is contained in the spark plug code, which is deciphered as follows:
Spark plugs from Bosch
Example:
| F | R | 8 | D | WITH | 0 |
| 1 | 2 | 3 | 4 | 5 | 6 |
(1) W = thread M 14 x 1.25 with flat tight fit, SW 21 (spanner size); F = thread M 14 x 1.25 with flat tight fit, SW 16 (spanner size); H = thread M 16 x 1.25 with conical tight fit, SW 16 (spanner size).
(2) R = with a resistor for suppressing interference. Thanks to this, the operation of the ignition system does not cause radio interference.
(3) Heat index. The heat index scale includes numbers from 06 – "cold" to 13 – "warm". Index 8 corresponds to the old heat index of 175 (earlier marking), 7 – 200, 6 – 225, etc.
(4) B = length of cut part 19 mm, normal spark discharge path; D = length of cut part 19 mm, extended trajectory of spark discharge; L = 19mm threaded length, extremely elongated spark discharge path. Threaded length information applies to flat tight fit spark plugs. Supplement with "T" (e.g. "DT") = 3 ground electrodes.
(5) Central Electrode Material: No indication = Cu-Ni alloy, C = Ni-Cu combined central electrode, S = silver central electrode, P = platinum central electrode.
(6) O = Standard spark plug with reinforced center electrode.
Spark plugs from Beru
Example:
| 14 | F | 8 | D | AND | 0 |
| 1 | 2 | 3 | 4 | 5 | 6 |
(1) Thread diameter in mm, here: M 14 x 1.25.
(2) F = Flat Push Fit, K = Tapered Push Fit, R = 5kOhm/10kOhm Noise Suppression Resistor.
(3) Heat rating index, decoding, as with Bosch.
Publication taken from an online resource (MercedesMan.ru)
(4) Length of the cut part, decoding, as with the Bosch company.
(5) Central electrode material: U ="ultra" (Ni-Cu - combined central electrode), S = silver central electrode, P = platinum central electrode.
(6) O = reinforced central electrode; R = with protective resistor (to reduce the burning of spark plug electrodes).
Heat ratings of spark plugs for engines used in the C-class
| Spark plugs of the following companies | ||
| Bosch | Beru | Champion |
| F8 DCO | 14 F-8 DU0 | C 10 YCC |
The distance between the electrodes in spark plugs is 0.8 mm.
Attention! Technical development does not stand still. Therefore, it is possible that other spark plug heat ratings will be suitable for individual new engines. Therefore, it is recommended to check which spark plugs are installed or which spark plugs are prescribed according to the service manual before purchasing new spark plugs. In case of doubt, ask specialists about the actual spark plug heat ratings.