The glow number index indicates the degree of heat load of the spark plug. The lower the heat index, the higher the heat load. Therefore, the spark plug can better dissipate heat, thereby preventing harmful pre-ignition (engine knock). However, a candle with a high heat 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 during driving (traffic in the city, driving short distances in winter). Usually, "cold" spark plug (glow index starting from 06) applied 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 even more so when using silver central electrodes, the thermal conductivity and, thereby, the thermal load increase.
The glow index is contained in the spark plug code, which is deciphered as follows:
Bosch spark plugs
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 (turnkey size); F = thread M 14 x 1.25 flat tight fit, SW 16 (turnkey size); H = thread M 16 x 1.25 tapered tight fit, SW 16 (turnkey size).
(2) R = with noise suppression resistor. Due to this, the functioning of the ignition system does not cause radio interference.
(3) Heat index. The scale of glow numbers includes numbers from 06 - "cold" up to 13 - "warm". In this case, the index 8 corresponds to the old incandescent number 175 (earlier marking), 7 - 200, 6 - 225, etc.
(4) B = 19 mm cut length, normal spark trajectory; D = 19 mm cut length, stretched spark discharge path; L = 19 mm cut length, extremely extended spark discharge path. The threaded length information refers to spark plugs with a flat tight fit. Letter Complement "T" (For example, "DT") = 3 side electrodes.
(5) Center electrode material: not specified = Cu-Ni alloy, C = Ni-Cu combined center electrode, S = silver center electrode, P = platinum center electrode.
(6) O = standard spark plug with reinforced center electrode.
Beru spark plugs
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 tight fit, K = tapered tight fit, R = 5 kΩ / 10 kΩ noise suppression resistor.
(3) Index of glow number, decoding, like that of Bosch.
(4) The length of the cut part, decoding, as in Bosch.
(5) Center electrode material: U="ultra" (Ni-Cu - combined center electrode), S = silver center electrode, P = platinum center electrode.
(6) O = reinforced center electrode; R = with protective resistor (to reduce the burning of spark plug electrodes).
Heat numbers of spark plugs for engines used in the C-class
Company spark plugs | ||
Bosch | Beru | Champion |
F8 DCO | 14 F-8 DU0 | C 10 YCC |
The distance between the electrodes in the 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 certain new engines. In view of this, it is recommended that before buying new spark plugs, check which spark plugs are installed or which spark plugs are prescribed in accordance with the service manual. In case of doubt, ask the experts for the actual glow ratings of the spark plugs.