The value of the glow number indicates the degree of thermal load on the spark plug at certain operating loads. The lower the glow number of a candle, the higher its ability to withstand thermal loads. Such a candle removes heat better, preventing detonation combustion. A candle with a high degree of heat resistance has the disadvantage that it needs a high self-cleaning temperature. Therefore, such candles are covered with soot faster, especially in conditions when the engine does not have time to reach its operating temperature while driving (driving in urban areas, short distances in winter). Usually, «cold» spark plug (with glow number from 06) installed on «hot» engines that require significant output and are intended for operation with high loads.
The increase in thermal conductivity and heat resistance is achieved due to the copper core in the central electrode. The highest indicators of heat resistance are achieved due to the core of the central electrode, made of silver.
The heat number is indicated in the designation of the candle. The designation of the candle is deciphered as follows:
Example: Bosch spark plugs
(1) W = M14x1.25 thread with flat seal, SW 21.
F = thread M14x1.25 with flat seal, wrench size 16.
M = thread M18x1.25 with flat seal, wrench size 25.
H = thread M14x1.25 with conical seal, SW 16.
D = thread M18x1.25 with conical seal, SW 21.
(2) R = with noise suppression resistor. This does not affect the operation of the ignition system.
(3) Heat number. We offer candles with a glow number from 06 («cold») up to 13 («warm»). In this case, the number 7 corresponds to the previous value of the glow number, equal to 150 175, 6 - 200.5 - 225, etc.
(4) A - thread length 12.7 mm, normal spark discharge trajectory.
B = 12.7 mm thread length, extended spark path.
C = thread length 19 mm, normal spark discharge path.
D = thread length 19 mm, extended spark path.
(5) = material from which the center electrode is made. Without indication - Cr-Ni (chromium nickel alloy), C = Ni-Cu cast copper-nickel center electrode, S = silver center electrode, P = platinum center electrode.
(6) 0 = standard spark plug with reinforced center electrode
(7) R = burn resistance 1 kΩ