Spark plugs (VW Golf 2)

The spark plug consists of a central electrode, an insulator with a housing and an outer electrode. Between these electrodes runs the ignition spark, which ignites the fuel mixture. The spark plug is responsible for starting the engine, idling, accelerating and maximum speed. For this reason, you cannot simply change the type of spark plugs installed by the manufacturer. The type of spark plugs is determined by their thermal value. This figure determines the degree of thermal load of the spark plug in the engine under certain operating conditions. Spark plugs for each engine are selected so that they reach, if possible, a self-cleaning temperature between 400°C and 800°C in all areas of rotation speed.

The higher the number that determines the thermal value of the spark plug, the higher its resistance to glow ignition and the lower its resistance to contamination.

The lower the figure defining the thermal value of the spark plug, the lower their resistance to glow ignition and the higher their resistance to contamination. With a high thermal value, the spark plug conducts heat better, and with a low thermal value, it conducts heat much worse. Using spark plugs with:

a) very high thermal value are the reason for the appearance of sediment on them. This is the result of the fact that the spark plugs do not reach the self-cleaning temperature, since too much heat passes through them to the walls of the head, i.e. the spark plug is very cold.
b) very low gross value is the cause of spontaneous ignition This is the result of the spark plug reaching a temperature well above 850°C, as very little heat passes through it to the walls of the head, i.e. the spark plug is very hot.

To give you an idea of the temperatures we are talking about, the temperatures surrounding the spark plugs while the engine is running are given below.

End of intake stroke 60°C
Ignition 350°C
Maximum values 3000°C
End of working stroke 1100°C

During idling or at partial engine loads (for example, stopping at an intersection or driving in a traffic jam). The spark plug temperature drops even below 350°C. Therefore, after working in such conditions, the spark plug is covered with black sediment. The more oil gets under the piston, the more of it.

The number that determines the thermal value is contained in the spark plug code.

Bosch type spark plug

Example: WR 7 D C R (1) (2) (3) (4) (5) (6)

(1)

W = thread M 14x1.25, flat fit, for key 21
F = thread M 14x1.25, flat seat, for key 16
M = thread M 18x1.5, flat fit, for key 25
I = thread M 14x1.25, conical fit, for key 21

(2)

R = with anti-interference resistor.

(3)
A number that defines the thermal value. The thermal value scale is from 06 ("cold") to 13 ("warm"). The number 7 corresponds to the old thermal value of 175 (old designation), a: 6-20 5-225 etc.

(4)