If you ever have to replace spark plugs outside of routine maintenance, keep the following in mind.
Spark plugs and the entire ignition system as a whole are selected for this engine, and this helps to reduce the amount of harmful substances in the exhaust gases. To avoid engine failures, reduced power, poor throttle response, increased content of harmful substances in the exhaust gases and interference with radio reception, only spark plugs recommended by Volkswagen for this engine should be used. Particularly important characteristics of the spark plugs are the number of electrodes, the glow number and, in some cases, the ability not to interfere with radio reception.
There are two types of seals when installed in the cylinder head: flat seat and elastic (steel or copper-asbestos) with a gasket or with a conical seat without a gasket. When installing a new spark plug, first of all, it is necessary to check what kind of seal is provided in the cylinder head, as well as the diameter and length of the threaded part. If you install a spark plug with a long threaded part in a head with a short thread, then the protruding part of the spark plug will be covered with carbon deposits and resins, lead deposits. The spark plug will overheat, and the threads may melt. When unscrewing the spark plug, this can damage the threads in the cylinder head. With a short threaded part of the spark plug and a long one in the head, carbon deposits and various deposits clog the threads, and they melt. After prolonged work, you have to go through the threaded part in the cylinder head with a tap, but so that dirt does not remain in the cylinder.
Depending on their purpose, spark plugs have a number of design features. Thus, for cars and motorcycles operating in difficult road conditions, when water may get on the engine, and with increased requirements for the level of radio interference, shielded spark plugs with a hermetic seal of the high-voltage wire are used. In order to increase the stability of the spark plugs, several electrodes are sometimes made on the body or there are no side electrodes at all (with floating electrode).
Depending on the specifics of the working process, spark plugs are made with an insulator recessed inside the housing or with a protruding insulator (for combustion chambers with intensive turbulence of the mixture in the combustion chamber).
The most important parameter characterizing a spark plug is the temperature regime of its elements, primarily the central electrode and the surface of the insulator skirt.
The main problem in creating a spark plug for a high-speed engine is the fulfillment of contradictory requirements. On the one hand, it is necessary to ensure a sufficiently high temperature of the insulator skirt to prevent the formation of carbon deposits on it during long-term operation at low loads and idling, which leads to shunting, especially at low temperatures. When switching to high loads at high temperatures, the spark plug insulator self-cleans due to the burning of deposits on it.
On the other hand, at high speeds and heavy loads, the temperature of the central electrode and the lower part of the insulator should not exceed the upper limit at which glow ignition occurs (ignition of the mixture before a spark from the ignition system occurs). From cycle to cycle, ignition occurs earlier and earlier, the temperature of the parts increases until something burns out - at best, the spark plug, at worst, the piston. So a small spark plug can cause big trouble.
How can you prevent the possibility of pre-ignition? First of all, you need to ensure proper (according to the instructions) setting the ignition advance angles, without allowing too early ignition. With a contact system, the ignition advance angle may increase unexpectedly, for example, if one of the soldered contacts of the breaker bounces off.
The second condition is to always fill with gasoline corresponding to the engine boost level (compression ratio, nominal speed, filling, etc.). The lower the octane number, the greater the tendency to pre-ignition. But if low-octane gasoline is poured in, then you cannot go to high loads at high RPM.
The third condition, sometimes the most important, is to install spark plugs with the required heat rating in accordance with the engine manufacturer's recommendation.
The most convenient designation of the heat rating, at one time adopted by most European companies, is (in seconds), after which, during testing on a special installation, glow ignition begins. In recent years, most companies have switched to a conventional designation of the glow number of spark plugs.
The original text is available on the website VWMANUAL.ru
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