If it is ever necessary to replace spark plugs outside of scheduled maintenance, the following must be considered.
Spark plugs and the entire ignition system as a whole are matched to this engine, and this helps to reduce the amount of harmful substances in the exhaust gases. To avoid engine failures, reduction of its power, deterioration of acceleration, increase in the content of harmful substances in exhaust gases and interference with radio reception, only spark plugs recommended by Volkswagen for this engine should be used. Particularly important characteristics of candles are the number of electrodes, 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 seated and resilient (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 which seal is provided in the cylinder head, as well as the diameter and length of the threaded part. If you install a candle with a long threaded part in a head with a short thread, then the protruding part of the candle will be covered with carbon deposits and resins, lead deposits. The candle will overheat, possibly melting the threads. Turning the spark plug out can damage the threads in the cylinder head. With a short threaded part of the candle and a long one in the head, carbon deposits and various deposits clog the thread, it melts. After a long work, you have to go through the threaded part in the cylinder head with a tap, but so that the dirt does not remain in the cylinder.
Depending on the purpose, candles have a number of design features. So, for cars and motorcycles operating in difficult road conditions, when water can get on the engine, and with increased requirements for the level of radio interference, shielded candles with a hermetically sealed high-voltage wire are used. In order to increase the stability of the candles, several electrodes are sometimes made on the body or there are no side electrodes at all (with floating electrode).
Depending on the features of the working process, candles are made with an insulator recessed inside the body or with a protruding insulator (for combustion chambers with intensive turbolization of the mixture in the combustion chamber).
The most important parameter characterizing the candle is the temperature regime of its elements, first of all, 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 conflicting requirements. On the one hand, it is necessary to ensure a sufficiently high temperature of the insulator skirt in order to prevent the formation of soot on it during long-term operation at low loads and idling, leading to shunting, especially at low temperatures. When switching to high loads at high temperatures, the spark plug insulator self-cleansing due to the burnout of deposits on it.
On the other hand, at high speeds and high load, 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 even before a spark from the ignition system occurs). From cycle to cycle, ignition occurs earlier and earlier, the temperature of the parts rises until something burns out - at best a candle, at worst a piston. So because of a small candle there can be big troubles.
How to prevent the possibility of the occurrence of incandescent ignition? First of all, you need to follow the correct (according to instructions) setting the ignition timing, preventing too early ignition. With a contact system, the ignition timing may suddenly increase, for example, if one of the soldered breaker contacts bounces.
The second condition is to always fill in gasoline corresponding to the level of engine boost (compression ratio, rated speed, filling, etc.). The lower the octane number, the greater the tendency to glow ignition. Well, if you still fill in low-octane gasoline, then you can’t go to high loads at high speeds.
The third condition, sometimes the most important thing, is to install candles with the required glow number in accordance with the recommendation of the engine manufacturer.
The most convenient designation of the glow number, at one time adopted by most European firms, in time (in seconds), after which, during tests on a special installation, glow ignition begins. In recent years, most firms have switched to the symbol for the glow number of candles.
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