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spark plugs

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he heat range

The heat range of a spark plug is a subject by itself, where the single words are easily mistaken. The selection of a plug with a matching heat range must always be seen as a matching to the actual thermal conditions that occur during continuous operation. The different properties of the engines with respect to load, work principle, compression ratio, speed, cooling and fuel make it impossible to get along with a "one type fits all plug". Best spark plugs for our application in rc-cars are e.g.: Champion RZ7C or NGR CMR7A
The same spark plug which would heat up strongly in one engine would stay at a relatively low medium temperature in another one. In the first scenario the mixture would ignite uncontrolled at hot parts of the plug (surface ignition), the other case would be the insulator quickly fouled so much that misfiring occurs.

Below about 500 degrees Celsius carbonaceous combustion residues will deposit onto the insulator and create a shunt. The ignition voltage then takes the easiest way via this layer of soot and will be grounded to the spark plug thread without generating a spark.

Because these residues are easily burned at high temperatures the plug should reach a temperature above 500 degrees Celsius as soon as.
By different designs with corresponding heat ranges, the manufacturers try to keep the temperature at the insulator nose always within the range of 500 to 850 degrees Celsius. The range from 850 to about 1000 degrees is a safety area where there is still no surface ignition; electrode wear however will increase rapidly. Only when the temperature rises even higher the dreaded surface ignition will start.
A relatively "hot" pug takes up a lot of heat by design, and derives little heat to the cylinder head. So it quickly reaches the self-cleaning temperature in an engine that operates at a moderate heat level, a "tame" low power engine.
A so-called "cold" plug takes up very little heat and even directs much of it straight to the cylinder head. This makes it suitable for highly modified engines that tend to run hot.

You should definitely always follow the engine manufacturer's recommendations for the spark plugs. Exceptions may be very high ambient temperatures or modified engines. When trying different heat ranges, always keep a look at the plug's face (described in next section) to read the spark plug conditions.
This picture shows the design of different heat ranges.

Fig.1 spark plug with high heat range ("hot plug"). Large insulator nose surface absorbs much heat. Low heat dissipation.

Fig.2 spark plug medium heat range. Insulator nose surface smaller than with "hot plug". Less heat absorption. Better heat dissipation.

Fig.3 spark plug low heat range ("cold spark"). Small insulator nose surface absorbs little heat. Heat dissipation is very good.
The spark plug appearance

Spark plug appearance provides information about the operating performance of the engine and spark plug. The appearance of electrodes and insulators of the spark plug is evidence of the operating performance of the spark plug and to the mixture composition and the combustion process of the engine.
Judging spark plug condition is an essential part of engine diagnosis. A reliable statement, however, is subject to the following important condition: Before the spark plug faces are evaluated, the car must have been driven. A previous prolonged idling, particularly when the engine is started cold, can cause soot and cause false reading of the spark plug appearance. The car should be run over a distance of about five laps. This engine should be operated with varying speeds. A longer idling before turning off the engine must be avoided.
These pictures show the different spark plug appearances.

Fig. 1: Blackened or oily: insulator, electrode and plug body are covered with a velvety-black, oily coating. Spark plug did not reach the necessary self-cleaning temperature (500 to 850 ° C). Reason: Gas mixture (carb setting) too rich, dirty air filter or wrong heat range. Results: shunt firing, poor cold start characteristics. Remedy: Adjust mixture leaner, clean air filter, if necessary, clean or replace spark plugs.

Fig. 2: Light grey electrode. Cause: Thermal overload (for example by sucking in false air), spark plug of incorrect heat range or lean mixture. Results: misfiring, power loss, engine failure. Remedy: adjust mixture more towards rich side; where appropriate install new plug with the correct heat range.

Fig. 3: Normal: insulator nose with brown to greyish tan color. The ideal case, i.e. the engine, ignition and heat range of the plug are completely in order. The slight crust on the ground electrode is meaningless.
Spark plug installation

When installing the spark plug into the engine, note the following: The contact surface on the spark plug seat and engine must be clean. Today's spark plugs do not need graphite or graphite lubricant on the thread. They are treated with an anti-seize lubricant. A seizure is not possible since the threads are nickel plated.

When tightening the spark plug the hex transfers the torque to sealing seat and thread. If by excessive torque or tilting the spark plug wrench the spark plug housing is tweaked, the insulator may come loose. Therefore, the torque may not exceed the value specified for plug and engine.
It is therefore appropriate to go with the following rules of thumb: insert spark plug into the cleaned thread and hand tighten until the gasket meets the cylinder head, and then put on the spark plug wrench.
Tighten new spark plug after the first rotation inhibition by a quarter turn. Already used spark plugs should only be tightened to the equivalent of about 5 minutes on a clock or an angle of about 30°.

When tightening or removing the spark plug the socket wrench must be held straight, otherwise the insulator is usually pushed away or pushed aside, and the spark plug becomes useless.
Over-tightening a spark plug may damage the thread in the cylinder head, a problem especially with aluminum heads; when the plug is not tightened enough heat conductances may be inadequate.

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