The spark plug’s job is simple- to ignite the fuel/air charge in the cylinder by producing a spark. The timing and duration of the spark is controlled by the rest of the ignition system (be it a magneto, a mechanical points/distributor setup or electronically-controlled coil packs). None the less the design and condition of the spark plug has a big effect on the efficiency and performance of the engine.
Traditional spark plugs consist of a copper core surrounded by a ceramic insulator. The core is connected to a steel terminal at the top of the plug, where the high tension lead from the rest of the ignition system attaches, and ends at an exposed steel ‘primary’ electrode. The lower part of the ceramic insulator is surrounded by a steel body with a thread so the plug can be fitted to the engine’s cylinder head. The body also includes a second ‘earthing’ electrode- a small finger of metal positioned under the primary electrode.
When the electrical charge is fed to the spark plug it travels down the copper core, being prevented from running to earth by the ceramic insulator. The only way for the charge to reach earth is to jump across the gap between the primary electrode and the earthing electrode. The charge creates an electrical arc (the spark) between the electrodes as it jumps the gap, which ignites the fuel/air mixture in the cylinder.
That is the essence of a spark plug and what it does, but there are many variables at work. Setting and maintaining the gap between the two electrodes is crucial to producing a healthy spark. The needs of modern ignition systems mean that many spark plugs are manufactured with gaps much narrower than those specified for classic cars with traditional points-and-condenser systems, even if the plugs themselves are suitable. The gap should be checked with feeler gauges or a dedicated plug gauge and adjusted to the specified amount.
By altering the amount of the insulator that is in contact with the steel body, the cooling properties of the plug can be matched to different requirements. Regardless of the type of vehicle they are fitted to all spark plugs operate most effectively in a temperature range of between 500 and 700 deg. C. This is hot enough for the plug to burn off any by-products of combustion or oil deposits but not hot enough that the plug is damaged.
The terminology of spark plug temperatures is a little misleading. A ‘cold’ spark plug is one intended for use in engines with high combustion temperatures- it is a cold plug because it has greater self-cooling properties and can maintain its optimum temperature even as engine internal temperatures rise. Alternatively a ‘hot’ plug can be used on under-stressed engines as they work better in cold starting conditions and when run under partial load.
Modern petrol generally produces higher combustion temperatures than the fuel available when a classic car was new, so many cars will benefit from having a slightly colder spark plug fitted than that specified. Fitting a plug that is too cold will do no harm to the engine but will reduce performance and have a tendency to foul. A plug that is too hot will suffer rapid electrode erosion through overheating and may promote dangerous pre-ignition or pinking which can lead to internal engine damage.
Because they are easily removable and are the centre of the combustion process spark plugs make a useful ‘window’ into what is going on inside your engine. A spark plug of the correct temperature rating in an engine with the correct mixture will show pale brown/grey deposits on the insulator and the underside of the earthing electrode, but the actual electrode faces will be clean. Black, sooty deposits point to an engine that is running rich or a spark plug that is too cold. A very clean plug with blistering on the ceramic parts and eroded electrodes show that the plug is running too hot- either the mixture is lean or the plug is of the wrong temperature rating.