All cars have an ignition coil(s), triggered either electronically or by a traditional points distributor. When triggered, the coil sends a burst of high voltage electricity down the plug lead to the spark plug. This arcs (jumps through the air) from the centre electrode to the outer electrode, which is earthed to the engine block. The spark created by this arcing electricity is what ignites the air/fuel mixture. The spark voltage required to jump across the electrode gap is influenced by: plug gap and the shape of the electrodes. If your ignition system produces inadequate voltage to make a spark, a misfire will occur.There are 3 main types of plugs: steel, copper/steel and platinum. Each of these metal types has a different "arc over" voltage. Plugs with a lower arc over voltage will fire more easily than those with a higher arc over voltage, but at the same time they will generally have lower spark energy so they don't give as much power.Platinum plugs have a lower arc over voltage than steel or copper/steel plugs, and generally last longer.
Plugs are designed to operate with a certain amount of residual temperature (centre electrodes insulation), known as the heat range. On hotter plugs the insulation sticks out from the plug whilst on colder plugs the electrodes are recessed right up inside the plug so the hot insulation won't pre-ignite the air/fuel mixture.
Cold plugs cause less problems with pre-ignition, but also foul up easily, whilst hotter plugs don't foul up as easily, but are not suited to highly tuned engines.
