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Crying foul

It doesn't seem that long ago when all that was needed to fire up your racer was to change one set of spark plugs for some 'hotter' ones, once the engine was warmed up and ready to go. The procedure used to be commonplace, and while the one set was rather less than cherished, the racing plugs were treated with respect, cleaned carefully and replaced in their little box ready for the next event. The evolution of the spark plug may have moved on, and many exotic (and some not so exotic) materials used, but the essential fact remains that in order to get the best out of your ignition system, the precise design and heat range needs to be established.

The necessity of changing plugs in those days was dictated by the somewhat narrower heat ranges of the plugs on offer. Attempts at firing from cold on the racing plugs was never attempted, for it was certain to foul the plugs through excessive oiling while cold or through excessively rich mixtures. The ability to change plugs in an instant and diagnose misfiring cylinders quickly therefore became second nature.

Racers these days may have never had it so good with the advent of the wide heat-range plug but even so, the ability of any engine builder to select the correct plug for the task is still a skill worth having.

Essentially it's all down to the design of the plug, and its ability to retain the heat of combustion at low speed and dissipate it as the speed rises. If you like, a spark plug is a compromise: if it dissipates heat too easily - which, paradoxically, we call 'hot' or cold-running plugs - then combustion deposits will build up on the electrodes, causing the plug to 'shunt' or lose the power of its spark. If it doesn't dissipate the heat fast enough (a 'cold', or hot-running plug) then the electrode may overheat and introduce pre-ignition to the combustion mixture, resulting in reduced performance, electrode wear and possibly even engine damage. The trick therefore for any engine tuner is to keep the spark plug temperature in the 500-900 C range or thereabouts throughout.

Given thermocoupled spark plugs and suitable instrumentation, determining the correct temperature range is easy enough, but a skilled engine tuner can actually 'read' the plug and extract so much more than just its temperature. For instance, looking at the annealing zone on the ground electrode can give not only an indication of the heat in the plug at the time, but viewing the condition of the porcelain centre cover can also indicate if the ignition timing is correct. Running what is generally known as a 'plug cut' - when the engine is switched off at wide-open throttle - little black specks can indicate a plug that's slightly too hot or an engine consuming oil. On the other hand, silver specks on the porcelain can indicate the onset of detonation, something that is often difficult to detect for the inexperienced. Retarding the engine slightly and checking the ground electrode for heat range might seem to be a good idea at this point.

But 'plug cuts' are also well known for reading the mixture. With modern, cleaner-burning race fuels, however, this is more difficult than it once was, and the shades of grey or brown on new plugs that will need to be used have to be carefully interpreted against previous experience.

There's more to a spark plug than you might at first think.

Written by John Coxon

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