Pre-ignition means that the mixture is set on fire before the spark occurs, being ignited by a red hot spot in the combustion chamber such as an incandescent particle of carbon; a thin piece of protruding metal; an overheated spark plug, or a bight red hot exhaust valve. The result is reduction of power and overheating

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Source: http://www.classicar.com/index.php?option=com_content&view=article&id=36:pre-ignition&catid=17:engine-tech&Itemid=23