PREIGNITION is the uncontrolled firing of the fuel-air charge before the normal spark. Potential preignition causes include a cracked spark plug, carbon or lead deposits in the combustion chamber, or a burned exhaust valve. Basically anything that can cause the fuel-air mixture to ignite prematurely. DETONATION occurs in reciprocating engines when the fuel-air mixture explodes instead of burning. This is commonly caused by using a lower-than-recommended fuel grade. Detonation is most likely to o

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