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By Randy Tragni

Chapman Report - October 1986


Detonation is a condition in which the charge (mixture) is ignited by compression and rapid chemical reaction. This results in pressure-waves which vibrate the cylinder walls causing the characteristic clatter sound familiar to diesel engine owners.


An engine suffering from detonation experiences a non-uniform flame front. Within the combustion chamber this causes components in the power stroke to experience forces which are not present in normal combustion. This combines with a higher internal temperature and causes severe component failure.


Detonation is not the same as pre-ignition though it can be a cause of it. Pre-ignition is caused by "hot spots" in the combustion chamber which ignite the charge prior to the spark. These are usually caused by carbon deposits from rich combustion processes, but can be caused by failing components due to material melting. This is the result of severe detonation over some period of time.


There are several factors which can contribute to the engines susceptibility to detonation. Some common ones are:


The Combustion Chamber; Hemispherical heads are one of the least susceptible designs. In general designs with squelch areas are more likely and flat head the worst.


Spark Plugs; the hotter the plug the more prone to detonation. It is also worth noting that multi-spark plugged heads are marginally better.


Spark Timing; the more advanced the timing the greater susceptibility to detonation. By retarding the spark it is possible to stop detonation, but at the cost of efficiency.


Octane; this reaction is directly proportional to the compression ratio of the engine. Ideally, one could increase the octane until detonation ceased, though this can not be accomplished easily in California. Many club members suggest either octane booster (some are much better than others) and/or mixing premium unleaded with leaded regular.


Inlet Temperature; Cooling the gas entering the combustion chamber reduces the tendency to detonate. This has a similar effect as a lean charge mixture thus increasing the reaction time.


Atmospheric Humidity; increasing the humidity can have advantageous effects against the engines susceptibility to detonate. First, increased humidity raises the charge density giving a less volatile mixture thus increasing the reaction time. Secondly, the increased humidity tends to cool the combustion chamber during the intake stroke lowering the internal temperatures. The easiest way to have a controlled increase in humidity is by use of a vapor injection system. There are two brands which I know of, Edelbrock and Spearco. I have recently installed the Spearco system on my Weber head TC. Installation takes about three hours and is very straight forward. It is worth noting that J.C. Whitney sells the Spearco system for a mere $47, less than half what you would pay locally.