Detonation is a chemical rate-of-reaction phenomenon that results in instantaneous, rather than progressive, oxidation of the fuel-air mixture (the "charge"). Detonation causes cylinder gas temperatures and pressures to greatly exceed normal levels. Pistons, bearings, connecting rods, crankshafts, head gaskets and valve seats are tortured by detonation. Detonation will occur at excessive values or combinations of the following:

• static compression ratio
• inlet pressure or boost
• advanced ignition timing
• lean mixture
• low grade fuel

The first of these five causes of detonation is excessive static compression ratio. This ratio determines the final (compressed) pressure and temperature of the charge. Near the end of the compression stroke, the charge is ignited by the spark. Cylinder pressures and temperatures are increased to effectively drive the piston downward, applying torque to the crankshaft. The proper combustion event is characterized by a flame front progressing through the unburned charge to oxidize the fuel at a finite rate.

If the unburned charge attains high enough pressure and temperature (termed the autoignition temperature) then oxidation of the charge will occur at an infinite rate. This is descriptive of an instantaneous combustion event and is termed detonation. Detonation in an internal combustion engine can produce instantaneous pressures in excess of 3000psi, in contrast to the normal operating peak pressure of about 800psi. Shock waves are generated and gas temperatures increase at a nearly infinite rate. The combustion chamber, valve faces, piston face and cylinder walls are considerably cooler than the cylinder gas temperature; hence the gas transfers heat to these members at a high rate. This heat transfer to the surrounding surfaces cools the gas, which causes it to contract and decrease in pressure. The thermal energy of the gas is irreversibly lost through heat transfer to the surrounding surfaces. Shock waves, impinging on the surrounding surfaces, lose much of the kinetic energy of the gas to those surfaces. The thermal and kinetic energy of the gas is lost to irreversibilities including heat transfer and sound generation. Engine performance will suffer from detonation. The fuel's chemical potential energy is released to the cylinder gas, which in turn, irreversibly loses the energy to the surroundings. There is little of the potential energy left over to drive the piston down. The detonation, shock wave generation and abatement and heat transfer occur over only a few degrees of crankshaft revolution; the depleted cylinder gas has no more energy to convert to work.

Detonation is detrimental to engine performance, but it is disastrous to engine components. High pressures can severely overload the structural members of the engine, including the piston, connecting rod, crankshaft, bearings, cylinder head and fasteners. High heat transfer rates will cause localized component temperatures, which exceed the melting, or even vaporization temperature. Aluminum specks on the insulator of the spark plug evidence mild detonation. The aluminum piston underwent surface vaporization at the face, the vaporized aluminum condensed and solidified on all internal engine surfaces. Since the spark plug insulator is white, the gray aluminum may be easily recognized. Severe detonation will erode a hole in the piston, beat the bearings out of the crankshaft and connecting rods and break components from structural overload.

Recently, detonation has been controlled quite successfully with oxygen sensors and knock sensors. Recently manufactured, well-tuned passenger cars and light trucks seldom suffer from detonation. But a large market segment is not presently served: high performance and racing applications.

Many drag racing applications do not employ oxygen and knock sensors because the technology was not built into the selected powerplant. Aftermarket high-performance accessories can expose a critical weakness of another system which is not capable of preventing detonation. An example is the installation of an aftermarket turbo- or super-charger. The increased air mass entering the cylinder during the intake stroke must be accompanied by proportionally more fuel. If the fuel system is incapable of providing adequate additional fuel, the engine will detonate. During high performance engine tuning, the engine variables are adjusted to produce maximum power. This performance maximum occurs just at the verge of detonation. Maximizing all of the first four of the five engine parameters, listed above, will result in maximum power. Exceeding the limit of any one of the above, or excessive combinations thereof, will result in detonation.

The high performance engine tuner must juggle the engine parameters to produce acceptable power, with a margin of error to the onset of detonation. Such sources of error could include: drop in ambient temperature, drop in ambient humidity, engine overheating or "hot-spotting", lack of fuel flow (e.g., fuel line restriction, undersized or poor performing fuel pump, exposed fuel pick-up), poor quality fuel, etc. During testing or racing conditions detonation is likely to occur. Sadly, under these conditions, engines are often destroyed in the blink of an eye. Once the engine is running and large amounts of power are generated, the tuner/operator has limited control over detonation, short of shutting down the engine, this presumes the detonation was audible and detected. This will help to limit the amount of damage done by detonation. Upon subsequent, mandatory engine dissassembly and inspection, the locations and causes of detonation may be investigated and a potential cause identified. This will be accompanied by parts replacement.

Don't take our word for the causes and consequences of detonation. To the left are links to authoritative websites which provide different explanations and viewpoints, but mostly the same conclusions. Detonation will destroy your engine, the more highly tuned your engine is, the more it will cost you in damages, and the quicker you will find out about the effects of detonation.