Engine detonation

What is detonation?
Detonation is an erratic form of combustion that can cause head gasket failure as well as other engine damage. Detonation occurs when excessive heat and pressure in the combustion chamber cause the air/fuel mixture to autoignite. This produces multiple flame fronts within the combustion chamber instead of a single flame kernel. When these multiple flames collide, they do so with explosive force that produces a sudden rise in cylinder pressure accompanied by a sharp metallic pinging or knocking noise. The hammer-like shock waves created by detonation subject the head gasket, piston, rings, spark plug and rod bearings to severe overloading.

Different ways to prevent detonation

Try a higher octane fuel.
The octane rating of a given grade of gasoline is a measure of its detonation resistance. The higher the octane number, the better able the fuel is to resist detonation. Most engines in good condition will run fine on regular grade 87 octane fuel. But engines with high compression ratios (over 9:1), turbochargers, superchargers, or with accumulated carbon deposits in the combustion chamber may require 89 or higher octane fuel.

Check for loss of EGR.
The Exhaust Gas Recirculation (EGR) system is one of the engine's primary emission controls. Its purpose is to reduce oxides of nitrogen (NOX) pollution in the exhaust. It does this by "leaking" (recirculating) small amounts of exhaust into the intake manifold through the EGR valve. Though the gases are hot, they actually have a cooling effect on combustion temperatures by diluting the air/fuel mixture slightly. Lowering the combustion temperature reduces the formation of NOX as well as the octane requirements of the engine.

Keep compression within reasonable limits.
A static compression ratio of 9:1 is usually the recommended limit for most naturally aspirated street engines (though some newer engines with knock sensors can handle higher compression ratios).
Compression ratios over 10.5:1 may create a detonation problem even with 93 octane premium gasoline.

Check for over-advanced ignition timing.
Too much spark advance can cause cylinder pressures to rise too rapidly. If resetting the timing to stock specifications doesn't help, retarding the timing a couple of degrees and/or recalibrating the distributor advance curve may be necessary to keep detonation under control.

Check for a defective knock sensor.
Many late model engines have a "knock sensor" on the engine that responds to the frequency vibrations characteristically produced by detonation (typically 6-8kHz). The knock sensor produces a voltage signal that signals the computer to momentarily retard ignition timing until the detonation stops.

"Read" your spark plugs.
The wrong heat range plug can cause detonation as well as preignition. If the insulators around the electrodes on your plugs appear yellowish or blistered, they may be too hot for the application.

Check for engine overheating.
A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Overheating can be caused by a low coolant level, a slipping fan clutch, too small a fan, too hot a thermostat, a bad water pump, or even a missing fan shroud.

Check the operation of the heated air intake system.
The thermostatically controlled air cleaner's job is to provide a carbureted engine with hot air when the engine is cold started. This aids fuel vaporization during engine warm-up. If the air control door sticks shut or is slow to open so that the carburetor continues to receive heated air after the engine is warm, the added heat may be enough to cause a detonation problem.

Check for a lean fuel mixture.
Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or the induction plumbing downstream of a fuel injection throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter or a weak fuel pump.

Remove carbon deposits.
An accumulation of carbon deposits in the combustion chamber and on the top of the pistons can increase compression to the point where detonation becomes a problem.

Check the boost pressure.
Controlling the amount of boost in a turbocharged engine is absolutely critical to prevent detonation.

Change your driving habits.
Instead of lugging the engine, try downshifting to a lower gear and/or accelerating more gradually.