Detonation is the rapid and uncontrolled combustion of the air/fuel mixture. Detonation can occur in the cylinder when operating on a fuel of inadequate octane rating, or with ignition too far advanced. It is informally called "pinging".
In discussions on this condition, we will refer to detonation as an abnormal combustion process. Normal combustion is a controlled amount of air/fuel ratio mixture entering each cylinder at the correct operation chamber temperature.
The compressed, "charged" mixture is then ignited at a precise moment by the ignition system that starts a small flame kernel, which is followed by a smooth controlled burning. Anything that alters the air/fuel ratio, chamber temperature, or ignition timing, will affect the combustion process.
The following are items that will usually effect the combustion process: Camshafts that alter valve timing from OEM specs, a different combination of pistons or cylinder heads altering compression ratio, wrong octane gasoline, engines that burn oil, a non-functioning EGR system, vacuum leaks, or restricted exhaust systems.
Listed below are some of the results of an abnormal combustion process in a gasoline engine:
- Piston, ring land, or piston ring breakage
- Head gasket armor distortion and burn through
- Tuliped intake valves
- Burned, cracked, or distorted exhaust valves
- Cylinder head cracks in combustion chamber
- Connecting rod bearing damage on upper shell first; leads to complete engine failure
- Wrong heat range on spark plugs
- Using oversized fuel injectors
- Wrong turbo charger
- Changing the computer or reprogramming it
Once detonation starts, it propagates with each combustion process. As the temperature and pressure in the cylinder increases, creating a violent uncontrolled burning, the exhaust stroke is no longer capable of evacuating enough temperature from the combustion chamber.
Modern engines have more electronic controls for engine management systems than engines built even 10 years ago. Those modern controls, when operating correctly, prevent detonation by retarding the ignition system. Engines without electronic spark control, rely on drivers to sense detonation, and correct the problem.
If you rebuild your engine, and it had a detonation problem, then you could damage your newly rebuilt engine. All systems and sensors need to be operating properly. The compression ratio has to be low enough for the type of fuel you run in your engine.
Symptoms typically include: melted spark plugs, and/or piston and ring damage, tulip valves, or burnt valves.
Possible causes of Detonation include, but are not limited to:
- Excessive engine advanced timing
- Mass air flow sensor contamination - lean fuel condition
- Restricted or defective exhaust gas re-circulation (EGR) flow, EGR valve or switch
- Low fuel pressure - lean running condition