AEM 30-71XX Infinity Stand-Alone Programmable Engine Mangement System Full Manual - Manual (Page 281)

EFI Components 275
© 2014 AEM Performance Electronics
The color or condition of the spark plug is a general indicator of how rich or lean the engine is
running and also if the engine is exhibiting signs of detonation if it is not audible. This is a plug
color chart (supplied courtesy of Denso) of plug conditions.
Detonation and Pre ignition
It is important to understand the mechanisms that cause knocking and pre ignition to set up an
ignition map that is suitable for the engine. Auto ignition, also known as knocking, pinging, or
detonation, is generally caused by improper combustion in an engine. An internal combustion
engine runs properly when the spark-initiated combustion wave expands rapidly but smoothly
throughout the combustion chamber. Combustion knock is caused by spontaneous ignition in the
hot unburned portion of the fuel mixture (typically referred to as end gas) in the combustion
chamber. The remaining charge portion is compressed first by the upward piston movement and
then by the moving flame front. Knocking is the almost instantaneous ignition of part of the
remaining mixture. This mixture auto ignites because the rapidly rising pressure and temperature
caused by the piston movement and the expanding gas from the flame front are sufficient to
ignite the remaining gasses. To illustrate the loads imposed on the engine components by
knocking, note that normal combustion speeds are about 12-25 m.s-1 while knocking
combustion speeds may be as high as 250–300 m.s-1.
The next illustration depicts the difference between a normal and abnormal combustion process.