AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 184)

Page 184 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
has a lesser tendency to misfire under the extreme pressures of a racing engine combustion
chamber. Also there are spark plugs made with exotic fine wire highly conductive center
electrodes that require less energy to fire such as the Denso Iridium that are well suited to
racing conditions. The following is a chart of gap sizes for various engines on gasoline:
Naturally Aspirated up to 11.0:1 CR 1.1mm (.044”)
Naturally Aspirated 11.0:1 to 14.0: 1.8mm (.032”)
Forced Induction to 20-PSI .7mm (.028”)
Forced Induction to 40-PSI .6mm (.022”)
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.
Engine Knock (Detonation) and Preignition
It is important to understand the mechanisms that cause knocking and preignition 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