AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 185)

Page 185 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
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. If the gasoline-air mixture
auto-ignites somewhere in the cylinder (other than at the spark plug) just after spark ignition, the
auto-ignition combustion wave can collide with the spark-initiated combustion wave, causing the
vibration we hear as a knock or ping. Depending on its intensity, knocking combustion may
range from barel
y audible “pinging” to a rather violent thumping. The point at which the
knocking becomes damaging to the engine is dependent on the components used in the engine.
If sustained knocking occurs, then the pistons may be damaged. When knocking reaches a
violent thump, engine operation should be ceased or at minimum the load and temperature
reduced to prevent engine damage. Light knocking that happens during acceleration is less
harmful and may not damage the engine. Knocking tendency is increased by the following
design or operational characteristics:

High Engine loads encountered while towing a vehicle.

Using low octane gasoline in a high-compression engine.

Too much timing advance for the type of fuel being used.

Higher air density, (this can be caused by starting a calibration at high altitude and then
traveling to a lower one, or the addition of forced induction).

Increased temperatures and pressure in the combustion chamber due to inadequate
engine cooling.

Excessive inlet air temperature.

Spark plugs with an improperly high heat range.

A non-central spark plug location in the combustion chamber.

An elongated combustion chamber design.