AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 187)

Page 187 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
very small shiny beads on the porcelain of the plug, there is a good chance that the engine is
knocking. Retard the timing to eliminate the knocking, enrich the mixture, or use a higher
octane of fuel.
The combustion chamber plays an important role in the amount of ignition timing that can be
used for the various operational phases of an engine. The most common type of combustion
chamber design used in contemporary engines is a four-valve/cylinder, pent roof chamber.
There are many other types of combustion chambers, such as a wedge, hemispherical, and
canted valve, to name a few. Ideally, it is best to initiate flame propagation at the geometric
center of the chamber. However, this is typically impossible to do because the spark plug is
usually located at the top or edge of the combustion chamber. The reason it is desirable to have
the flame front start at the geometric center of the combustion chamber is because there is less
chance of autoignition of the mixture.
A pent roof combustion chamber places the spark plug near the center of the combustion
chamber. Wedge combustion chambers are the most sensitive to spark knock because of the
distance the flame front must travel within the bore prior to the power stroke.
When ignition occurs, a flame kernel starts at the spark plug electrode and expands across the
combustion chamber. As the front progresses across the chamber, the hot expanding gas
compresses and heats the end gasses and mixture at the opposite end of the chamber. If the
pressure, and consequently temperature, inside the chamber increases beyond the flash point
of the end gas, autoignition occurs. Because the pent roof type of chamber has a spark plug
that is nearer to the geometric center of the chamber, the flame travels more evenly across the
combustion chamber, leaving very little end gas that can be compressed or ignited by the
advancing flame front. Thanks to the short distance the flame front has to travel in pent roof
type chambers, ignition timing usually does not have to be as advanced as much as a wedge
type chamber, to achieve maximum torque. We find that there is usually about five to eight
degrees less timing required for pent roof combustion chambers than for wedge types.
Charge motion, which is comprised of intake swirl and squish; increase combustion speed when
compared to a standard combustion chamber. The combination of these factors increases the
mean effective pressure (MEP), lowers fuel consumption, and delivers smaller cycle-to-cycle
variations at full throttle operation.
In addition to the performance benefit of effective charge motion, the resulting factors reduce Hc
emissions, with a slightly increased NOx component. At partial load the benefits of charge
motion vs. a standard combustion chamber are similar, but due to the decrease in density of the
intake charge, not very.
The combination of swirl and squish is greater than either squish or swirl alone. Swirl improves
mixture preparation and is mostly responsible for reducing ignition delay and cycle-to-cycle
variations. Swirl is accomplished through the inlet port design or by using shrouded valves in
the combustion chamber. As engine speed increases, the swirl motion increases along with it.
Cycle-to-cycle variations decrease with increasing swirl action.