AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 188)

Page 188 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
Squish is accomplished via a small gap between the head deck and the piston top. The
decrease of this gap drives the inlet charge toward the spark plug electrode. An additional
benefit of having this small gap is the reduction of end gas volume at the extreme edges of the
cylinder. This reduces the tendency for spark knock and leads to a reduction of Hc emissions.
Cycle-to-cycle variation refers to a situation where an engine operates on the threshold of
knocking or detonation throughout the engine cycles and is based on the average peak
pressure of all of the cycles. The cycles with lower peak pressures may not be prone to
detonation, while those with higher peak pressures may detonate with increasing intensity as
peak pressure increases. Increasing this cyclic variation will increase the number of cycles that
detonate, and decreasing the number leads to less cycles that detonate.
Ignition Delay is the point at which perceptible inflammation of the mixture and a pressure rise in
the combustion chamber occurs. The time between when spark occurs at the spark plug and
when inflammation of the mixture occurs is the delay time. This delay is caused by the chemical
reactions that take place when the rate of reaction after the delay accelerates to an extent that
noticeable combustion and a rise in cylinder pressure and temperature occurs. The chemical
reaction within and on the surface of the initial flame kernel causes energy to be released during
the ignition delay period. When the flame front is at this initial stage of propagation there are
energy losses via conduction, radiation, and convection of heat. If too much of this energy is
lost, the mixture will fail to propagate and a misfire will result.
As compression ratio increases the requirement for charge motion in the chamber is reduced.
Engines that have high compression ratios generally require reduced ignition timing
requirements due to the increased flame speed. The design of the combustion chamber in a
high compression engine is necessarily small. The compactness of the chamber imparts its
own charge motion and squish into the mixture.
We have included some sample ignition timing maps for gasoline engines that are typical for
use on a street car. These examples include a naturally aspirated engine, forced-induction
engine, and a high-compression engine. These are conservative samples that should be
used as a starting point only.
The tuner of the engine must use good judgment when
selecting the appropriate timing map for the engine.