AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 99)

Page 99 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
Spark Tuning General Concepts
The ignition timing or spark timing value refers to the distance in crank degrees between the
spark plug firing event (initiating combustion) and the uppermost point of the pistons stroke (top
center TC, top dead center TDC, etc.)
If the start of the combustion process is progressively advanced before TC, the compression
stroke work transfer (which is from the piston to the cylinder gases) increases. If the end of the
combustion process is progressively delayed by retarding the spark timing, the peak cylinder
pressure occurs later in the expansion stroke and is reduced in magnitude. These changes
reduce the expansion stroke work transfer from the cylinder gases to the piston. The optimum
timing which gives the maximum brake torque - called maximum brake torque or MBT timing
occurs when the magnitudes of these two opposing trends just offset each other. Timing which
is advanced or retarded from this optimum gives lower torque. MBT timing defines the
optimum point (maximum brake torque) for peak cylinder pressure relative to crank angle.
Certain operating conditions, combustion chamber designs, fuel types, etc. will prohibit an
engine from reaching MBT timing before detonation occurs. Detonation, or spark knock is
caused by the combustion gases spontaneously combusting at the outer edges of the piston
before the normal combustion flame front reaches those areas. This causes pressure spikes
that can damage the engine over time.
Every engine will have different spark timing requirements. Unlike fuel tuning where you can
take advantage of inexpensive, commonly available wideband oxygen sensor kits, spark tuning
requires expensive laboratory equipment for complete optimization. Spark tuning is best
accomplished on an engine dyno. If that is not practical, a chassis dyno is the next best thing.
Even with the use of a dyno though, spark tuning requires experience to know where to start
and what kind of changes to make. Ignition timing values can range from 50+ degrees BTDC to
15+ degrees after top dead center for certain extreme conditions on race vehicles.