Ignition tuning – AEM 30-71XX Infinity Stand-Alone Programmable Engine Mangement System Full Manual User Manual
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Tuning Guide
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© 2014 AEM Performance Electronics
Ignition Tuning
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 TDC, 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 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. But
even with the use of a dyno, 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.
Ignition Synchronization
The ignition timing measured with a timing light must be synchronized with the displayed value in
Infinity Tuner. This is one of the most important steps in the setup process. Without it, the
commanded ignition timing may not be accurate.
It’s a good idea to disable the fuel injectors and/or fuel pump during this step as well. Until the
timing is sync’d, it doesn’t make any sense to inject fuel just to fire it at the wrong time. The
ignition coil(s) must be connected and in working order.
for more detail.
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