AEM 30-71XX Infinity Stand-Alone Programmable Engine Mangement System Full Manual User Manual

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EFI Components

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Wasted Spark

Wasted spark systems employ one coil for two cylinders. The term “wasted spark” comes from
the fact that each plug fires every engine revolution. On a 4-cycle engine, the piston is at Top
Dead Center (TDC) two times for every cycle; once for firing and again during the overlap phase.
The wasted spark coil fires one plug Before Top Dead center (BTDC) and another plug just
before the overlap phase (at the latest part of the exhaust stroke before the exhaust valve
closes). Wasted spark systems have a higher potential for spark intensity because the duty of
charging and discharging is split between the coils, which allows for more charge time per coil.
Additionally, wasted spark systems build up less heat in the coil, making it more reliable. Wasted
spark systems have been in use since the mid 80’s on GM cars and on motorcycles for
considerably longer than that. There are no moving parts, no complicated relationships with a
cap and rotor to maintain, and they deliver very accurate spark timing. Furthermore, multi-channel
spark amplification systems to enhance spark duration or intensity are available for wasted
spark ignition systems.

Direct Fire

Direct fire systems employ one coil on each spark plug and is the most reliable system used
today, (this type of system is used on most modern cars). Each coil fires sequentially in the
cylinder firing order. The charge time for each coil is twice as long as those of a wasted spark
system, which allows direct-fire, coil manufacturers to build compact, lightweight coils that retain
sufficient spark energy. There are no moving parts to wear out and no HT leads that will
deteriorate. The lack of HT leads in direct fire systems is a major advantage for an EFI-equipped
car because there is a very low incidence of noise due to leaking or improperly routed wires.
There have been incidences of the terminal from a direct-fire coil (that attaches to the spark plug)
cracking and subsequently causing Radio Frequency Interference (RFI) or “noise” to the ECU.
This will cause engine operation problems, but it should be noted that these cases are extremely
rare.