Capacitor Discharge Ignition (CDI)

Inductive Ignition versus CDI Systems

In an inductive ignition system, the coil is charged at battery voltage for a period of time—known as 'dwell time'—prior to firing. The dwell or charge time is controlled by the ECU and should match the coil being used in order to not over or under charge. Undercharging reduces available spark energy, while overcharging can cause overheating of the coil and/or ignition module. Inductive ignition systems produce a spark, at a lower voltage with a longer duration compared to capacitor discharge ignition systems.

A CDI, Capacitor Discharge Ignition system is charging constantly and sends a large voltage charge (380–450 V) to the coil. The spark produced is extremely short in duration and at a much higher voltage than an inductive setup. Note that inductive coils should not be used with a CDI system; CDI compatible coils are required.

There are three main engine running characteristics to consider:

High cylinder pressures
Generally, higher cylinder pressures require more voltage to initiate a spark. Boosted or nitrous injected engines create tremendous cylinder pressures that increase resistance to lighting the ignition spark. CDI systems are most often used on these engines.
High RPM
The time available to charge the coil in an inductive system reduces at higher RPM. If the time available is shorter than the time required for a full charge, coil power and, as a result, performance will be reduced. A CDI system might be required.
Lean mixtures
The shorter spark duration in CDI systems might not be sufficient to ignite enough of the mixture to propagate the flame front through the cylinder. Inductive ignition will perform better in this setup.
Most vehicles, including high performance road and race applications use an inductive ignition system. Generally, if your engine can run correctly on an inductive setup, it is better to leave it that way and install a CDI system only when your engine, due to high RPM or cylinder pressure, requires that you do so.

Charging the Coils
The ignition signal is provided to the ignition module that controls the coil. Choose either inductive or capacitive discharge ignition modules with the correct number of channels.