Fundamentals

Ignition systems for internal combustion engines fall into two basic design categories. The most common is the Inductive system which is used on almost all production car engines. The other is called a Capacitive Discharge Ignition. This type of ignition, often referred to as a CDI unit, is used for high performance and race applications. There are distinct differences between the two types of ignitions and their applications.

Inductive Ignition

The Inductive Ignition is used on most production cars. The positive side of the coil is fed by the car's 12 volt battery and is then charged by grounding the negative side via a distributor contact breaker or a transistorized module. The charging period, called dwell time, is followed by a ground side release from which spark energy is generated. This ignition method works well on production vehicles that have relatively low cylinder pressures and when the engine operates at relatively low speeds.

Racing engines operate at much higher speeds and therefore require faster coil charging times (dwell time), and as a result makes it difficult for the Inductive ignition type to charge completely and generate optimal spark energy. Most race engines also run higher cylinder pressures and when combined with the lower spark energy/voltage generated by the inductive type ignition can frequently cause misfires and reduced overall performance.

Capacitive Discharge Ignition

This type of Ignition, also called a CDI, is used in most motorsports applications. It functions by using an amplifier to step the battery voltage from 12 to 450 volts which then charges a large internal capacitor. The charge time is very short and efficient, and is not effected by high engine speeds as is the Inductive type ignition. When triggered, the highly charged capacitor quickly releases all of its energy into the coil to generate a very powerful spark voltage.

A CDI system typically doubles the available spark voltage and delivers 5 times more spark energy than the best Inductive Ignition systems on the market. An additional benefit of the CDI is that it produces full spark energy even when battery voltages are as low as 10 volts whereas an Inductive Ignition output is significantly reduced under these conditions. The ability to create higher spark energy insures optimum combustion and greater performance in all engines running high cylinder pressures and speeds.

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