If there is a single part of the ignition that can holds some mystery to its operation, it is the coil. In a stock ignition system, 12 – 14 volts are delivered to the coil via the positive and negative terminals. Within the wink of an eye, it sends and output of 15,000 volts or higher! But there is no slight-of-hand taking place here – it’s all a carefully crafted lesson in electronics. And one that can be tuned to meet a variety of output requirements.
A coil consists of two sets of windings made up of insulated wires that surround an iron or similar metal core. The primary windings are generally made up of several hundred turns of a heavy wire while the secondary set consists of a much smaller gauge wiring that makes up thousands of turns. Coil manufacturers use a ratio between the secondary and primary winding numbers as a specification such as 100:1, which would mean 100 secondary turns to every one of the primary. This is a commonly used specification you will see in the coil information given by many manufacturers.
In a typical factory-style inductive ignition, current from the battery flows through the thicker primary windings when the switching device (points or magnetic pickups) is closed. This creates a magnetic field that builds strength thanks to the help of the iron core. When the switching device opens (the trigger signal) the flow of current is broken and this magnetic field collapses over to the thousands of secondary windings. During this collapse, the voltage is stepped up, creating the higher voltage that is required to jump the spark plug gap and ignite the air/fuel mixture. This process is called inductance.
When shopping for a coil, keep in mind that the windings, materials and resistances of the coils can be altered to design coils for the street or for racing ignitions. Also there are a variety of sizes and shapes available today. Most manufacturers will explain what application the coils are used for so you should have no trouble finding a coil for your application.