How Inductive Ignition Systems Work

10 min read

How Inductive Ignition Systems Work

10 min read

Since electricity is not something we can physically see, the ignition system of our cars sometimes holds a little mystery or suspicion in its operation. It’s important to realize that electricity, especially in an automotive setting, is not magic. And it’s also important to realize that the ignition is key to your engine’s performance. Your engine’s overall performance is counting on that spark for complete combustion. Once you have a better understanding on how the ignition works and what each component actually does, things will begin to all make sense – and you’ll be able to build the right ignition for your engine. Key goals of the ignition system:

  • Produce and deliver a high-voltage spark from a low voltage supply source (the battery)
  • This spark must be distributed to each combustion chamber as the piston nears top dead center on the compression stroke of the piston
  • Control and even alter when the spark occurs in the cylinder to meet different engine demands
  • Deliver a spark that has enough voltage and energy to ensure combustion of the fuel mixture
  • Be able to reliably accomplish these goals throughout a variety of rpm, load, temperatures and conditions

Quick Overview

A typical (meaning stock) 12-volt automotive ignition system operates by taking in a low voltage with high current from the car’s battery and changing it into a higher voltage with lower current to jump the spark plug gap to propagate combustion in the cylinder. This process of changing low voltage to high voltage, called induction, takes place in the coil. From there, the high voltage spark is transferred to the distributor and on to a spark plug wire which must deliver the spark to the cylinder that is coming up on the compression stroke.

Two Sides to Every Ignition

To help better understand the operation of the automotive ignition system, lets break the system into two sides: a primary side and a secondary side.


The Primary Side:

The best way to define the primary components is that they are the parts that deal with the low voltage from the battery (the 12-volt side). Note that all of these parts use conventional wiring, since they’re carrying lower voltages (except the battery cables that connect to the starter and alternator). This includes the battery itself, the ignition switch, a switching device and the wiring that connects to the coil’s negative and positive terminals. The coil works both sides of the fence since it is the point where the primary and secondary systems meet as it takes a high voltage in but sends out a high voltage spark.

The Secondary Side:

The spark plug wire that connects the coil to the center terminal of the distributor (called the coil wire) is where the secondary side of the ignition begins. In stock systems, there is generally anywhere from 10,000 – 20,000 volts bolting through this wire. This spark zips through the wire and transfers to the rotor tip and then jumps across a small gap to another cap terminal. From there it moves out of the cap, through a spark plug wire, and finally across the spark plug gap. It is important to note that these secondary components, the cap, rotor and wires, are maintenance parts. Depending on your use, they should be inspected at least once every cruise season and numerous times during a race system. More of each of these components and systems is found through our website.

Ignition Maintenance

Even though we’re dealing with electronics, it’s important to inspect your ignition system. Remember that the ignition system, at least the high voltage side, requires maintenance. Over time, the rotor tip wears as does the carbon contact of the distributor cap. Spark plug wires also grow weary over time from delivering voltages that can range from 10,000 to 40,000 volts. Plus, your wires operate in a harsh, nasty environment with extreme heat cycles, moving parts, oils and fluids all working against them.

  • It is recommended to inspect the cap and rotor at least once a year. If you live in an area with high humidity, you may want to inspect it more often.
  • Visually inspect the cap and rotor for wear of the cap terminals and the rotor tip.
  • Look for traces of carbon tracks where spark scatter occurs.
  • Visually inspect the plug wires for burns or tears. Also, it is a good idea to periodically check the resistance of the wires so see if there is a wire with excessive resistance indicating a break in the conductor.
  • MSD’s Spark Guard, PN 8804, is a dielectric grease that helps isolate the spark at the plug wire terminal and cap connection.


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