Ignition System Basics
No Ford engine project is complete without a properly functioning ignition system. This system has the important job of delivering the spark to the required cylinder at precisely the right time. Any deviation from the micro-second accuracy required and the engine won't be running correctly. If you can't get the ignition dialed in exactly then all the high-performance hardware in the world won't add anything to your engine's performance. The classic Ford cars we love were delivered with breaker-point distributors as original equipment. After a few thousand miles of wear, you'd begin to notice subtle changes in the way the engine runs as the points wear and the dwell, or point gap, changes. While many of us now circumvent these problems by installing a more modern system, the requirements for a correctly running engine are the same, so it's important to understand the basic principles regarding how you car's ignition system functions.
Whether you've got a Pertronix ignition or not you still have primary and secondary sides to the ignition system and you'll still have to have the correct engine timing. A breaker-less ignition setup, like the Pertronix, eliminates the mechanical variable of wear on the point set but the voltage to the ignition coil must still be delivered in the same manner. New cars are freed of these types of concerns because a computer controls the ignition system and there is no mechanical dimension to their function. However, on our 289-powered '67 Fairlane the distributor and its functions are still very relevant to our world. Let's examine some of the more basic concepts related to your vehicle's ignition system.
This diagram illustrates the basic structure of the standard ignition system as delivered
Primary And Secondary Ignition
Your classic Ford ignition is divided into primary and secondary ignition systems. The juncture between the two systems is the ignition coil. The primary system carries the 12-volt DC current from the battery through the ignition switch directly to the positive side of the ignition coil. This charge goes to the coil directly during start up, and through a resistor (which lowers the voltage) when running. The resistor prolongs the service life of the point set.
The ignition coil has both primary and secondary wiring in the form of winding circuits. The coil primary winding consists of more than 100 turns of insulated heavy copper wire. The primary circuit wire goes into the coil through the positive terminal and loops around the primary windings and then exits through the negative terminal.
The coil secondary winding circuit contains perhaps 20,000 coils of fine copper wire. The secondary windings sit inside the loops of the primary windings. As the 12-volt current flows through the primary side of the coil it passes through the outer windings and out to the breaker points. When the points are closed, the circuit is complete. In this condition a strong magnetic field is produced inside of the coil, with the outer windings becoming a powerful electromagnet. When the points open, the circuit is interrupted and the magnetic field inside the coil collapses. When this happens the inner, or secondary, windings in the coil react by producing a powerful electrical charge in the 20,000-volt range. This transition represents the juncture between the primary and secondary systems.
Once the 20,000-volt spark is produced, it's managed by the secondary ignition system. The big electrical charge travels out the top of the coil and through the coil wire to the center terminal of the distributor cap. The current then arrives at the rotor, which will deliver the charge to the correct spark plug at exactly the right moment.
Once the required spark is produced there are several variables that must be managed in order to assure that the spark is delivered to the spark plug at the correct time. Two of the most important factors are engine timing and breaker-point dwell. Both involve the distributor.