Summed up, the magnetic line of force builds while the points are closed. We call these distributor degrees "dwell." On a breaker point ignition, the dwell period is also referred to as the distributor "cam angle." This is the number of cam degrees between the point set closing and reopening again. The primary (battery or resisted) voltage moves slower around the fine wire of the coil's internal secondary winding. As the points open, the abrupt change in the magnetic field's force sends the high voltage of the secondary winding into the soft iron core of the coil.

The iron core takes the spiked voltage and sends it to the high-tension spark wire that connects the coil to the distributor cap. This voltage can range to 25,000 volts in an original breaker point coil, 50,000 volts or more in an HEI ignition. The rotor now delivers spark to the aligned spark wire contact in the distributor cap. In turn, spark travels through each spark lead to its spark plug. This spark is sufficient to fire across the spark plug gap under high cylinder pressures.

In this whole process, the coil becomes very hot. One means of cooling the coil is oil. An "oil-filled" coil is much like an oil-filled transformer; the oil serves as a coolant. Also critical to cooling is the resistor or "ballast resistor." Six-volt ignitions do not generate excessive heat at lower engine speeds--but 12 volts may.

To compensate for the hotter coil temperatures at lower engine speeds, the resistor on a 12-volt system lowers the available voltage to the primary winding of the coil. As the engine speed increases, along with quicker coil firing cycles, some types of resistors cool slightly. This raises the primary voltage, providing higher spark plug firing voltages at higher engine speeds.

The Importance Of Proper Ignition Tuning
The breaker points need the correct gap and dwell settings to provide the coil with sufficient primary current. Too narrow a gap, and the points will arc and burn out prematurely. Too wide, and the dwell will not be long enough to saturate the coil properly. Ideally, points should always be set with a dwell meter, using the factory recommended dwell setting.

A contributor to breaker point life is the condenser. If the condenser is defective, when the points open to interrupt the current flow through the primary circuit of the coil, current will arc across the point gap. To prevent point arcing, the condenser stores current then sends that current backward against the primary winding to aid in the coil's rapid collapse of the magnetic field and line of force.

Points, condenser, rotor, and spark plugs have been traditional tune-up parts. For breaker point and HEI distributors, there is also periodic distributor cap, rotor, spark wire, and spark plug replacement. The HEI or electronic ignition module is a somewhat common replacement part as well. With a better understanding of how the ignition system functions, troubleshooting and tuning will be much easier.