The point ignition systems used in cars with 12 volt ignition systems had to have the current draw reduced across the points to prevent arcing, pitting and early destruction of the contacts on the points. In order to reduce the current resistance was introduced into the ignition system. This was accomplished by either an external ballast resistor or a resistor wire in the circuit. Ignition coils are also available with built in resistance, which do not require a ballast resistor or resistor wire.
The total resistance used, that reduces the arcing and pitting on the points and also provide an ignition system that supplies enough voltage to create a spark in the spark plug gap, is around 3 ohms.
The currents of the various resistances used in ballasts and coils are shown in this table. The engine stopped condition is with the points closed or negative side of the coil grounded:
[SIZE=11pt]Because the tachometer in older Ford cars[/SIZE][SIZE=11pt] is also inlin[/SIZE][SIZE=11pt]e in the ignition cir[/SIZE][SIZE=11pt]c[/SIZE][SIZE=11pt]uit the current flowing through the igni[/SIZE][SIZE=11pt]tion als[/SIZE][SIZE=11pt]o flows through the tachometer. The tachometer has a small transformer that the ignition system current flows through and the pulses created by the coil turning on and off creates a small pulsed voltage that the circuitry in the tachometer converts into a variable voltage that moves the tachometer needle. This small transformer was designed to function with the 2.7 to 3 ohms found in the typical older Ford ignitions and more importantly, the current draw. A high current draw may destroy the transformer.[/SIZE]
While the electronic aftermarket ignitions don’t have points, and can be designed to run on battery voltage, those with factory tachometers have to be aware of the loads placed on that little transformer. There are two ways to get around this limitation and not fry the transformer. One is to get the MSD adapter, Part No. 8920. The other is to get Rocketman to convert your tachometer to a modern three wire design, eliminating the need for adapters or worries about if your system will function properly.
Realistically, all that is needed for our cars is a properly functioning stock coil, which supply around 25,000 volts. Our cars, with a 0.35" gap and 10:1 compression, and not fouled with oil or carbon, require about 12,000 volts to fire the spark plugs. The wider gaps, that came about as car makers needed to improve the efficiency of the engines, required higher voltage coils for the wider spark plug gaps.
There is a lot of advertising about 30,000+ volt coils. In general these voltages are only produced in controlled or ideal conditions. An interesting thing about coils is that they just produce enough voltage to bridge the spark plug gap. The total potential voltage is governed by the saturation current in the primary windings, the winding ratio between the primary and secondary windings, and the iron core. The primary side is charged when the points are closed (or connected to ground by an electronic ignition). As soon as the points are opened (or the ground removed) the magnetic field created by the primary windings collapses almost instantly through the iron core and secondary windings creating the high voltage necessary to bridge the spark plug gap. As soon as the spark is created the secondary field is discharged and the voltage does not get any higher. So, if you have a 40,000 volt coil and your sparkplugs only require 15,000 volts to fire the remaining 25,000 volts is wasted, along with the energy required to charge the coil windings.
