Since your truck has a 12 volt electrical system, I would stay with a ballast resistor for your electronic ignition system unless the coil has internal ballast resistance. The original contact ignition system and front-end wire harness was designed to operate with a steady-state (static) primary current of 4-5 amps with a ballast resistor and about 1.5 ohms of coil primary resistance. Static current is the steady-state current flow that results from the system voltage and system resistance, but does not consider the brief opposition to current caused by inductive reactance in the coil as primary current rises after turn-on. The importance of this point is that the ignition system reaches the static current level when the coil primary winding has reached magnetic saturation, which occurs near the end of the dwell period, but occurs earlier at low engine speed- below about 1500 RPM. Now, if you leave out the ballast resistor, you will increase the primary static current, approximately double, to steady-state of 8-9 amps. The wire harness will probably handle that for short-trip driving, but prolonged engine operation (several hours) may result in the front-end wire harness overheating. This heating condition would be aggravated if the Pertronix system increases the dwell angle from the original 30 or so degrees. Increasing the dwell angle increases the time interval during which primary current flows, which in turn increases the power (heat) dissipated by wire resistance. So I guess the short version of this is that if the coil primary resistance is around 1.5 ohms (or less), I would advise using the external ballast resistor. Coil primary resistance of about 3 ohms indicates internal ballast resistance, and you could do without the external resistor.
All this leads to some points, mostly of academic interest:
First, the only ways to raise coil secondary output voltage are to increase the primary magnetic field strength, increase the number of windings in the coil, or both. Increasing the primary field strength is typically accomplished by reducing the primary resistance, which increases primary steady-state current (and heat dissipated by wire resistance). Increasing the secondary windings in the coil then will get you a higher secondary voltage available and higher spark current (and more erosion on the spark plug electrodes). Usually, but not always, high-output coils use the combination of lower primary resistance (1 ohm or less) and higher secondary resistance- usually more than about 20 K ohms.
Secondly, and more of academic interest, is that the Pertronix system appears to be a “hot-coil” design, which is the same as a standard contact ignition system. “Hot coil” means that the coil is connected directly to power, and ignition current switching takes place on the ground side (as with a contact system). Hot-coil systems are the most common electronic ignition systems, but in the early days of electronic ignition systems, there were “cold-coil” systems, in which the current switching took place on the power side of the coil, and the ground side of the coil was connected directly to ground. There was an early Delco-Remy system of that type, and I believe there was an early Ford system also of that type- not sure about the second one.
I realize all this goes beyond the original question, but maybe this helps explain why things are the way they are.
Harvester
