Hi Jerry,
Yeah.... I knew that primary current on HEI systems was considerably more than breaker point systems. My poor tired brain did not remember it being that high, but it makes perfect sense - considering how much larger the distributor feed wire is on HEI.
This can have ramifications, however. Back in 86 or 87, my 85 Corvette was recalled to replace the HEI coil and module. My understanding was that under certain conditions, the coil in that integrated distributor design was getting too hot, so they had to throttle back a little on the primary current. I'm sure that at the time, the Corvette was probably worst case, since the reduced air flow through the engine compartment probably led to some fairly high under hood temperatures. Combine this with a distributor tucked in close to the firewall, and a customer that lives in Arizona, and the result was probably a design that was pushed to the limit.
I personally always liked that integrated design. I thought it was clean, self-contained, and did not have a coil wire. However, I always got the feeling that the ignition system guys preferred the eventual evolution that utilized the separate coil, which provided much better cooling, and was probably easier to package.
One of the interesting take-aways from that paper is on page 4: "
To answer this question, you have to appreciate that the expression for the induced emf tells us it is not possible to switch off the current instantaneously: as Δt → 0, emf → ∞! What happens in practice is that, when the voltage gets high enough, the air breaks down (ionizes) in the strong electric field near the point where the current is interrupted, and a spark is produced "Although they do not explicitly include it, they are obviously referring to V = L (di/dt). So, as the author states, as the value of t goes down, the value of v goes up - to the point where it sufficient to ionize the point gap, and act just like a spark plug.
As you mentioned, the switching circuitry in the HEI module must be designed to handle the "V" caused by the coil being switched off. My only question still, is that according to the author of the paper, that capacitor is needed to provide a resonant RLC circuit in order to boost the coil output. I'm wondering if this is perhaps not as important of a factor as the author maintains. (You know those academics…. ).
I too, have seen those RF suppression capacitors on vehicles as well. I used to see them sometimes on alternators, or the old electro-mechanical voltage regulators. Some vehicles even used them on the brake light switch. The inherent inductance in the brake light bulbs (coiled filaments) would cause enough surge when the pedal was released to cause an audible "switch pop" in the radio speakers.
My truck, which is a former military vehicle, is loaded with suppression capacitors. They are everywhere, along with star washers on the front sheet metal bolts, and extra ground straps to help ensure ground integrity. In fact, if you look at the registration number on WWII vehicles (the number typically seen on the hood) the "-S" suffix stands for "suppressed". The Army was obviously very concerned about radiated emissions.
Lastly, if you are trying to determine total primary circuit resistance (wires, ignition switch, connectors, etc.) this measurement is not difficult to make. Just detach the ignition wire at the coil, and insert an amp meter and power resistor in series to ground (you could even use the coil, I guess). Activate the circuit and measure the current, along with the voltage drop across the entire circuit (or individual parts, if you are looking to isolate certain sections). Using ohm's law, you can then calculate the circuit resistance using the measured voltage and current. We used to do this all the time for running cranking system control circuit resistance tests (aka the starter solenoid feed circuit).
Best Regards....
