coil without ballst resistor?

Coil and ballast resistor choice is really all about the current draw the particular ecm is designed for. Coil current is hard to measure, so most folks talk about voltage. Voltage is easy to measure, but doesn’t tell you much. This leads to a lot of confusion when swapping parts around.

Let’s talk about the current on a stock Mopar electronic ignition. Instead of trying to measure something (current is pulsing, so it is hard to measure) let’s do a little simple Ohms law calculations.

There are different ballast resistors, but I believe 1.2 ohms is a common value used with Mopar ECMs. It is in series with the coil, which is about 1.5 ohms. This presents a 2.7 ohm load to to the ECM (1.2 + 1.5 = 2.7).

Let’s assume the ECM supply voltage is 13V. Using ohms law, the current in the coil (and ballast resistor) is 13V / 2.7 ohms = 4.8 amps. All these things have tolerances, so it would appear that the Mopar ECM is designed to operate at about 5 amps.

The stock coil and ballast resistor provide about a 5 amp load to the ECM, so everything is good until you start switching parts.

if you substitute a 3 ohm coil, now you have a total resistance of 4.2 ohms, and the current goes down. Usually this will reduce your spark. This is a situation where removing the ballast resistor makes sense. That would get your current back up around the 5 amps the ECM is designed for.

If you substitute a 0.7 ohm coil, now you have a total resistance of 1.9 ohms which drives the current up. Now your current is about 7 amps. Can the ECM handle it without burning up? I don’t know, but I suspect this is one reason why ECMs fail. Here is a situation where a larger ballast resistor would work.

The above calculations are “nominal” in value, intended to show how ECM, coil and ballast resistor compatibility works.

For aftermarket ignition systems, all the same calculations apply. If you know amps or ohms the unit will drive, then you can figure out what to do.

As also stated above there are lots of other factors that will affect how hot you spark is. Two coils that have the same resistance and pull the same current could have very different outputs due to other factors. I have not considered that here.

My point in this long winded explanation is that if you understand the basics of how to match ECMs with coils and ballast resistors, you will be able to get your system operating within its design limits and have good performance and durability. You can have really expensive and high performance ignition components, but if they are not properly matched, you may not get what you paid for, and you may even lower your performance.