What coil..?

In order to understand all this coil swapping stuff, it is much easier to talk in terms of current rather than voltage. Folks talk about voltage because that is what is easily measured.

The higher the current in the coil, the more energy is available for the spark. There are other factors, but we are just working on coil swapping for this discussion.

In the stock mopar system, you have a 1.5 ohm resistor and a 1.5 ohm coil, those add for a total of 3 ohms. Using Ohms law, 12 volts divided by 3 ohms is 4 amps. This is what the system is designed for.

If you take the ballast resistor out, then 12 volts divided by 1.5 ohm coil is 8 amps, doubling the current in both the ECU and the coil. One or both of them are likely to be damaged. This is why my car kept burning out coils. The previous owner had it wired so that the ballast resistor was in the start circuit, not the run circuit.

So, if you want to swap coils with a mopar ecu, you need to keep the current about the same, at 4 amps, to keep from damaging the ecu.

This all means if you buy a 3 ohm coil, the current would be 12 volts divided by 3 ohms = 4 amps, so you would be fine without a ballast resistor.

If you buy a 1.5 ohm coil, then you would need a ballast resistor of about 1.5 ohms to keep the current correct.

Disclaimer: I have used nominal values for illustration. Some will say that the 12 volts is really 14, and the 1.5 ohms may really be 1.6. While all that is true, I just picked numbers for easy math for demonstration of the concept.
14 volts divided by 3 ohms is 4.7 amps.

Electronic systems are designed to operate in a range, so 4 or 4.7 amps probably doesn’t matter, and certainly 1.4 to 1.6 ohms really doesn’t matter. Doubling the current from 4 to 8 amps does matter.

If you understand what current you ecu is rated for (whatever brand it is) Then you can make an intelligent decision about the coil you choose.