HEI upgrade?

67Dart273 said

The 7 volts is feedback coming from the ballast. It doesn't do a thing as the switch contacts are open. How does this prevent backfire?

I don't know about "all years" but at the very least "some years," the dark blue most certainly goes cold when the key is in "start."

Click to expand...

The dark blue gets voltage along with the coil from the brown wire in start too. Current will flow through the ballast resistor in either direction.
Ever turn ignition switch off and one again during run ? We use to blow the mufflers off school buses like that. LOL
So there never is even a split second between start and run when the ignition has no power, by design.