What is the correct ballast resistor for this car?

[Bill Crowell]

Standard Mopar EI distributor, orange box and MSD Blaster 2 coil.

MSD says to use a .8-ohm ballast with this coil.

My friend Mark, NM9Stheham (who is a smart guy), says that Chrysler recommends using a .5 or .6-ohm ballast with their EI. (I don't doubt that Mark is right.) To me this shows that Chrysler thought its EI would like a little higher voltage than the Blaster 2 coil would like to see.

It seems to me that the voltage the coil primary wants should be determinative, since too much voltage on the coil primary will cause it to draw too much current and could burn it up.

OTOH, I wonder if the Mopar orange box works the way it is supposed to with a .8-ohm ballast, because it is going to be getting a little bit less operating voltage than Chrysler says it wants.

I would very much like to have your thoughts on this. Thanks very much.

 

[67Dart273]

I would use the recommendation for the coil

 

[BillGrissom]

Agreed. The coil is what matters, not the ECU. The combined resistance of ballast + coil is likely the same as in Chrysler's factory setup. Chrysler later (1980's?) had "ballasted coils" where all the series resistance needed was in the coil. This is when they still had primitive transistor-only electronics which couldn't run a low-resistance coil directly. Later electronics, like GM's HEI and Ford's TFI can pulse a low-impedance coil, like an e-core type. They have circuitry for "dwell management". Chrysler's version was buried inside the engine controller (PCM), so can't retrofit to our cars like a GM module.