A 1970 Superbird was originally equipped with a contact point ignition system. The original point system or a Chrysler factory type ignition requires a ballast resistor for current limiting as part of the design of the system. The current limiting is required for the coil and the electronic control unit (ECU), or contact points in the point system. MSD states that a ballast resistor is not required for their coil, but one can be left in the circuit. The ECU, or points, still requires it to not exceed its current carrying capacity. MSD does supply its own 0.8 ohm resistor, part number 8214, for point systems, or as otherwise required. Bypassing the resistor could cause ECU failure.
If the current "factory electronic ignition" is a Chrysler ignition using production parts or a conversion kit using production style parts, the ballast resistor likely measures either 0.5 ohm for a re-used points system single resistor, or similar replacement, or 1.25 ohm single resistor for a later production electronic factory, conversion kit resistor similar replacement, or possibly a 1.5 ohm single resistor in some aftermarket resistors. If using a dual ballast resistor, the numbers for the primary circuit resistors follow the same values.
The Chrysler 1960 - 1979 production coil primary resistance at 70-80 degrees Fahrenheit measures 1.6 - 1.79 ohms for the Prestolite 2444242 type coil and 1.34 - 1.55 ohms for the Essex/Echlin 2444241 type coil (part numbers 2495531 and 2084847). The Chrysler 1980 - 1989 production coil primary resistance at 70-80 degrees Fahrenheit measures 2 ohms (part number 4176009).
The coil and other components for direct current limiting based on the resulting cumulative resistance of the primary circuit. The cumulative resistance of the primary circuit including the ballast resistor, the coil primary winding, the wiring, the connections, and the effective junction resistance of the switching transistor or contact point resistance, should be such that the direct current (neglecting dynamic impedance factors) is less than or equal to the capacity of the series connected components, under the applied primary voltage. Depending on the applied voltage, primary current should be kept at four to six amps, with spikes less than 10 amps for production ECUs, or three to four amps with spikes less than six amps for production contact point sets.
For estimating purposes, the coil and ballast resistor are the primary resistances to consider. With good wiring and connections, their resistance is negligible. For these same purposes, the effective resistance and voltage drop across the transistor, and the resistance of healthy contact points can be considered to be zero.
MSD Blaster 2 coils have a primary resistance of 0.7 ohm. If a ballast resistor of 1.2 -1.5 ohms is used, the total current through the switching transistor of the ECU, depending on the applied primary voltage will remain within permissible limits. If the ballast resistor is a 0.5 ohm unit, the total current through the switching transistor of the ECU, depending on the applied primary voltage, may push these limits. If the ballast resistor is eliminated, or bypassed, the total current through the switching transistor of the ECU, depending on the applied primary voltage, will exceed these limits, and ECU failure may occur, depending on the ECU itself.
