[onZedge]

I've had my MSD 6200 6a for about 35 years now, so I know I can help. The function of the ballast resistor is to limit the charging current to the primary side of the coil. The Porsche factory coil has the ballast resistor built into it.

OK, on to MSD stuff. The MSD Blaster coil does NOT have a built in ballast resistor. If you are using the MSD 6a box, you do NOT need a ballast resistor with the MSD Blaster coil because the MSD 6a box has the ballast resistor built into it. IF YOU EVER BYPASS the MSD 6a back to factory ignition AND you still want to use the MSD Blaster coil with the factory setup, YOU MUST USE A 0.8 ohm BALLAST RESISTOR in-line with the coil (+) or you WILL burn out the coil, or worse, the coil driver circuitry in the Motronics DME.

Tech stuff: Using just the Blaster coil with the factory setup, MSD wants you to limit the charging current to 15 Amps. How? With an external ballast resistor.

So if I=E/R, where:
I is current in amps
E is voltage in volts
R is resistance in ohms

I=12 volts/0.8 ohms = 15 amps !!

Now you can easily see that without a ballast resistor, as the resistance lowers toward zero ohms, the current rises toward infinite amps resulting in a failed $45 coil, or worse, a failed $$$ Motronics DME.