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why is there a black wire running to the dizzy from the ballast resistor ?
Datsun-Nissan EI distributors have two wires. Often they are B/W and L (blue). The B/W wire goes to a 12V source, and the closest handiest place to get this in a Datsun 1200 is at the ballast resistor.
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12V EI coil, is just another name for the replacement 12 amp coil such as the MSD Blaster correct?
MSD Blaster is simply a replacement coil. It is nothing special except it comes in Red or Chrome. 12V and 12A are two different things.
MSD Blaster that you hear the racers using refers to MSD Blaster multi-spark (CDI) ignition system, rather than the MSD Blaster coil.
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Is there no difference in performance with keeping the ballast and running the jumper wire or completely removing it as you say?
It looks cleaner to remove it. There is no performance difference.
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Will I notice improved performance going from the old 4 amp (A series) coil to a 12 amp one?
No you probably will not notice. But it will allow you to re-gap the spark plugs from the normal 0.035 inch (0.9 mm) to 1.1 mm and you might notice it run a little smoother. The larger gap will fire lean mixtures better and should improve fuel economy slightly and power slightly at full throttle.
The real and very noticeable advantages are not increased power. They are reliability and staying in tune longer. See
EI for more details.