What is CDI?

Ignition boxes such as MSD, Accel, and Miller Ignition use capacitors charged to high voltage to create spark energy rather than the collapsing magnetic field of a standard breaker points ignition. Capacitor discharge ignition has the ability to fire fouled plugs as well as generally improve combustion vs. regular ignition systems due to their much higher peak currents (up to 5x) and fast voltage rise time. Although multiple sparks are a big advantage an additional advantage is due to the fact that the sparks are created through capacitor discharge vs. a collapsing magnetic field. 

A common criticism of CDI is that the spark duration is very short however Miller Ignition is similar in duration to most inductive ignition systems. This is due to a unique design which rings the discharge capacitor dry as well as takes advantage of energy remaining on the coil after the initial capacitor discharge. 

I truly believe that most well-made capacitor discharge ignition systems are a worthwhile addition to a classic car. Whether that be a 50 year old Delta Mark 10B or a new multi spark system such as Miller Ignition, MSD, or Accel. Miller Ignitions advantage is allowing multiple sparks to redline as well as longer duration sparks. Due to multi sparking all the way to redline as well as using multi-phase AC sparks under all circumstances there is a minimum of 8 opportunities for the air fuel mixture to be ignited. 

For those in need of a 6V unit please follow the link below;

Noble Associates (wbnoble.com)