 One factor often not taken into consideration when discussing CDI spark energy is the actual energy provided to the spark gap versus the energy applied to the primary side of the coil.As a simple example, a typical ignition coil may have a secondary winding resistance of 4000 ohms and a secondary current of 400 milliamperes.Once a spark has struck, the voltage across the spark gap in a running engine drops to a relatively low value, in the order of 1500-2000 volts.