ORIGINAL: cu. in.
I believe that the confusion could be reduced or eliminated if an EE or manufacturer could briefly describe the design and coil differences between the two circuits so that we all could be on board here.
i am neither but i do understand the diffs. my terminology may be a bit off so feel free to correct me.....
KETTERING: the energy for the spark is stored in the coils primary winding and then dumped through the secondary to increase its voltage. a high inductance coil works best for this and requires a high number (100-200) of primary windings and because of the need for the correct ratio between primary and secondary you end up with an astronomical number of windings in the secondary. add to this the need for good insulation between each layer of the secondary and the heavy iron core and you get a big, heavy coil. if you try to miniaturize the coil (like the modelelectric and gettig coils) you start giving up reliability.
CDI: something between 150-400 volts is stored in a large capacitor....when the ignition is triggered this energy is sent through the coil that acts only as a step-up transformer to get the high voltage needed for the spark. this is the same system used in flashes, strobes and stun-guns. low inductance is best here so you don't need more than 20 winds on the primary of the coil. the massive insulation can also be tossed out and a small ferrite core used.
the coils used in some of the CDI ignitions (magneto and battery) weigh less than 1/2 oz.!
whew
dave