I'll stand by my assertion that primary voltage doesn't affect the spark plug voltage. It does affect the duration of the spark, since a coil of the same winding can't store as much energy with half the input voltage.
I also thought the same, as in the coil outputs such a high voltage even hooked up to a 6 volt battery it should still function. It just doesnt make sense that a 6/12 volt battery would make a difference other then how bright your strobe light might be.
