Hi,
One thing to keep in mind is that these circuits are boost circuits even without
the coil secondary. This alone could generate very high voltages for the transistor
and other parts. Some sort of clamp across the transistor or some other way
to manage the high voltage kickback spike of the primary would be a good idea.
Another thing to watch out for is the primary DC saturation. This could cause
very high currents to flow during the normal pulse time.
The telltale sign is when the current in the primary ramps up as usual, but
then suddenly exponentially spikes up really fast. The sudden current rise
is due to the inductor core saturation and hence a sudden decrease in
permeability.
I dont have much experience with automobile coils so i dont know how much
leakage they design in at the factory nor how much primary resistance there
is. The primary resistance may be set to limit current in the event of saturation.
Since the coil primary ends up having some DC current flowing too, it would be
interesting to try to drive the primary with an H bridge
I bet there would be some huge peak to peak output!