How much voltage is too much

Voltage is fine. 8 ohm resistor, post #7??

One of the most important parts of a coil that never seems to get a mention is the purity of the elements in the steel core. It requires a large percentage of silicon. It has to magnetise & de-magnetise at very high rates, the greater the number of cylinders that it has to fire, the more important metal purity & percentage of alloying elements becomes. Hopefully when you pay more for brand name coils, this is what you pay for.