The volage drop is approximately 0.25%. The voltage required to spark is less than the coil can provide so that drop means nothing. This is assuming a 0.025" plug gap, normal resistor plugs, normal temperatures, proper fuel/air mixture, and 8.5:1 compression ratio giving about a 2M ohm gap resistance.

I thought that the air gap resistance drops significantly once the spark starts to jump which ionizes the air...

Dennis

Since current is flowing - let's say for the fun of it that the voltage required to start the current flowing across the gap is 12,000 volts. I=E/R so 12,000/2,005000 to start. So the initial current is about .006 amps. Now let's say that the effective resistance drops to 1M ohm once the current is flowing, so 12,000/1,005000 means that the current is now .12 amps. So the voltage across the resistor and gap are (E=IR):

Before: .006*5,000 = about 30 volts and the voltage across the gap .006/2,000,000 is about 12,000 volts - the coil voltage is 12,030 volts

During: .012*5,000 = about 60 volts and the voltage across the gap .012/1,000,000 is about 12,000 volts - the coil voltage is 12,060 volts

You know the plain old Lucas coils are capable of high voltage - the bike will run at high or low altitude, at as least 13:1 compression ratio, rich or lean, and even at the Norton top gap of .028". They probably won't spark standard plugs at a wider gap or even higher compression.

Any way you look at it the voltage across the plug gap will remain the same. If yo umake a coil that can produce a million volts, the volts across the gap won't change. When you have fancy plugs like surface gap, they has much higher resistance and therefore require a coil with higher capability.

Of course I'm using steady state numbers. A coil secondary collapsing is not like steady state - for one the voltage and therefore current ramps up and ramps down - it's like a flat-top spike and the it, to a much lesser degree goes the other way.