True, in a steady-state situation.
But the high voltage initiates the spark, which partially ionizes the mixture along its trail, reducing its resistance, and then a higher current flows at a lower voltage.
I do not know specifically exactly what sort of energy characteristics a spark has across a given gap, through a given mixture, under a given pressure, at a given temperature, using a given coil, for a given duration, as I am not an engineer, but all of these are known factors, and are used in modern cars to determine whether each cylinder has fired, each time, the whole time the engine is running, so they are knowable.
In general, though, a coil (or an alternator) will only make a certain maximum voltage when feeding in to a certain maximum resistance.
So, you may have a 50,000 volt coil, but if the above conditions (especially spark gap) are right, it will never reach that voltage, because the spark will initiate at a lower voltage, and then current will flow at an even lower voltage.
If conditions are different - more resistive mixture, wider plug gap, etc., the voltage may rise higher.
On the other hand, if you have a leaky spark plug wire, the insulation may hold current in at lower voltages, but start to leak at higher voltages, so under low load, with less spark resistance, the engine will run fine, but when you press the gas, spark resistance will increase, voltage will increase, surpassing the resistance of the insulation flaw, and bleed out, without ever rising high enough to initiate a spark.
You can often see this if you hook the car up to an oscilloscope.
All of that being said, newer (post-mid-1970s) cars with much wider gaps and much longer maintenance intervals generally have higher voltage coils to be able to run well even with excessively worn spark plug electrodes and giant gaps.
- Eric
edit: Damn. Mattax posted before me. And better.
