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.
Ignition coils do not operate the same way as power transformers, since the input is not sinusoidal. Inductors (coils) are current-law devices, that is the voltage induced is proportional to the instantaneous change in current. V=di/dt * L. The primary takes current through the coil and the magnetic field saturates once the points have been closed long enough. At that point no more energy can be stored in the coil and the rest of the energy ends up as heat. When the points open, the magnetic field collapses because the current went away. Since the points are open, all the energy has to flow out through the secondary side (ignoring what is stored in the condenser, that is just there to keep the points from burning out). However, there is a gap in the secondary circuit, which is the spark plug. All that energy wants to get out so the voltage builds until the spark plug arcs across, and then it all dissipates and the cycle starts again when the points close.
Coils are rated for voltage in what is the maximum they will throw, but in reality the system runs at whatever the spark plug makes it.
Resistor plugs and wires surpress interference, but they also serve another two important functions. They raise the voltage across the plug, and they also slow down the release of energy from the coil and make a longer duration spark. Once there is enough current flowing across the spark gap to ionize the air, the resistance of the gap drops rapidly. The resistance of the wire or plug electrode only allows enough current to flow to keep the spark lit and it uses the energy stored in the coil in a more efficient manner.
Most standard ignition coil systems work best with ~5000 ohms resistance between the coil and the plug electrode.
Last edited by 4ontheFloor; 01/15/2008 9:46 AM.
