Ignition system

coil produces 20,000 volts and it takes about 7,000 volts to jump the spark plug, so where does the 13,000 volt go?

Losses in the spark plug wire and the spark plug itself. The wires are resistance wires to lower the Rf signal produced that causes static in the radio and cause electronic engine control problems. The typical spark plug wire has about 1000 ohms or resistance per foot. There is also a resistor in the spark plug that helps reduce Rf signals, too.

It just follows along with the other volts.

When the coil is triggered, the voltage rise in the secondary winding begins. This rise is not instantaneous. When the voltage gets to the point that it can overcome the TOTAL resistance in the circuit, current flows (the spark plug fires). That’s as high as the voltage ever gets…The coil may be capable of producing 20,000 volts or more, but once current begins to flow, the voltage climbs no higher.

The higher the cylinder pressure (measured in PSI) the more resistance the plug gap has. At wide open throttle, maximum cylinder pressure, much higher voltage is needed to fire a plug than is needed in a high vacuum state…As plugs age and their gaps widen, voltage demand increases. Also, as plugs age, the porcelain insulator begins to break down allowing a small amount of current to flow to ground through this path. This bleeds off voltage and the voltage may never reach a high enough value to fire the plug. A classic high speed misfire…

When you say coil produces 20,000 volts, do you mean the max it can produce? Coil (secondary circuit) will produce what’s needed, no more. Less resistance in circuit, less voltage will be required. Voltage has to overcome rotor-to-cap contact point gap, resistance in plug wires, and air gap (spark plug gap). Resistance of plug gap is dependent on 2 factors: fuel mixture richness (leaner the mix, higher the

resistance); and combustion chamber pressure just before combustuion (higher the pressure, higher the resistance). Also, and I could be wrong about this, but since secondary winding is so long and so skinny, even though it’s copper, it has appreciable resistance. Longer the conductor, higher the resistance; skinnier the conductor; higher the resistance. Not to be nosey, but why do you ask ? Please post back. Thanks! KS