So it's time to replace the coil. That one at the auto parts store is sure to do the trick. Because it's yellow and it has a shiny sticker that says it's a supermegavoltfireballthunderspark coil and like most performance parts, it will make you go faster, if only because it lightens your wallet by so much.
Okay, Okay - Let's talk voltage first, since this is the main entrance for most people's trip down the garden path.
Q: How much voltage do you need? A: Enough for a hot spark. Q: How much is that? A: .........uh, isn't more better?
Now that some of you have been insulted, let's try to put some real numbers to the problem. Suppose you have a motor with 9:1 compression and an air/fuel mixture of 14.7:1. It's a nice cool day and your driving down the coast about 25 feet above sea level. You've just installed a new cap and rotor, a fresh set of spark plugs gapped at .035", and a new set of plug wires. For good measure, you just changed the oil and washed the car, so it's really running sweet.
So how much voltage do you need?
Oh, about 12,000 volts (12Kv).
What about when you nail it to pass the Good Sam going 35 in the 65 zone? Okay, maybe 14Kv.
But that monster coil you just installed is still putting out 60,000 volts to the plugs just like it says in the magazine ad, right? Nope, sorry. See, once the voltage has built up high enough to jump the plug gap, its job is basically done. After the plug fires, the voltage required to sustain the arc is much lower than the firing voltage. At this point, what's important is to shove as much current across the gap as possible.
When you get home you discover your annual smog check is due today. So you run out and turn the mixture screws to lean out the motor. Firing voltage just went up to 14Kv. But the motor won't run right because there are fewer fuel molecules to interact with the spark. So you open up the plug gaps to .045". Firing voltage just went up again, maybe to 16 or 17Kv.
So just how do you get 60,000 volts (or even half that) to the plugs? You don't, except maybe in the lab. You see, high voltage is a strange beast. It tends to crawl over things or go through things you'd expect would stop it. If you kept opening the plug gaps, you'd find it increasingly difficult to get the voltage to the plug. At about 25KV, it would much rather run down the outside of the plug though the oil and dirt left from your fingerprints when you screwed it in or arcing through the tower of you new coil.
Does this mean 60,000 volts is complete fiction? Well, that depends on your view of reality. If you string together two car batteries in series (24 volts) and fire the coil a few times with no load attached, and it makes 60Kv just before it dies, is that coil not in fact capable of producing 60,000 volts?
One thing you will never see on a coil box or ad is "This coil is capable of producing up to 30,000 volts when measured in accordance with SAE specification XYZ " Even more enlightening would be a graph of how the coil voltage falls off with rpm. Of course this would be death in the marketplace. Can you imagine the shiny yellow coil promising nothing short of the ability to arc weld, next to the one that says "well, I start out at 30,000 volts and go down from there - buy me". Which would you choose?
So by now the question in your mind might be "If it takes so little voltage to fire the plugs, why do I need even a 30Kv coil?" Three important terms to keep in mind: Secondary Available Voltage, Required Firing Voltage, and Reserve Voltage. Secondary Available Voltage is what the secondary side (or high voltage side) of the coil is capable of producing - say 30Kv. Required Firing Voltage is what it actually takes to jump the plug gap - perhaps 14Kv. Reserve Voltage is the difference between the Available and Required voltage - 16Kv (i.e., what's left over).
So what good is this reserve voltage? Well, as the spark plugs begin to wear and loose the sharp edges on the electrodes, the required firing voltage may go up by 1 or 2Kv. Likewise for the cap and rotor. Inspected your plug wires lately? Burned or broken conductors, usually by the crimp area will still function, but may require an addition 3 to 4Kv to overcome the additional gap.
Therefore, one could assert that the primary benefit of a high voltage coil is to increase the service interval of the ignition components, keeping the vehicle in tune longer. This statement will no doubt bring howls from the turbonitrousblowninjected crowd, but that's not really the focus here. Most people's experience is with passenger cars (ouch! It still hurts from when someone called my high school ride that - a nice '69 Dart with fat tires, loud exhaust, and really cool stripes), that are unlikely to be substantially affected by a performance coil.
