Ignition coils are elementary devices . . . or at least they used to be. A good old-fashioned coil works like a transformer. It takes the 12 to 16 volts available from the battery and transforms it into 12,000, 20,000, or even 40,000-plus volts, or whatever is necessary to jump the gap at the spark plug. Simple enough, but it can become a bit more convoluted (we'll look at the complexities later).
First, let's consider what's inside a common coil: Coils have three posts - a large center tower, which is the connection for the secondary wire to the distributor cap and two smaller threaded posts for the primary wire connections. The primary wires are typically marked positive (+) or "Batt" and negative (-) or "Dist." It is possible to reverse the polarity and the car will still run, but coil output will be reduced dramatically. Internally, the coil has an iron core. The primary wires are wrapped around this core. The ratio of "turns" of primary and secondary wire surrounding the core determines the amount of step-up power (between battery voltage and output voltage) the coil has. Because the step-up from the relatively low-battery voltage to the output voltage is high, the primary "turns" are small, while the secondary "turns" are considerable.
The most common way to describe a coil is by secondary voltage. The bigger the number the stronger the spark, correct? Maybe not. The massive voltage numbers tacked on to some coils are simply theoretical. Worse yet, high secondary voltage figures should not be used to evaluate the performance of a coil. It's the current or amperage that does the major work in the ignition system, not the voltage.
Secondary voltage numbers are almost always used as a coil comparison benchmark. There's a good reason for that: As mentioned, the secondary voltage numbers can end up "big." And in the mind set of most people, bigger is usually better, especially when it comes to spark. In most cases, advertised secondary voltage figures are numbers that are obtained in a laboratory, but with no real loads placed upon the coil. In the real world, the 50,000, 60,000, and higher voltage figures may well be impossible to reach, simply because most of today's ignition components, such as distributor caps, rotors, and wires, are only capable of handling 35,000 to 45,000 volts.
Turn and Burn
While that might sound confusing, there are some answers to the coil dilemma. Here's how it works: As mentioned previously, coils are designed with different internal "turn ratios" (sometimes called winding ratios). A coil with one form of winding might make the engine easy to start. But at the top end (high-rpm range), the coil won't function properly. Another coil winding ratio might be best suited for use at high engine rpm, but it won't work well to fire the spark when starting the engine. Because of this, most coils are a design of compromise.
In the end, the real job of the coil is to supply sufficient voltage to the spark plug to jump the gap. This is technically called "arc-over" voltage. An excess of arc-over voltage in a coil (high advertised voltage number) usually means that it has fewer internal "turns" on the secondary coil windings. Fewer turns on the secondary coil windings usually means the coil will produce less spark current. It is the spark current and the duration of the spark that ignite the air-fuel ratio in the combustion chamber - not the voltage!
Of course, there's a catch (isn't there always?). When the spark plug gap is increased, so is the need for a larger arc over voltage number. Because of this, each and every "combination" might require a slightly different coil specification.
Which Coil Is Right for You?
There is no universally "correct" coil. Instead, the coil should be matched precisely to the job. The idea is to purchase a coil that is recommended by the manufacturer to work with your particular ignition system (according to Mallory, especially important with CD ignition systems), and today, one of the newest coil configurations you'll come across is the "E coil" assembly.
According to industry experts, coils have always had to compromise voltage output against current output. More voltage increases the initial ionization of the spark plug gap, but lowers the amount of current or heat that follows across the plug gap. When a coil is designed to produce more current, the voltage output generally suffers, which taxes the ease of ionizing the gap. In order to accomplish a combination of high voltage and current, the latest coils incorporate a special "E-core" winding design. This is an efficient design that more closely resembles the windings in a true transformer. In this system, less loss occurs during the transfer of electricity due to the closed core of the coil.
Given this technology, an "E" style coil produces spark with both high voltage and current. Another benefit is the coils run extremely cool, even at high rpm. This is due to the efficiency of the design, as well as the large laminations (Laminations are the heat sink-style "fins" on the outside of the coil body). Does this new coil design technology work? Most certainly. The following is a comparison between Mallory's well-proven conventional canister coil (PN 29216) and their latest Promaster E Coil (PN 30440):
Promaster E Coil
Primary Resistance 0.7 ohms 0.5 ohms Secondary Resistance 8.9K ohms 8.9K ohms Maximum Voltage 51,000 volts 51,000 volts Inductance 6.6 mH 3.56 mH Turns Ratio 99:1 99:1 Peak Current 190 mA 260 mA Spark Duration 400 uS 250 uS Note: Spark tests were done per SAE J973 using a HYFIRE part number 685 for both coil assemblies.
As you can see, the E-coil design produces the same maximum voltage, but the more important peak current numbers are much higher. This means you can really have your cake and eat it too. The newest coils are less of a compromise than older designs, and allow good starting and strong top end (high-rpm) performance.
So What Does This All Mean?
Companies such as Mallory, MSD, and others all offer a wide choice of coils. In all cases, the common advice from manufacturers is to match the coil to the application and to the ignition system. The old process of bolting on the coil with the biggest voltage number simply doesn't work. Consult the ignition manufacturer. You won't regret it.