You may be surprised that there is quite a bit of confusion, myths, and advertising hype surrounding ignition coils. We are in contact almost every day with people who have made bad choices in coil selection, do not understand how a coil releases energy to the spark plugs, and who need to be educated (and sometimes de-programmed) to what is truly best for their application. Choosing the correct coil characteristics to match your application is important. We will help you with some information in this article.
At the bottom of this page is a selection guide with links to more product details, pricing, and where you can place your order.
I guess we should first explain the function of the ignition coil and how it works. The ignition coil is the component of the ignition system that is able to generate the high voltages needed to create the spark that ignites the compressed air/fuel mixture in your engine. A simple definition is that the ignition coil is nothing more than a special type of electrical transformer. A transformer is an electromagnetic device that couples AC (alternating current) energy from one circuit to another. But, know that a transformer cannot transfer DC (direct current) energy. When you use a transformer (ignition coil) on an automotive or marine application it can be used either as a "simple transformer" or as a combination "energy storage unit/transformer".
This sounds confusing if you do not understand how your ignition works, but to keep this as an easily understandable article, consider that on most all applications the coil is a combination storage unit/transformer. If you have added a CD (Capacitive Discharge) amplifier to your ignition system your coil would now work as a transformer only. The coil must only convert the energy from the CD unit into a form of energy that will fire the spark plugs.
As you can see by the image at right there are two separate windings inside of the coil. These windings are made from fine copper wire and you have primary side windings and a secondary side windings. The secondary side has hundreds of times more turns of wire, or windings, than the primary side.
On a conventional ignition system when you have the key ON there is voltage that is going through the positive (primary) side of the coil. This primary side of the coil stores the energy in the form of a magnetic field. When this charging/storage circuit is broken by the trigger in the distributor (breaker points, Unilite® module, or Hyfire® ignition amplifier) there is a surge of energy, or rise, that is created when the magnetic field on the primary side collapses. All of this energy engulfs the secondary side of the coil and creates a high voltage current, with available output depending on the number of windings and coil design. This is also known as the turns ratio. It is the turns ratio and wire size which dictate the efficiency and output of the coil. For example, a large wire diameter (windings) with less turns ratio has a faster recovery time. This energy is released to the distributor and out through the plug leads to the spark plugs, searching for a place to ground out, the same way lightning from the sky is seeking ground.
The coil must do this repeatedly, and reliably, keeping up with the demands of the ignition system and engine. The higher the RPM, the faster the coil must build - release - and recover, then do it all again. On a typical 8-cylinder the demand seems overwhelming if you actually look at the numbers.
First imagine an average 700 RPM idle speed. The coil must cycle itself 2800 times per minute, or 46.67 times per second:
700 RPM x 8 cylinder (÷2) = 2800 cycles per/minute
Watch how it changes when you are cruising down the freeway at 2,500 RPM:
2,500 x 8 (÷2) = 10,000 cycles per/minute
10,000 ÷ 60 = 166.67 cycles per/second
At a 7,500 RPM while racing or at a shift point in a performance engine:
7,500 x 8 (÷2) = 30,000 cycles per/minute
30,000 ÷ 60 = 500 cycles per/second
When you take this information into account you may not realize that the spark energy is fighting its way through many resistance areas (cap, rotor, rotor gap, plug wires, spark plug resistance, spark plug gap, and cylinder pressure). Also consider that the coil under load or acceleration is going to be working harder than at a steady-state RPM. There will be more load on the engine during acceleration which will increase the cylinder pressures inside the engine. This requires the coil to work that much harder. To top that, I almost forgot another important factor ...heat. As the coil windings get hotter they cannot do their job as efficiently. That is why most coil housings are filled with oil as a coolant. Newer design epoxy coils have a different design with a different type of copper winding and a special epoxy surrounding the windings that is used as a sealer and insulator.
When a coil fails it is usually due to a hot spot on one or more locations of the windings that creates a failed connection, or lost continuity. This can be found by using an ohm meter and looking for an "open" test result, or failed continuity. If the coil has been overworked, had to fight off bad connections or shorts in the electrical wiring, overheated, or just plain abused by not being strong enough handle what you ask of it, you will see coil failure. If a new coil has a bad spot in the windings for example, it will fail fairly soon. If you have a coil that has been working fine for years and then dies, you should probably look elsewhere for the cause.
There are some common terminologies you should be familiar with regarding coil selection. But keep in mind that many coil manufacturers do not publish this data, over-advertise the data (lie), or confuse you by leaving data out of published reports. The main characteristics of a coil you should know are the primary inductance, primary resistance, turns ratio, secondary resistance, and lastly, the core configuration.
Since the energy first gets to the primary side of the coil, lets discuss the primary Inductance and primary Resistance.
The primary inductance (in a non-CD ignition system) determines the amount of stored energy that will be available in relation to the supplied current, the useable RPM range of the coil, and also affects the rise time of the spark voltage. The higher the inductance value, the more available the output energy, but it comes with the price of a slower rise time. On a CD ignition though, the primary inductance determines the peak primary current (and resulting secondary current). A coil with a lower inductance means a higher peak primary current and faster rise time.
The primary resistance (in a non-CD ignition system) effects the maximum current that coil can achieve. The greater the primary resistance, the lower the current and available energy. When using a CD ignition the primary resistance is not as important, but it causes parasitic energy losses as the peak current goes up. The primary resistance is calculated based upon the resistance of the coil's primary windings in conjunction with any external resistance (ballast resister, resistance wire) if used. There are some coils that are manufactured specifically with a very high resistance value. Motorcycles are one example that use high resistance coils, but Mallory also makes higher resistance coils that are designed to work best when combined with a Mallory electronic distributors. By including a higher resistance within the coil (known as self-ballasted or internally resisted coils) an external resistance component is no longer necessary. On internally resisted coils you will find that they do operate at a much higher temperature due to the resistance load being handled inside the coil housing. This can exceed 200° F. The only benefit of an external resistance is that you can fine tune it, though you never need to worry about this on most applications. NOTE: Never use any internally resisted coil with a CD amplifier.
Moving on to the Turns Ratio, this is the ratio between the number of primary turns (a turn being one 360° wrap around the magnetic core of the coil) and the number of secondary turns within the coil. The turns ratio affects the maximum peak output voltage, maximum spark current, and has a limited affect on the rise time. The greater the turns ratio the higher the output voltage and slower the rise time, but the lower turns ratio offers a greater spark gap current.
The Secondary Resistance affects secondary current and overall efficiency, with also a limited affect on rise time. The greater the secondary resistance the slower the rise time. But more importantly is that the greater secondary resistance lowers the maximum spark gap current, and drastically affects the coil's overall efficiency.
The last stop for coil terminology is the Core Configuration. The definition of core configuration is that of how the magnetic path inside the coil is designed. The most common path used is the "I" core, or Rod Core. This is as simple as what you see in the image above, but some modern coils are using a more continuous magnetic path configuration. Using a "Double-C" core or an "EI" core design (also known as E-core) improves the efficiency of how much primary stored energy is transferred the the secondary. This design can improve efficiency of the coil (and confuse you with the published numbers), and it also saves the manufacturer in costs through fewer required windings, and in reducing the overall dimensions of this style coil.
You now know that the secondary output energy which a coil delivers to the spark plugs is controlled and managed by a variety of factors, and that matching the coil to the application is very important. Other components affect the output from the coil, but there is a line that you should not cross to attempt to get more energy from your coil. To be able to do its job the coil must not run too hot (NOTE: Internally resisted coils run considerably hotter than normal ignition coils), have enough time to rise-fire-recover, and be triggered accurately and efficiently. But, changing a few other components can have sometimes positive results, while at other times cause lost power and efficiency.
For example, opening up the plug gap is a beneficial tweak to many engines in that it allows the spark kernel to be larger, offering a more complete burn. But, if the gap is too big the coil and other secondary ignition components cannot do their job as well. This also occurs with too much gap between the rotor tip and terminal in the cap, and with plug wires that have too much resistance. All of these issues can make the coil work harder than intended, or affect the accuracy, causing misfires and lost power.
You should first know what type of ignition you are going to use. There are coils available that can serve double duty (CD ignition and conventional breaker point or electronic), but you should choose the coil responsibly. When you select a coil for your application, do you know if it is going to be the best choice, or if it is going to work right at all? This is tough to answer sometimes, but we have come up with a simple chart to help you decide what is best. Note that there are variations to the rule, and if you want to know more, read the Ignition Coil article in our Tech Zone section. It offers the same information as above, but expands on many more details. If you have more questions, use the live chat link above our our contact form.
IGNITION TYPE
OPERATING RANGE
DESCRIPTION
COIL #s
Breaker Point
up to 6,000 RPM
A point ignition typically wants a coil with higher inductance (7-10 mH) and a primary current of no more than 4 amps, which means a primary resistance of about 3 ohms.
You will lose a bit of low end performance so as to gain upper RPM capability, but on class restricted applications that spend all their time at upper RPM levels you will need a slightly different coil.
Right off, the electronic trigger gives better overall performance and allows you to get away with a lower resistance coil and gain energy at both low and high RPMs. There are numerous coil offerings that fill the needs of this application. Space restrictions choose the round or E-coil designs. Performance applications will use the Promaster series. Don't want to use a ballast, use one of the internally resisted versions.
Inductive amplifiers (like Mallory Hyfire® III series units) have current-limiting coil driver in them. This allows the module to control low RPM voltages and still allow the unit to provide higher current output at upper RPMs. This is similar to GM HEI, Ford TFI, and other designs that use a current-limiter.
CD ignitions thrive on maximum spark output, by getting the spark there faster and more powerful than anything comparable. A good CD box will work the coil pretty hard, especially the good multi-spark units. A basic street application should use the MAL-29440 or MAL-30440. A more powerful street/strip application should use the MAL-29625 or MAL-30625 for their overall better performance.
This is a very powerful ignition that is to be used ONLY on racing and serious street/strip applications. Only one coil can keep up with it. Do not use this coil on anything other than with this series ignition amplifier.
I guess we should first explain the function of the ignition coil and how it works. The ignition coil is the component of the ignition system that is able to generate the high voltages needed to create the spark that ignites the compressed air/fuel mixture in your engine. A simple definition is that the ignition coil is nothing more than a special type of electrical transformer. A transformer is an electromagnetic device that couples AC (alternating current) energy from one circuit to another. But, know that a transformer cannot transfer DC (direct current) energy. When you use a transformer (ignition coil) on an automotive or marine application it can be used either as a "simple transformer" or as a combination "energy storage unit/transformer".
