You may be surprised that there is quite a bit of confusion, myths, and advertising hype surrounding ignition coils. Almost daily we are in contact 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 the best ignition coil for their application. Choosing the correct coil characteristics to match your application is important. We will help you with some information in this article.
I 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 required 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".
NOTE: A transformer is used on magneto ignition systems. An ignition coil and transformer are NOT INTERCHANGEABLE! Please do not think you can use a transformer in place of an ignition coil as part of your vehicle's ignition system.
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 box 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 (quickly drawn, and not internally accurate) image, there are two separate windings inside of the coil. These windings are made from fine copper wire and there are 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. The 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, Mallory Unilite® module, GM HEI, or aftermarket 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. The available current output depends on the number of windings and coil design. The coil design and winding is also 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 (in the coil winding) with less turns ratio has a faster recovery time.
NOTE: Available current is NOT always the same as what is necessary to jump the spark plug gap and ignite the air/fuel mixture - this voltage is usually much less. I will describe more on this topic later.
The energy that is released to the distributor and out through the plug leads to the spark plugs is searching for a place to ground out, the same way lightning from the sky is seeking ground. Hopefully, this released energy finds the gap between the center electrode and ground strap of your spark plugs!
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 energy - release the energy when told - and then recover to do it all again. On a typical 8-cylinder engine the demand seems overwhelming if you actually look at the numbers.
First, we can look at 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
2,800 cycles ÷ 60 seconds = 46.67 cycles per/second
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) to reach and jump the spark plug gap. Also consider that the coil, when the engine is operating 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 a fine 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.
Coil failure is not as common as the guy behind the parts counter may like to claim. 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 fault (open circuit) 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, poor power supply, overheated, or abused by not being the correct coil to 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.
One last thing on coil failures. One of the engineering values of an ignition coil is its resistance. Further down this article you will see some comparison numbers. Ignition coils are often considered "internally resisted" when their resistance value exceeds 1.4 Ohms. The higher the resistance value of the coil, the hotter it is going to run - sometimes near 200°F (93.3°C)! This added heat can increase a coil's failure rate (more internally resisted coils are prone to failure versus standard coils). Another concern, NEVER use an internally resisted coil on applications using an ignition amplifier (Mallory Hyfire, Accel, Crane, MSD). Coils that overheat can cause serious damage to your vehicle!
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. One reason a manufacturer my provide incomplete or inaccurate results is that their coil is no different than the one of their competitors. The only difference is the box it came in, the label on it, and maybe the case color, finish, or design (round versus ribbed for example). This happens because there are only a handful of coil manufacturers, and many companies, whether it be the aftermarket or OEM, are buying their coils from one of these manufacturers and then enclosing the component in their own branded packaging. 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-capacitive discharge "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 P.I. 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 or 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 and others also offer higher resistance coils that are designed to work best when combined with a specific electronic trigger distributors (Mallory Unilite or their Magnetic Breakerless Ignition for example). 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 (93.3°C). The only benefit of an external resistance source 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 an inductive or capacitive ignition amplifier box.
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 will be. 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 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 to the secondary side of the coil. This design can improve efficiency of the coil (and further confuse you with the published numbers), and it also saves the manufacturer in costs through fewer required windings, and also in reducing the overall dimensions of this style coil.
Coil Output Energy
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 energy from the coil, but there is a line that you should not cross in an 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 sometimes present 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 must work harder and may not be able to do their job as well. This will also add to wear or various ignition components, and the system may fail to deliver the spark on each and every firing cycle. This failure can also occur 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 its reliability and accuracy, causing misfires and lost power.
It is a common misconception that advertised coil output is what we receive on each and every firing cycle in our engines. Advertised output ratings are always far from what actually occurs as the ignition system operates. You have to look at a few events in the engine that help decide secondary coil energy, the energy that actually reaches the spark plug. A few simple example include:
- Compression and cylinder pressure (how much is there, and is it amplified by aggressive camshaft profiles, a supercharger or turbocharger, nitrous oxide, or exotic fuels)
- Overall engine condition
- Type of ignition (breaker point, electronic, hall-effect, magnetic, is an amplifier in use, and if so, what type)
- Resistance in secondary ignition components
- Spark Plug Gap
One item that many people do not know (or refuse to believe due to the absurd advertising they have been exposed to) about ignition systems can be said in a relative simple sentence, "The spark energy that leaves the coil and reaches the spark plug, jumping the gap and igniting the fuel mixture, is only going to be enough to do just that, no matter the coil's output rating." What that means, is that it does not matter if you had a bazillion-volt coil on your vehicle, if it only took 10,000v, 20,000v, or 8,000v to jump the gap and ignite the fuel mixture, that is all you're going to get on that cycle. These voltage levels can be seen clearly when your vehicle is connected to an oscilloscope that is monitoring spark energy. There is a little more to it that that, because the type of trigger (coil driver or coil control) also plays a part in secondary energy output.
If you have a simple breaker point distributor on your vehicle, you will never see more than 20kv (20,000 volts) of secondary energy. Even if you have a dual point ignition, your output is still under 20kv. A dual point system simply allows for a longer rise time (charge time) on the primary side of the coil for more efficient upper RPM use versus a single point system. When we upgrade to an electronic ignition system (electronic trigger such as the GM HEI, Mallory, Accel, Crane, Pertronix) we are now looking at greater secondary output, better coil management, and more accurate triggering (telling the coil when to charge-release-recover). The Mallory Unilite for example has a rating of 32kv, and the GM HEI is even higher. This does NOT mean that each firing cycle is going to see those voltage levels, but as RPM, engine load, and other factors affect the need for greater energy, they are better qualified to do the job.
Why should I upgrade to electronic ignition and a high output coil if you are saying I'm stuck with low voltages anyway?
That is a common question, and the answer may seem vague or that I'm avoiding it, but I have already answered it. Reliability and accuracy! There are many times during the operation of our engine when the voltage demands increase to where we would lose power or efficiency without the better electronic components, and anyone who has driven an application with both a breaker point and then an electronic ignition feels the power difference, and knows they spend less at the gas station after upgrading to electronic ignition. Added engine load (towing, racing, a freeway on-ramp, even pulling away from a stop sign) increase the amount of energy required to "jump the gap". The quality of fuel, engine condition, altitude, and other factors will also affect the secondary voltage demands. Adding an ignition amplifier can benefit many applications as well, but I will discuss the benefits (and negatives) of using ignition amplifier units in a separate article.
In conclusion, the most expensive, flashiest-packaged, highest output rated coil is not always the best. Choose the coil that matches your use and application. Now that you know a $100.00 coil on a stock or mild application is going to do nothing but fondle your ego and look cool in the engine bay, it might actually cost you horsepower and engine efficiency. Choose wisely. I often suggest the choice of coil using non-technical means. Choose by resistance (to match the rest of your ignition), and by intended use (RPM, and where you are using the vehicle). Keeping it simple is often better than trying to weigh all the technical numbers. The manufacturers usually provide this information in their specifications.
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, and we have classified the systems in the chart below. This Ohm rating mentioned in the chart is PRIMARY RESISTANCE.
I like to tell people that coil selection is similar to buying shoes. You would not wear rubber flip-flops to climb Mt. Everest, and you would not be very comfortable wearing winter boots to cross the Mojave Desert in August. When you purchase an ignition coil you need to consider a few things, and you select your coil with a similar thought process as how you buy your shoes. Ask yourself:
- What is my intended use? (stock, street/strip, towing, racing/off-road)
- What is my intended RPM? (under 6,000, up to 8,000, over 10,000)
- What other ignition system components am I using? (amplifier box, timing controls)
- Where is the vehicle going to be operated - This will decide if an epoxy coil is a better choice for vibration protection? (street, off-road, marine)
||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. Common round coil for either breaker point (1.4 Ohms) or combination point/electronic (0.7 Ohms).
||Beyond 6,000 RPM
||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. A canister coil (0.7 Ohm) or preferred performance coil (0.6 - 0.75 Ohms).
|Electronic, Mallory Unilite or M.B.I.
||Up to 8,000 RPM
||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 allow you to choose the round or E-coil designs. Performance applications are recommended to use the Mallory Promaster series coils. If you do not want to use a ballast, use one of the internally resisted coils. *See Warning in Article Above*
|GM HEI and Ford TFI
||Up to 7,000 RPM
||GM HEI coils are offered in both internal (coil in cap) and external designs. Performance applications should consider a conversion kit that removes the coil-in-cap and allows the use of an external coil. For standard HEI ignitions (still using the HEI module), the coil values are 0.3 - 0.5 Ohms. The lower value is for higher RPM use. The Ford TFI coils are also offered with values between 0.3 - 0.5 Ohms.
|Inductive Amplifier Systems w/Electronic ignition
||Up to 7,500 RPM
||Inductive amplifiers (like Mallory Hyfire® III series units) have a 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.
A canister coil (0.7 Ohm) or preferred performance coil (0.6 - 0.7 Ohms). Inductive amplifiers manage the coil differently, so you can use a canister coil in these applications.
|CD (Capacitive Discharge) Amplifier Systems
||Up to 8,000+ RPM
||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 still use a high quality performance coil (not the round canister coils, no matter the brand) with a rating of 0.5 -0.7 Ohms .
|Pro-Series CD Amplifier Systems (Mallory Hyfire VII, Crane HI-6DSR)
||Up to 12,000 RPM
||These are a very powerful ignition systems that are to be used ONLY on racing and serious street/strip applications. Only one type of coil can keep up with them. Do not attempt to use an incompatible coil on these systems. Common coil values are in the neighborhood of 0.1 Ohms.