Drag racers are constantly searching for ways to either go faster down the quarter-mile or to make those quarter-mile blasts become more consistent. There are plenty of ways to make more horsepower when building an engine and there are plenty of ways to pursue greater consistency once the drivetrain is installed. However, not many drag racers are aware that both ETs and consistency can be influenced by choosing an aftermarket ignition coil with the right electrical characteristics to complement your aftermarket ignition system. Just choosing a high performance, high voltage coil isn't enough-you must also choose a coil with the proper electrical characteristics. You may be surprised to learn that through proper coil selection, you can actually custom tailor the spark inside the combustion chamber itself, tuning your car's quarter-mile performance in the process.
By choosing a coil for its electronic characteristics, you can actually tune your strip performance to produce better 60-foot times or better top speeds, thus shaving your ETs and making your runs more consistent. We will reveal the coil tuning secrets in this article known only to certain tightlipped pros. When you are done, you will benefit from the same advantages the pros enjoy when getting the last tenth of performance from their cars.
Ignition systems have not changed a whole lot in the last 50 years. Most systems rely upon a distributor and an ignition coil to create the necessary spark energy and time its delivery to the combustion chambers. Most racers know that the spark plugs fire the air/fuel mixture with anywhere from 15,000 to 30,000 volts. Since battery voltage is only 12 volts, you might wonder how you end up with tens of thousands of volts. The source of this increased voltage relies upon the principle of electromagnetism. Almost all stock ignition systems in use today store the spark energy for combustion in the electromagnetic field of the ignition coil. Ignition designs that use the electromagnetic field of an ignition coil to store spark energy are technically known as "Kettering" ignition systems, named after its original designer 75 years ago. Today, however, these designs are more commonly called inductive ignition systems.
Early scientists learned that if you coil many loops of electrical wire around a tube and send electricity through them, you will create a strong magnetic field. If you place another coil of wire in close proximity to the first coil, the magnetism created when you send current through the first coil will momentarily "induce" a proportional current and voltage in the second coil. Even though the two coils, each containing multiple loops of wire, are not physically connected to each other, electricity can be induced to flow in the secondary coil by the action of magnetism from the primary coil. The property of a coil that allows voltage to be induced is called inductance, and inductance is the principle upon which hundreds of millions of ignition systems have been built over the years.
TO EVERY THING, TURN, TURN, TURN
Ignition coils used in automotive ignition systems contain a primary coil which surrounds a secondary coil located inside it. These two coils are contained in a single housing, and most racers simply refer to them as "the coil." The loops of wire that form each of the two coils inside the housing are called "turns" by electrical engineers, and the ratio of turns between the primary and secondary coils is very important.
Even though only 12 volts is supplied to the primary coil, it is a phenomenon of electrical flow that when you break a high current circuit, a momentary voltage spike will occur. This explains why the ignition contacts on older "points"-style ignition systems will arc voltage as they open, causing the points to break down over time. A small condenser is placed in the circuit to "suck" this voltage spike into it, protecting the points from damage and increasing the durability of the ignition points.
Typical points ignition systems produce a 250-volt spike in the primary side of the ignition system when the points open. This 250-volt spike feeds the primary coil that then induces a corresponding voltage in the secondary coil. By winding more turns of wire on the secondary coil than on the primary coil, a voltage step-up can be produced. The degree of voltage step-up that can be induced in the ignition coil is generally proportional to the turns ratio. For example, if there are 100 secondary turns for every one primary turn, there will be a 100:1 turns ratio, causing a 100:1 voltage step-up ratio.
In the example above, two hundred fifty volts "in" will produce 25,000 volts "out" given a 100:1 turns ratio. In reality, a 100:1 turns ratio is quite typical of many brands of ignition coils on the market. You can increase the voltage step up even more by increasing the turns ratio, but there is a point where the turns ratio gets too big, and secondary voltage can actually go down. It is also important to note that as voltage output is increased, the current output is decreased. Further, as you increase the turns ratio, other electronic properties are affected, such as resistance, reactance, and impedance. Without getting into these properties, suffice it to say that to make the best high performance coil is not simply a matter of getting the turns ratio up as high as you can.
While electricity itself moves fast, it takes time for the changing magnetic fields in the coil to develop the full potential current and voltage. This is a way of saying that the induced voltage (stepped up voltage) does not develop instantaneously. To keep things simple, let's think of the coil as an energy storage device that can be "charged up" and "discharged" in a manner similar to a battery. It takes time for the coil to charge to its full potential, a condition we will call saturation. Similarly, it takes time for the coil to discharge some quantity of its electrical energy as it fires a spark plug.
The time that the ignition system gives to the coil for charge-up is called "dwell." With a points ignition system, dwell is fixed and is measured in terms of degrees of distributor rotation, typically 30 degrees for V8 engines, which is 60 degrees at the crankshaft. As engine speeds go up, the crank rotates faster and faster, and quite obviously it takes less time to spin through 60 degrees of crankshaft displacement. Therefore, the higher the engine speed, the less time is allowed for coil charge-up between spark firings for all inductive ignition systems.
All inductive ignition systems can eventually reach an engine RPM point where there is not enough dwell time for the coil to recharge before the next spark plug is to be fired. Exactly where this RPM point will be depends upon the type of ignition system used and the specific characteristics of the engine in question. When this RPM point is reached, there is insufficient initial spark energy to jump the spark gap and ionize the air/fuel mixture, and the coil will be unable to fire that cylinder, generating a miss. If you increase RPM further, additional misses will be generated and eventually the ignition system will be unable to fire any combustion chamber. This is called "crash," and engine performance hits a wall.
The dynamics of this phenomenon are such that, typically, if one cylinder is skipped, all eight can be skipped. (See Chart 1 for more on this condition.) The engine literally shuts down and coasts as engine RPM drops. When the coil has sufficiently recharged it then refires, the engine comes back to life, and engine speed picks up again.
Further, the onset and frequency of crash is RPM sensitive. Put another way, the higher the engine RPM, the more your ignition system is likely to crash and the more frequently your engine will coast rather than put power to the rear wheels. Obviously, this is bad for consistency and low strip times!
DON'T DWELL ON IT
The shortcoming of inductive ignition systems that rely upon points is that the coil dwell time is not adjustable during operation. Once you set it at 30 degrees at the distributor (60 degrees at the crank), it cannot react to meet the needs of the engine in operation. In reality, 30 degrees of dwell is too long at low RPM and not long enough at high RPM for optimum operation.
At low RPM, 30 degrees of coil dwell time can be 2 or 3 times longer than really needed, causing excessive heat buildup in the ignition system and unnecessarily consuming electrical power from the alternator. This can cause as much as a 1 MPG loss in engine efficiency and can shorten the life of the electrical components in the ignition system.
At high RPM, on the other hand, 30 degrees of dwell is insufficient recharge time. This leads the engine to miss, producing a loss of power, and eventually causing the ignition system to crash, as described earlier.
What to do?
HEI IGNITION SYSTEMS
Because of these problems, engineers among the big three auto manufacturers introduced electronic ignitions in the 1970s which offered "smart" electronics. The ignition module in a GM HEI, for example, contains what is called a "dwell predictor circuit" that shortens coil recharge time at low RPM, say only 15 degrees, and lengthens it at higher RPM, say all the way up to 40 degrees. This staves off crash to a higher RPM, and raises the total spark voltage available to the plugs from the secondary side of the ignition coil.
However, even the electronic HEI ignition systems in use today are still Kettering inductive systems, and they will still suffer crash when used in high RPM racing applications.
No matter whether you have a points ignition or an electronic ignition, both are inductive, and all inductive ignition systems are sensitive to what is called roll off. Somewhere between 3000 and 3500 RPM, the input current to the primary begins to drop off because the recharge time is insufficient. As a result, the output voltage from the secondary to the spark gaps at the plugs begins to drop. As engine RPM continues to rise, output voltage continues to decline. Because it gets harder and harder to ionize the air gap at the spark plug as engine speed goes up, the decline in output voltage is exactly the opposite of what an engine needs in order to run effectively at high RPM.
Obviously, for drag racers, engine speeds routinely get into the 5000 to 7000 RPM range, so rolloff can be a real concern.
FAT, DUMB, AND HAPPY
If you are using an inductive ignition system, whether single points, dual points, single or multiple coils, and run at speeds above 3000 RPM, the chances are real that your engine is encountering spark rolloff, and is being starved of sufficient spark energy at elevated engine RPM. Because it happens at high engine speeds, rather than at idle, the intermittent misses are frequently undetected by the driver. You are unaware of the performance loss and therefore may not perceive the need to fix the problem. Perhaps you launch hard, pull strong in the midrange, but tail off in the last 100 yards. You chalk it up to poor engine breathing at the top end. It could well be rolloff and a poorly tuned ignition system.
Many racers replace the stock ignition system with a high performance aftermarket design as part of the high performance buildup of their race cars. There are many brands available in the aftermarket, all of which employ a variety of design philosophies, but one of the most common and most successful aftermarket ignition system is offered by Autotronic Controls Corporation in El Paso, TX, known as the MSD.
An MSD ignition is not a purely inductive ignition. It does not rely upon an ignition coil to store the full spark energy necessary for combustion. As a result, it is not limited by the design constraints inherent in a purely inductive system. This opens doors to the racer that were previously tightly shut.
Instead of using the coil to store the energy, the MSD design uses an electronic device called a capacitor charged to more than 450 volts, which then is discharged through the coil to feed the spark plugs. The capacitor can be "charged" far faster than a coil, and under ideal conditions will receive its full charge without encountering roll-off all the way to 15,000 RPM or so.
By discharging the capacitor into the coil, the function of the coil in the overall ignition system changes. It now becomes part of a special circuit that can resonate at a particular frequency. The details of resonant electronic circuits that employ coils and capacitors together can get quite complicated. Fortunately, it is unnecessary for the drag racer to understand such sophisticated electronics. All you need to know is that, when you go to an MSD capacitive discharge (CD) ignition system, a very good thing happens. Your coil becomes a tuning device for your ignition system, and by swapping in different coils with different levels of inductance, you can tune the actual spark in the combustion chamber to improve power and strip performance!
SPECIFIC PARTS AND RATINGS
Inductance is measured in a unit called the "Henry." Most automotive ignition coils, from purely stock to full-on racing units, are rated at between 4 and 10 thousandths of a Henry, called 4 to 10 milliHenrys ( abbreviated 4mH).
An MSD CD ignition system is sensitive to what is called "leakage inductance." Without going into the complicated theory behind this property, let us say that as the overall inductance of a coil changes, the leakage inductance changes too. Thus, by having an idea of the leakage inductance rating of different coils, you can select that coil with exactly the right leakage inductance to produce the best engine performance for your specific engine and racing application.
MSD offers four high performance ignition coils: coils: its line of traditional, canister-style Blaster 2 Coils, a drag race-only Pro Power coil, and a new line of HVC (High Voltage and Current) coils. All the Blaster coils have the same electronic and inductance characteristics, making your coil selection a simple matter: the Blaster or the ProPower. ACCEL and Mallory have produced a variety of coils over the years and recently Crane and Holley have entered the performance ignition scene, so there are many coils from which to choose.
BIG AND SHORT, SMALL AND LONG
Every race engine (and every street engine as well) is different from any other. Certain cylinders run hotter than others. Certain combustion chambers have more carbon build up than others. Some valve guides leak more oil than others, while some valves seal better than others. Carburetors are notorious for delivering nonlinear air/fuel ratios as engine speeds change, and all intake manifolds suffer from uneven mixture distribution to the cylinders. For these reasons, and many more, every race engine experiences slightly different combustion characteristics, even those built to the same specs. This is further complicated by the chassis dynamics and weight characteristics of each race car, which affect how the engine power is applied to the track. Combine all these factors with different driver styles, and you can see that every driver, engine, race car, and track is unique, requiring unique solutions to the ET and consistency challenge.
By tuning your MSD ignition system through coil selection, you can tune the shape of the spark delivered to your plugs. You can have high peak spark intensity but shorter spark duration, or lower peak intensity but longer duration. Because of the combustion chamber variables that make each engine unique from all others, certain engines will respond better with one type of spark while others will perform at their best with a different type of spark.
The key is to find exactly that type of spark that produces the best performance in your specific engine and your specific car.
COIL IMPACT ON SPARK SHAPE
Changing to a coil with a different leakage inductance rating, measured in mH, will change the spark duration and peak spark intensity. A low leakage inductance coil will provide a faster spark with a higher peak intensity and will be capable of running tthrough a higher engine RPM. A high leakage inductance coil will offer a longer duration spark and a lower peak spark current and will work better with engines that do not rev as highly by helping at lower engine speeds. These generalities will hold true when such coils are combined with CD ignition systems, but will have only a minor effect when mated with standard inductive ignition designs.
Using the two common MSD ignition systems (the MSD 6 and the MSD 7AL) as an example, the street/strip MSD 6 units offer a tuning range that allows you to as much as double the spark current between a high leakage inductance and a low leakage inductance coil. With the MSD 7AL series ignition, the coil tuning range opens up to a whopping tenfold ratio! By simply changing from a high to low leakage inductance coil, or vice versa, you can have a tremendous influence on spark intensity and duration, resulting in altered combustion and measurable effects on the drag strip.
HOW TO DO IT
Before tuning your MSD ignition with various ignition coils, start by opening up your spark gap as far as you can go. Generally speaking, the wider the spark gap, the better the performance of the engine. Bigger gaps initiate the flame kernel better, and start a more robust flame front in the combustion chamber for better power production.
Progressively open the gap until your elapsed times start to fall a little, then back off the gap a smidge. This should get your gap to its optimum opening. Be aware, by the way, that your gap could possibly already be at its optimum, or might even be too wide, so be prepared if you see your times drop rather than improve as you open the gap. This is not too likely, but is possible.
The track surface and weather can change as you make repeated treks down the strip, and these changes could potentially fool you into making the wrong conclusions about your spark gap. Therefore, use the following A-B-A test procedure.
Make three runs down the strip, checking them for consistency. If you are satisfied with the consistency of each run, open the plug gap and make three more passes. Average the results of each group of three passes and use these averages to determine your results. Part way through your testing, set the gap back to an earlier setting and see if your results go back to what they were before. They may not match exactly, but if the trend is in the correct direction, you can rely on your testing results.
With your optimum spark plug gap finalized, you can now begin to tune your track performance through coil selection. Bring a high leakage inductance and a low leakage inductance coil with you to the track. consult the ignition companies catalog, Web site or tech line to narrow your choices of coils to test with. An example would be between MSDs Blaster 2 Coil, PN 8202, with an inductance of 8mH and 100:1 turns ratio, compared to its HVC Pro Power, PN 8251, with 1mH and 85:1 turns ratio.
While youre investigating coil specifications for your tests, be sure to check out what coils are recommended for your ignition. Not all coils are compatible with every ignition! For instance, the MSD Pro Power Coil, PN 8201, cannot be used with a 6 Series Ignition and will result in poor performance and possible damage to the ignition (due to the coils low resistance and the drivers of the ignition). Also, many ignition manufacturers design coils specifically for their ignition components.
Make at least three runs to get to the point where your times are consistent and you are satisfied with them, and then switch coils. Make three more passes and average the results. Then reinstall the first coil again and make three more passes. Do this in order to ensure that weather and track factors have not affected your time slips. The results of your third group of three passes should sum up your first three passes.
Assuming these passes are once again consistent and you are satisfied with the results, compare the performance of the car with the two coils. Which one is better? If the high-leakage inductance coil produces better track performance, swap in a third coil with even higher leakage inductance. Conversely, if the low leakage inductance coil works better, swap in a third coil with even lower leakage inductance. As you continue raising or lowering the leakage inductance, you will eventually reach a point of optimum track performance.
GETTING A RISE
There is one last aspect of coil selection that you should be aware of and that is coil rise time. Once the signal is given to discharge the ignition energy through the coil to the combustion chambers, certain coils retard the passage of the spark energy more than others. The amount of retardation that takes place is partially affected by each coil's rise time, and the lower the leakage inductance the faster the rise time and the less the retardation of the spark energy.
Thus, as you change the shape of the spark in the combustion chamber by altering the leakage inductance of your coil, you are also introducing a change in the ignition timing itself. If your engine responds to a higher leakage inductance coil, the spark will be delivered to the plugs later. On the other hand, if your engine responds to a lower leakage inductance coil, the spark will be delivered sooner. The difference in timing due to the retarding effect of different coils can be as much as a few degrees from one extreme to another.
Putting it all together, track testing for optimum spark gap, optimum leakage inductance, and optimum coil rise time can result in solving your specific problems and improving your competitiveness. You can make the choice to improve your launches and 60-foot times via a lower peak intensity but longer duration spark. While you might lose some speed at the top end, you could potentially gain a better ET by launching harder and carrying that speed the rest of the way down the track.
On the other hand, if you have excess HP at launch and spin the tires, you may want to sacrifice some bottom end power. In this case, you might choose a higher peak intensity coil, but one with a shorter duration. This type of coil can reduce or eliminate tire spin at launch, helping you launch harder and with more consistency, and offer improved top end performance to boot.
The objective in all cases is to produce a lower ET and greater consistency. You can actually tune your ignition system to overcome shortcomings in chassis hookup or top end breathing and can reduce the variability in your track performance and improve your consistency this way too.
As with everything else when it comes to racing, small improvements can often lead to big results. Drag racing is an arena where hundredths and even thousandths of a second can make the difference between victory and also-ran. Coil tuning can play as important a role in quarter-mile performance as dialing in the carb jets and bumping the ignition timing around. Most people have never realized that the coil can be a potent tuning tool when combined with an MSD or other high-performance ignition. Spend some time at the track to find out what your engine wants, and like the pros, the benefits will be yours.