What Does the Ignition Coil Do?
Discounting for the present the battery and spark plug and a host of interesting ancilliary pieces such as the stop switch, an ignition system has three major important parts. One part amplifies the energy coming into the system so that the voltage going out is high enough to bridge the spark plug's 0.030-0.040" air gap. Another part times when this output is delivered to the plug. A third part switches, that is, conducts electricity to turn the system on and off at the appropriate times. So, amplifier, timer, and switcher. Critical functions.
The amplifer in the ignition system is the ignition coil. Many folks refer to it as simply "the coil," but for one thing an ignition coil isn't the only wire coil on your vehicle, but one of many, including the alternator, starter, and others. For another thing, an ignition coil is actually made up of two coils. Just like a model train transformer, an ignition coil has a primary winding (or coil) and a secondary winding (or coil). The primary winding is the half of the ignition coil that connects to the battery, the secondary winding is the half that connects to the spark plug. In between, the aforementioend amplification takes place to convert the battery's low voltage to the high voltage needed by the spark plug. The actual conversion process is called "mutual induction.". It's rather technical and not all that useful to our discussion.
What is a "High Performance" Ignition Coil?
Ignition coils being production items, no manufacturer puts one on their vehicle that is more powerful than necessary, so typically they are constructed with a ratio of primary to secondary windings of, say, 50:1 (example only). That is, the primary's thick wire is coiled onto itself, say, 150 turns while the secondary's thinner wire has 50 times as many turns (7500, in this example), resulting in an increase in voltage, just as in a transformer. The two windings are wrapped one around the other, concentrically, with the primary winding being on the outside, usually. (Why "usually" is not important, but some ignition coils are built the other way, with the primary winding in the middle instead of on the outside.) The center of the ignition coil, around which the two windings are wrapped, is a steel core. This core gives the magnetic field that appears inside the ignition coil a concentration point, which strengthens the field, improving ignition coil performance.
The traditional high performance ignition coil is one that has a different windings turn ratio than stock. So instead of the 50:1 mentioned above, the ratio might be 70:1. That is, the secondary winding has many more turns than the stock ignition coil, resulting in a higher voltage output. This costs money of course, so some makers of high performance ignition coils take a different tack, and get that 70:1 ratio by reducing the primary turns instead of increasing the secondary turns. This is legitimate, and the result is exactly the same, except less money is spent, the coil doesn't end up being larger, no larger shipping boxes are needed, etc. But as with everything, there is no free lunch. The primary winding is connected to the battery, remember, and being smaller than stock means less resistance than stock, which means more electrical current, more loading of the ignition parts connected on the battery side -- most notably the points -- and of course, more heat and wear. Metallic contact points can withstand this added strain, though with accelerated wear, but the electronic boxes that replaced the points on later machines' electronic ignitions cannot, quickly overheating and failing. This is the main reason the sellers of high performance ignition coils offer two main choices, low-ohm and high-ohm models. The low ohm is suitable for points ignitions, the higher for electronic ignitions. This matter of primary resistance is also why fitting automotive ignition coils to motorcycles isn't always a good idea. Automotive ignition coils, though rated for higher voltage, have supplemental protective circuits supporting them and preventing the vehicle's electronics from overheating despite the ignition coils very low ohms primary resistance.
How Does a High Performance Ignition Coil Help Performance?
A high performance ignition coil helps engine performance four important ways. First, the higher voltage allows for a larger spark plug gap, which results in a more robust initial flame kernal at the start of combustion. The result is a real-world engine torque increase. Second, having more voltage on tap means the voltage required to bridge the spark plug gap gets there faster, leaving less time for voltage diversion through the spark plug's inevitable carbon deposits. Third, the higher voltage potential creates a stronger "push" on the electrical stream to the plug, resulting in increased electrical current, i.e. more energy, more snap. Fourth, with more voltage available, there is more in reserve for non-standard situations such as two-up plus camping gear and trailer while going uphill on poor fuel on a hot day with too low tire pressures and a 20 mph headwind. :-)
Why So Much Voltage?
This last thought brings us to the subject of how much voltage is commonly used by the spark plug. The fact is the voltage needed to bridge the plug's gap is not constant but always changing, and is nowhere near the level of the ignition coil's voltage potential. That is, a 30,000 volt ignition coil virtually never fires its spark plug at 30,000 volts, but more often at 5,000-15,000. How can this be, and why the extra then? When a spark plug is about to fire, what happens is the air inside its gap is of course not conductive and must be made so. It is actually temporarily made conductive, and this is called by a fancy name that has to do with atoms and such, "ionization.". It simply means that the air is prepared to flow electricity. A pretty mysterious thing, this. Think of it as the air molecules getting so heated and excited by the rapidly building plug voltage that the result is voltage can flow through this agitated air to jump the plug's gap. How much voltage is required to make ionization happen depends on several things ranging from the amount of cylinder compression to how worn the plug's electrodes are. But in round numbers you will be safe to think 5,000 volts at idle. As soon as the throttle is used however this goes up, and if the transmission is put into gear then load comes into the picture and the requirement goes up even more. So let's settle on about 15,000 volts for a bike in a state of cruise. Now go uphill and the requirement increases. Be in too high a gear for conditions and it increases. Yank the throttle open hard and it increaes. Go downhill and it decreases, downshift to a lower geat and it decreases. So the actual voltage is all over the place while the bike is being ridden, and higher voltage ensures there is always enough and that it gets there fast.
What About Modern Bikes?
An interesting thing has happened in recent years. You now see high performance ignition coils being found on, and sold for use on, only older, non-fuel injected vehicles. Vintage carbed bikes, in other words. High performance ignition coils are not needed on modern vehicles, not because the factories have started putting better coils on their bikes, but because they are fuel injected. The main reason vintage vehicles benefit so much from better ignition coils is because their carburetion is heavily emissions spec and full of other fuel/air compromises that make carburetion far less than perfect. What a high output coil does in these cases is compensate by its superior ability to ignite less than perfect mixtures. That's right. Better ignition coils make up for poorer carburetion. It also works in reverse. Better carburetion makes fewer demands on an engine's ignition system (this is in fact one of the benefits of a properly rebuilt carburetor -- not only is carburetion good, combustion improves through more efficient ignition utilization too, as a by-product). The thing is, today's engines carburet so flawlessly due to fuel injection that mongo coils are no longer needed.
The Stick Coil Controversy
In fact, manufacturers are today installing weaker ignition coils than ever before in powersports history, not stronger ones. What do you think "stick" coils, or as they are known in the auto world, "coil over plug" ignition coils are? Wimpy, minimalist, cost-down ignition coils, that's what! Seriously. And in their usual way, the Internet forums have everyone convinced retrofitting stick coils to vintage bikes is an advantage! But it is far from that, a disadvantage in fact. Modern engines get by with weaker ignition coils because fueling is now virtually spot-on and no longer compromised. Stronger coils are not necessary.
One last thing. Without even changing your ignition coils you can do quite a bit to "freshen up" and improve your bike's ignition performance. For one thing, doing something about the common 2-3 volt drop that is present in most vintage motorcycles between the battery and ignition coil. This means at the least, ensuring good contact at the connectors, the keyswitch, and the engine stop switch (many if not most street bikes wire their coils through the stop switch). Some folks have even gone so far as to install a relay to shuttle voltage more directly to the coil and thus recover some of that lost voltage. It makes a difference.
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