® OEM ignitions compared

Powersports vehicles have employed a number of very different ignition systems. Here we'll explore the six most common OEM systems and compare them.

But first, to do that effectively, we have to set in place a filter or gauge. This filter consists of two things that are true of all ignitions, things that contrastingly describe their strengths and weaknesses. Far from arbitrary, this grid considers what really makes ignitions different. For you M.E.E.s, I'm not talking about millijoules or Lenz, Faraday, Mesh or Fleming rules. Just two very basic but important thumbprints. The first filter is the ignition system's power source. An ignition system's source is important because DC-powered ignitions behave one way and AC systems in another way. So source is important. The other defining ignition system metric, and one equally vital, is whether the ignition fires its spark plug by rising its primary or by collapsing it. These two things: (1) source and (2) rising or collapsing field, like nothing else define ignitions. They are more important than output voltage, more important than brightly colored half inch thick plug wires, more important than space-age whiz-bang spark plugs, more important than famous names, more important than that buzzword, "electronic"; more important than any other ignition system metric. These two metrics are the arbitrators of what makes ignitions good, better and best. The following ignition comparisons of the six most common original equipment ignition systems are made using the grid of these two all-important ignition system design characteristics.

1) Magneto ignition. Magneto ignition has a certain charisma, a kind of reckless, "tough guy" personna. Think ratty, bobbed, open-piped iron Sportster jumped on four or five times by a sweaty, 20-something, wife-beater-clad Stallone look-alike before finally roaring to life. Whatever its reputation, ironically, magneto is actually one of the worst ignition systems on the planet. One reason is that it is AC-powered. AC-sourced ignition systems are inferior to DC-powered systems right off the bat. At low rpm they are very weak and in addition they are hyper-sensitive to point cleanliness and precise timing. The reason for these handicaps goes back to the source. At low rpm there is far less voltage available than at higher rpm. Magneto's timing sensitivity is considerable also, and it's because its AC source rythymically goes through a zero point, the midpoint in the AC sine wave. If the ignition timing is just a little bit off, the points open closer to this zero spot than normal, and the result is vastly reduced voltage and even weak or no spark. That's enough to make magneto not so great. But there's more. Magneto ignition is at an additional disadvantage because of its collapsing field firing modality. That is, its ignition coil is turned on before the ignition event, then turned off, and then the spark plug fires. Click-click-snap. Magneto fires on coil collapse. The resulting handful of milliseconds' slower buildup of voltage at the spark plug means some of the electricity gathering there (why it's called "rise") has time to bleed off through carbon deposits, resulting in reduced firing voltage at the plug. These two things: the first AC sourcing resulting in lower starting voltage and fussy points maintenance, and the second the more deposits-challenged collapsing field design, make magneto the lowest performing and least desirable of all ignition systems.

Magneto works like this. AC is magnetically generated by a magnet spinning next to the ignition coil's primary winding, saturating it. When the points open, the primary's magnetic field collapses, inducing the coil's secondary winding, and the spark plug fires. Again, the plug fires on coil collapse; magneto is a collapsing field ignition system.

2) Energy transfer ignition. You may never have heard of the energy transfer ignition system, probably because you know it by another, false, name. It is almost always mistaken for magneto. However, energy transfer is very different from and was actually developed to replace magneto. Sure, they're both AC-sourced so equally low powered and intolerant of dirt and inexact timing. However, energy transfer has an edge on magneto, because unlike magneto, energy transfer is not a collapsing field system. It's a rising field system. Where magneto works on collapse of its firstly built-up primary when the points open, energy transfer sources from a separate, third winding ("generator") and instantly rises the field in the coil's primary at the moment the points open, getting a huge head start in getting its energy to the spark plug. It's so much faster that the voltage has far less time to bleed off, resulting in less voltage waste and a stronger spark. Energy transfer is found on most vintage offroad bikes, scooters and ATVs, though the manufacturer seldom calls it out by name.

In the energy transfer ignition system, the source coil puts out voltage which loops through a ground circuit until the points open. When the points open, the ignition coil's primary is instantly saturated, which induces the secondary and the spark plug fires. So the energy transfer system is a rising field system, which alone makes it superior to magneto.

It's important to distinguish magneto from ET. The visual clue is magneto has only two windings, both of them in the ignition coil. There is no separate source coil; the coil's primary winding does double duty as both source and primary. Energy transfer on the other hand uses three windings: a stand-alone generator coil plus the two windings in the ignition coil. You will see these differences in their schematics as well as in the physical parts.

3) Capacitor discharge ignition. Capacitor discharge ignition (CDI) is the king of ignition systems and different from all others. Two things make it unique. First, CDI runs its ignition coil on much higher source voltage than any other ignition system. In some examples the primary is fed as much as 400 volts. Second, before sending it to the coil, the system stores this energy in a capacitor. A capacitor is a lot like a battery; they both are voltage storage devices. But unlike a battery which releases its energy gradually, a capacitor gives up its charge all at once. A big dump. This massive burst into the ignition coil makes for a blasting effect on ignition. Then, add to these two unique CDI characteristics a third, the fact that the system is wired up for a rising field function and not the slower collapsing field, and you have a superlative ignition system with all the best technology and characteristics. And it's still the best, close to a hundred years after its invention.

Though developed in the 1930s, it wasn't until some brainiac 1950s car hot-rodders began playing with it -- vacuum tubes and all -- that CDI became widely known. The funny thing is, the rodders discovered CDI held no real advantage for them; they didn't have any problems CDI could solve. No lean fueling, no weak ignition power sources, and most importantly, no problem with plug fouling. However, this couldn't be said by snowmobilers. When the snowmobile guys discovered it, CDI was a godsend, a boon. It practically made the snowmobile a viable machine. With such dramatic spark plug power, CDI easily overcame the snowmobiler's major headache: the two-stroke engine's spark-plug-wasting oil-burning deposits. Those CDI-ignited snowmobile plugs fired no matter what! Just the thing for crudely-built pull-start vehicles in freezing temps far from camp. Probably even saved some lives! This is CDI's heritage and its real claim to fame.

Here's how CDI works. First, the source, whether AC or DC (both exist), fills up the system's capacitor with hundreds of volts. Next, the timer (pulser) tells the switcher (ignition module or "CDI box") to connect the capacitor to the ignition coil's primary winding, allowing the capacitor to instantly flood the ignition coil. This immediately induces the coil's secondary winding, and the spark plug throws a killer spark. Note that the plug fires on ignition coil buildup. In other words, CDI is a rising field system.

There is an often mistaken notion that CDI's advantage is its high spark plug voltage. Like so much "common wisdom", this is false. Does CDI often have high plug voltage potential? Yes. But that is not what makes it laugh at fouled spark plugs. It's CDI's super-fast voltage rise time that makes it impervious to plug deposits. Its spark plug sees arc-ready voltage in a third of the time of any other system.

However, this can also be a problem. CDI's biggest strength is also its weakness. While the absolute best in offroad applications, as soon you take CDI away from the offroad vehicle and attempt to make it an ignition suitable for a modern, emissions-controlled street vehicle, its star status quickly diminishes. CDI's one weakness is a spark duration that is one-third that of other ignitions. This is no big deal on abundantly-rich snowmobiles and high performance motocrossers. But on cleaner-burning road-going engines CDI is a liability. Lean mixtures don't do well with short duration sparks. To overcome this, engineers eventually found a way to make CDI fire multiple times, effectively lengthening its ultra-short spark duration and by that clever innovation making CDI suitable for leaner-burning street bikes. All successful examples of CDI in road use are this multi-fire version.

4) Kettering ignition. Kettering ignition, often called "battery/point", is nothing short of monumental in its history, longevity, and all-around usefulness. Charles Kettering, the founder of Dayton Electrical Laboratories Corp. (Delco) invented the system that bears his name in 1908. Yup, before the sinking of the Titanic, a mere handful of years after the first experimental radio transmission, and the very same year Henry Ford started production of the Model T. Think of it! Eons ago! Kettering is battery-powered and collapsing field. Battery (DC) power makes Kettering better than an AC system, but collapsing field keeps it from being a superlative ignition. It's a middling performer: better than magneto and energy transfer, but not as good as CDI. However, because it is both battery-powered and a collapsing field design, Kettering is famous for being a power hog. As rpm increase, plug voltage does not increase accordingly. Kettering struggles to keep up and voltage to the ignition coil drops precariously low. Therefore, dirty wiring connectors or a less than perfect battery can make Kettering perform below its best. However, due to its simplicity and ruggedness, virtually all vintage four-stroke Japanese road bikes have Kettering ignition, either in its original contact points form or its later transistorized form.

Kettering works like this. First, the battery saturates the ignition coil. Second, the points open, disconnecting the coil from power, causing the ignition coil's primary magnetic field to collapse. This collapse mutually induces the ignition coil's secondary winding, firing the spark plug. Kettering fires its spark plug on coil collapse; it's a collapsing field system.

5) Transistorized Kettering. Transistorized Kettering is often called "electronic" or "inductive" ignition. The points were replaced with a transistor, which did the switching duty, and by a another new part, the electronic trigger, or pulser, which took over timing. Two parts were necessary because standard Kettering's contact points in fact did two jobs. They switched and timed simultaneously. Other than this one change, there is absolutely no difference between early transistorized Kettering and standard points-equipped Kettering. Thus all of the original Kettering's characteristics are present in the newer transistorized Kettering: its voltage-hogging nature, its resulting sensitivity to voltage drop, and its collapsing field design -- all are the same in transistorized Kettering. Even standard Kettering's ignition coil's tendancy to overheat (due to its being always-on) is the same in transistorized Kettering. Moreover, advertising notwithstanding, transistorized Kettering offers no gain in engine power, no better starting, none of that. The only advantage is maintenance. And this is true of the aftermarket Kettering systems too, such as the Dyna S.

Transistorized Kettering's transistor is inside its own little box, called the control module, ignitor, or spark unit. Like all alternative-to-points systems, transistorized Kettering's transistor box is merely a switcher that connects and disconnects the voltage source to (and from) the ignition coil.

Here's how transistorized Kettering works. Battery power is sent to the control module (ignitor), whose transistor switches it on to the ignition coil. The ignition coil is then live, just like in standard Kettering. The system's pulser then tells the control module's transistor to disconnect the coil. The primary field's sudden collapse mutually induces the secondary coil, and the spark plug fires. Note that transistorized Kettering, just as with standard Kettering, fires when the ignition coil's field collapses; transistorized Kettering is a collapsing field system. Transistorized Kettering is presently the most widely used ignition system in the world on production street-going vehicles.

But be careful to not confuse transistorized Kettering with CDI. They are completely different systems, though many folks speak of them as if they were the same. Among a half-dozen other differences, Kettering is a collapsing field system, CDI a rising field system.

6) Transistorized magneto. As electronics became more widespread, the magneto system described at the beginning of this article lost its points, just as standard Kettering did. You can now find transistorized magneto on smaller-engined powersports products such as utility vehicles, scooters, smaller offroad vehicles, stationary engines such as portable generators, and lawn equipment. Everything is the same between transistorized magneto and standard magneto except the points are replaced by those two solid state parts, the pulser and the transistor.

I hope this comparison of powersports' six most common ignition system types enlightened your thinking, provided you with proper terminology, and gave you a sense of history as it relates to the powersports product.


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