OK… my friend Sportster Paul will understand my motivation, but suffice to say for you, gentle reader, this time out the story is intended to be more cathartic than anything else.
Even though we’ll get into ignitions, it all starts out with a supporting player in points ignitions, the auto advance unit. I’ve always had a real attitude problem where those damn things are concerned, since eight or nine times out of 10, they are the real problem with points ignitions. For those not versed in antiquities, an advance unit is comprised of a metal plate with strategically-placed posts and holes, a couple of small coil springs attached to pivoting counterweights, cradling a so-called points cam. The theory at work in this design is the springs hold the weights in towards the center of the mechanism to effectively retard the timing for easy starting (usually by kickstarter) and once running above about 2500 rpm the weights are flung out by centrifugal force to advance the timing for higher rpm road work. Two issues; well, make it three. First, when new they are pretty trouble-free, so forgotten by owner and rider until… second, they wear out… then third, in that state, cause hard starting, variable timing, backfiring, misfires and other such issues to the point that the points ignition becomes more trouble than it’s worth. Especially for those who aren’t into care and maintenance and would rather just ride.
One other thing—can’t replace ‘em if they’re obsolete, or shouldn’t… if of inferior quality!
One other “other” thing! Two-strokes, bless their stinky, smoky hearts, don’t use advance units. These old strokers have ignitions that are plenty good enough for high performance and do not require anything like the intensive maintenance that’s come to be mythologically associated with points. Sure, rubbing blocks wear and the spring band on points can “bounce” off the points cam at five-digit red-lines… but it amounts to nothing compared to the agro of auto advance in four strokes!
This is the position—the only position—I’ll take against these old-school mechanical ignitions. Because aside from that, they have advantages of their own (not least, they will fire with a near-flat battery and a good kickstarter). Fact is, electronic and, more recently, digital ignitions have little or no performance advantage over properly maintained points, beyond being “maintenance free.” But…
To point out
When points close, current flows through the coil’s primary winding, magnetizes the coil core, and stores magnetic energy. When points open current flow is interrupted, causing the magnetic field to collapse. This rapid cut-off induces a large voltage in the coil secondary windings. The faster the rate, the higher the voltage. An inductor (a.k.a. ignition coil) resists this change in current. As current collapses, making that nice, high voltage to fire the spark plugs, the other end is also generating high voltage trying to suck electrons across the open points. If the voltage gets high enough, current flows across an arc, and will stop collapsing and no high voltage will be generated and points simply fry! This is where the condenser (which every other industry in the world calls a capacitor) plays its role. When the points are closed, the condenser is also discharged. As the points open and the coil tries to suck electrons, the condenser acts as a reservoir, providing an electrical cushion, until the points get far enough apart to prevent formation of an arc. It’s a balancing act… and essentially a switch.
A pointless exercise
Almost all ignition systems work like this, so what if you could replace mechanical contacts with a solid-state device that is not subject to wear? Solid-state device? Okay, okay—a transistor or a microchip. You turn it on, the resistance drops, and current flows (just like the points closing). When it’s turned off, the resistance goes up (way, way up), and virtually no current flows. Since we no longer need to slow down the voltage rise to allow time for the points to get out of the way, the coil current can be switched off much faster. This results in a faster cut-off and is a higher secondary voltage. Additionally, since this thing is a solid chunk of silicon, there is no opportunity for creating an arc. Some early electronic ignitions (most notably Japanese in the early 70’s) were actually hybrids that used points to control the timing and a transistor to switch the coil current. Although the points lasted much longer, the system was far from maintenance free.They were prone to dwell shift due to rubbing block wear, contact corrosion near marine environments, insufficient current to prevent oxidation of the contact, etc.
So, the next step was to create some form of non-contact sensor to generate the timing information. The big three are: magnetic, optical, and Hall effect triggering.
Trigger happy
Magnetic triggering has been used by virtually every manufacturer since the mid-70’s and is still widely used today. Typically, a bar of steel is wrapped with several hundred turns of fine wire on one end. A small magnet is attached to the other end, and this assembly is mounted in the distributor facing the distributor shaft. Where the point cam would normally be, a small-toothed “reluctor” wheel is attached. As the teeth approach the coil, the flux from the magnet is pulled in close to the bar. As the teeth move away, the flux springs back outward, inducing a voltage in the pickup coil, used to drive a high voltage/high current transistor that switches the coil current. Used mostly in cars, magnetic triggering has limited ability to sense teeth that are very close together, which is necessary to gain the positional accuracy required by modern engine management systems.
Optical triggering consists of an infrared LED (light emitting diode) facing a phototransistor separated by a small gap. Through this gap a slotted wheel passes which alternately blocks and unblocks the light, generating position information. Since light will pass through a very narrow slot, a high degree of positional accuracy can be obtained. It was the only viable alternative to magnetic back in the 1970’s when most of the aftermarket ignition companies were founded. It was attractive chiefly because a simple trigger wheel could be fabricated out of plastic or other household materials and the output required minimal signal conditioning, unlike magnetic.
A so-called Hall effect trigger consists of a wafer of silicon through which a current is passed. When a magnet is placed in proximity to the wafer, the current tends to bunch up on one side of the silicon. This concentration is amplified and detected, indicating the presence or absence of a magnetic field. The advantages are, since it is an integrated circuit, it can be made very small with a number of features at minimal cost. It exceeds all current automotive temperature specs, and its accuracy is unaffected even when covered in water or muck. Hall effect has become the overwhelming choice as sensor technology evolves into crank angle sensors. These typically are placed to read the starter gear teeth on the flywheel providing the high degree of positional accuracy required for advanced engine management systems. Hall effect sensors are also widely used to sense wheel spin for traction control and on anti-lock brake systems.
Coiled up
Let’s talk voltage first, since this is the main entrance for most people’s trip down the garden path. That hot-rod, hi-perf 60,000-volt coil you lust after is more than likely useless overkill! See, once the voltage has built up high enough to jump the plug gap, its job is basically done. After the plug fires, the voltage required to sustain the spark is much lower. Regardless of air-to-fuel ratios, plug gaps or most any other things associated, you’ll never need more than about 17,000 volts to initiate combustion.
But… three important terms to keep in mind: Secondary available voltage, required firing voltage, and reserve voltage. Secondary available voltage is what the secondary side (or high-voltage side) of the coil is capable of producing—say 30Kv. Required firing voltage is what it actually takes to jump the plug gap—perhaps 14Kv. Reserve voltage is the difference between the available and required voltage—16Kv (i.e., what’s left over). So what good is this reserve voltage? Well, as the spark plugs begin to wear and lose the sharp edges on the electrodes, the required firing voltage may go up by 1 or 2Kv. Inspect your plugs and wires lately? Burned or broken conductors, cracked wires, and the like may require an addition 3 to 4Kv to overcome the additional gap. A 30Kv coil is an insurance policy… no more, no less.
Getting wired
The three most common spark plug wire types are metal core, resistor core and spiral core. Metal core wires consist of stranded copper or stainless steel conductors. Resistor core is generally constructed with a filament impregnated with carbon or graphite particles, and looks like a pencil lead. Spiral core looks very similar to resistor core, but has a very fine wire wrapped spirally around the core.
Metal core wires for the most part are obsolete due to the interference they generate with other devices and systems… like radios. However, they are still found on motorcycle engines (when used in conjunction with a resistor spark plug cap), in part due to their ability to withstand vibration. They are also used in some race applications, such as with magneto ignitions.
Resistor core has been the most commonly used suppression-type wire. Its job is to slow the discharge rate and dampen the oscillations that occur on the secondary side of the ignition… reducing the tendency of the wire to act like a radiating antenna. The core is somewhat fragile and will erode.
Spiral core wire has become increasingly popular in the last several years. Its function is also to reduce radio frequency interference (RFI), but by means of inductive reactance. As current flows through the wire, the spiral windings appear inductive, which by now you know means it opposes a change in current, again slowing the discharge rate and subsequent oscillations, but does not convert as much energy to heat as does resistor core wire.
Remember, whatever those enticing ads in the funny papers might say about “bigger, fatter, better…” chances are you’re good to go with the type of wire your Harley’s ignition system came with unless you’ve changed that system substantially! Meaning, wires should match the system requirements, and well-made and durable are the desired outcomes of any “upgrade.”
Digitalis
No, not the herbs… rather, 21st century Harley-Davidson ignitions which are all true digital systems with crank “position” sensors (sometimes cam sensors) and all the trouble-free trappings. This makes them the best way to light the fires in your Twinkie, ever devised by The Motor Co. Not a thing really needs to be changed or enhanced, beyond the possible exception of iridium spark plugs. Yay!
I only lament that along with the lack of need comes lack of options. For as long as most of us can remember… and even beyond living memory… imperfect ignitions could be improved. From the days of the magneto, on through many iterations of “cone” motor Sportsters (till ’03) and Big Twins (Evo), to the end of that era. No more! Well, I’ll ask you, where’s the fun in that?
We’ve gone from “have to do it” to “can’t do it.” A chore and a choice… removed at one stroke? From mastery to mastered. From participant in an uneasy alliance with physics and mechanics to digitized, abject Orwellian servitude in the name of … what… progress… precision… performance? We’re sold those notions, but didn’t the EPA actually twist the arms of manufacturers to fight the dreaded “smog?” Gap, dwell, the skills to “fix it and limp home,” a faded memory to rival what it was to hand crank your car to life. The lore will be lost in time. Make no mistake, I wouldn’t want to go back to the way things were, and I’m not saying we should… but I could. Could you?
So, till next time, I’ll just leave you with this: 90 percent of electrical problems are fuel related and 90 percent of fuel problems are electrical. The other 10 percent are both! Welcome to my world!
