ALL ABOUT IGNITIONS
When the points close, current begins flowing thru the coil primary. This current flow magnetizes the coil core, which acts as a concentrator, storing magnetic energy. As the core becomes more magnetized, magnetic field lines (called flux) spread out and envelop the windings. As long as current continues to flow, this flux will exist.
About this time, the points open and current flow is interrupted, causing the magnetic field to collapse. This rapid "cutting" of the windings by the flux is what induces a large voltage on the coil output. The faster the rate of this "cutting", the higher the voltage. You may remember this effect from back when your primary means of transportation was a bicycle. If you had a headlight you probably noticed the faster you pedaled, the brighter the light became. This was due to the flux from the permanent magnets in the generator cutting the windings faster and inducing a higher voltage across the lamp.
Getting back to ignitions - one of the fundamental characteristics of an inductor (which is a fancy name for ignition coil) is that it opposes a change in current. How? As the flux is collapsing back into the core; 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, an arc will form and bad things will happen.
Like what? Well since current is now flowing across the arc, the flux will stop collapsing and no high voltage will be generated. Further, the points will very shortly look like a pair of charcoal briquettes (if you can manage to keep the engine running long enough). This is where the condenser comes into play. The condenser (which every other industry in the world calls a capacitor) is that little metal cylinder mounted in the distributor with one wire connected to the points.
Like the inductor, it too has a fundamental characteristic. Namely, to oppose a change in voltage, and here's how. When the points are closed, the wire from the condenser is also grounded keeping it discharged. As the points open and the coil tries to suck electrons, the condenser acts as a reservoir, providing a source until the points have time to get far enough apart to prevent formation of an arc. So now all the problems are solved, right?
Well, not quite. Like most things in life, there are tradeoffs. If you were to squeeze a big, fat condenser into the distributor, the points would last a lifetime. This is because the condenser would supply so much current, the voltage would never get high enough to arc across the points. The down side is the magnetic field around the coil secondary winding would collapse so slowly, very little if any high voltage would be produced. Also, the car wouldn't run, which would tend to extend the life of the points.
Going in the other direction, we know that having no condenser causes a current flow in the form of an arc, with the same net effect on secondary voltage. Therefore, choosing a condenser means deciding how much secondary voltage you need and how much point burn you can live with.
From the previous section, you can see that the points are no more than a switch, which grounds and un-grounds the minus side of the coil. You undoubtedly have begun to suspect the major benefit of electronic ignition. Namely, replacement of mechanical contacts with a solid-state device that is not subject to wear. Solid-state device? Okay, okay - a transistor. So how is this accomplished?
Forgetting for a moment how you control this device or the underlying physics behind it, think of it as a variable resistor. 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 collapse of the flux, creating a higher secondary voltage. Additionally, since this thing is a solid chunk of silicon, there is no opportunity for creating an arc (or the erosion that results from it).
Of course the technically elite will quickly point out that the voltage will rise high enough to exceed the breakdown voltage of the device. For this reason, most ignition systems limit the coil primary voltage to the 400 - 500 volt range. Point systems typically hover around 250 volts.
So this takes care of all the problems? Not quite. The points not only interrupted current, but with assistance from the point cam, also controlled when to do it. Some early electronic ignitions (most notably Japanese vehicles of the early '70s) 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; dwell shift due to rubbing block wear, contact corrosion near marine environments, insufficient current to prevent oxidation of the contact, etc.
The next obvious 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. A fourth, called ECKO for Eddy Current Killed Oscillator (used by Lucas Electric) will be discussed, because sometimes it's fun to take long road into town.
Far and away the most popular technology has been the magnetic trigger. It has been used by virtually every auto manufacturer since the mid-seventies and is still widely used today. It's construction and operation is inherently simple: 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 wheel is attached. This is called a reluctor. As the teeth of the reluctor approach the coil assembly, 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. Sound familiar?
This voltage is then chopped / filtered / amplified and used to drive a high voltage / high current transistor that switches the coil current. It is a rugged, reliable system that holds up well in a high temperature, high vibration environment. Since it generates a signal without external power, it is especially easy to apply.
The magnetic sensor is gradually being phased out though. It 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 has seen very little use by automotive manufacturers (one or two years of the Nissan Sentra come to mind). The basic construction is an infrared LED (Light Emitting Diode) facing a phototransistor separated by a small gap. Thru this gap a slotted wheel passes which alternately blocks and un-blocks the light, generating position information. Since light will pass through a very narrow slot, a high degree of positional accuracy can be obtained. So why doesn't everybody use this method?
A couple of reasons, the optics of the LED and phototransistor must be kept fairly clean, particularly as the windows in the trigger wheel get smaller. Failure ranges from a subtle timing shift to complete inoperability. Also, LED's and phototransistors that are rated for the automotive temperature range are not available in low cost (required in cost sensitive applications).
Optical triggering has been used primarily by aftermarket ignition manufacturers. 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 Hall-Effect sensor consists of a wafer of silicon thru 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 of the Hall device are numerous. 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 underhood muck.
Hall-Effect triggering was widely used by Bosch on European spec vehicles since the late 1970's and was sporadically used in the U.S. as early as 1975. In the 1980's it became somewhat more prevalent, mainly on Chrysler imports. Ford was the first domestic manufacturer to embrace the technology with the advent of the TFI (Thick Film Integrated) ignition. Unfortunately, a good sensor technology was coupled with a marginal ignition module, as evidenced by the current class action lawsuit on behalf of owners of TFI equipped vehicles (not to worry though, Ford straightened this out with the TFI II).
Hall-Effect has since become the overwhelming choice for sensor technology as automotive manufactures migrate to 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 on anti-lock brake systems.
This is a technology that is still used today in the form of proximity sensors used in various commercial and industrial environments. Unfortunately, it didn't transition well into the automotive realm. Maybe it was just in the execution?
Basically it worked like this: a pickup with two coils was mounted in the distributor through which an oscillating current was passed. A plastic wheel was attached to the distributor shaft that contained very small iron dowel pins (one pin per cylinder). As the pins passed the pickup, an imbalance was caused in the pickup oscillation. This was sensed by the module (located elsewhere on the vehicle), which fired the coil.
To get the pickup to sense the pins, it had to be close, about .010"-.015". Unfortunately, the plastic rotor changed shape as it dried out from exposure to heat, causing the timing to be anybody's guess. It also had the nasty habit of cracking and flinging a dowel pin into the pickup. Since the pickup and module were tuned to work together, this meant replacing both. That was about $380 in 1972 dollars. Then there were the heat and vibration problems - but lets not be sadistic.
Lucas apparently learned the error of their ways for they began to stuff magnetic pickups in their distributors, and General Motors HEI modules in little black boxes and charge even more money for them.
So it's time to replace the coil. That one at the auto parts store is sure to do the trick. Because it's yellow and it has a shiny sticker that says it's a supermegavoltfireballthunderspark coil and like most performance parts, it will make you go faster, if only because it lightens your wallet by so much.
Okay, Okay - Let's talk voltage first, since this is the main entrance for most people's trip down the garden path.
Q: How much voltage do you need? A: Enough for a hot spark. Q: How much is that? A: .........uh, isn't more better?
Now that some of you have been insulted, let's try to put some real numbers to the problem. Suppose you have a motor with 9:1 compression and an air/fuel mixture of 14.7:1. It's a nice cool day and your driving down the coast about 25 feet above sea level. You've just installed a new cap and rotor, a fresh set of spark plugs gapped at .035", and a new set of plug wires. For good measure, you just changed the oil and washed the car, so it's really running sweet.
So how much voltage do you need?
Oh, about 12,000 volts (12Kv).
What about when you nail it to pass the Good Sam going 35 in the 65 zone? Okay, maybe 14Kv.
But that monster coil you just installed is still putting out 60,000 volts to the plugs just like it says in the magazine ad, right? Nope, sorry. 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 arc is much lower than the firing voltage. At this point, what's important is to shove as much current across the gap as possible.
When you get home you discover your annual smog check is due today. So you run out and turn the mixture screws to lean out the motor. Firing voltage just went up to 14Kv. But the motor won't run right because there are fewer fuel molecules to interact with the spark. So you open up the plug gaps to .045". Firing voltage just went up again, maybe to 16 or 17Kv.
So just how do you get 60,000 volts (or even half that) to the plugs? You don't, except maybe in the lab. You see, high voltage is a strange beast. It tends to crawl over things or go through things you'd expect would stop it. If you kept opening the plug gaps, you'd find it increasingly difficult to get the voltage to the plug. At about 25KV, it would much rather run down the outside of the plug though the oil and dirt left from your fingerprints when you screwed it in or arcing through the tower of you new coil.
Does this mean 60,000 volts is complete fiction? Well, that depends on your view of reality. If you string together two car batteries in series (24 volts) and fire the coil a few times with no load attached, and it makes 60Kv just before it dies, is that coil not in fact capable of producing 60,000 volts?
One thing you will never see on a coil box or ad is "This coil is capable of producing up to 30,000 volts when measured in accordance with SAE specification XYZ " Even more enlightening would be a graph of how the coil voltage falls off with rpm. Of course this would be death in the marketplace. Can you imagine the shiny yellow coil promising nothing short of the ability to arc weld, next to the one that says "well, I start out at 30,000 volts and go down from there - buy me". Which would you choose?
So by now the question in your mind might be "If it takes so little voltage to fire the plugs, why do I need even a 30Kv coil?" 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 loose the sharp edges on the electrodes, the required firing voltage may go up by 1 or 2Kv. Likewise for the cap and rotor. Inspected your plug wires lately? Burned or broken conductors, usually by the crimp area will still function, but may require an addition 3 to 4Kv to overcome the additional gap.
Therefore, one could assert that the primary benefit of a high voltage coil is to increase the service interval of the ignition components, keeping the vehicle in tune longer. This statement will no doubt bring howls from the turbonitrousblowninjected crowd, but that's not really the focus here. Most people's experience is with passenger cars (ouch! It still hurts from when someone called my high school ride that - a nice '69 Cutlass with fat tires, loud exhaust, and really cool stripes), that are unlikely to be substantially affected by a performance coil.
The three most common 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 communication systems. However, they are still found on some imports and 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. This has the effect of reducing the tendency of the wire to act like a radiating antenna. Its chief drawback is that the core is somewhat fragile and will erode open if nicked. This is commonly seen at the ends where the wire is stripped in order to attach the spark plug and distributor terminals. It also reduces the energy delivered to the plug somewhat, due to its inherent I2R losses.
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. Because this opposition to the current is in the form of "phony ohms", it does not convert as much energy to heat as does resistor core wire. This is primarily of benefit to some capacitive discharge ignition systems that have very high peak secondary currents.
So the choice is obvious, right? By now you probably can guess what the answer will be. Don't be misled by the cute display at the auto parts store that shows how much brighter the flashlight bulb is with the new SUPERWIRE than with the old, evil, low performance "stock" wire. This has little correlation to how the wire will actually work on the vehicle. Also, be aware that while there are some well made spiral core wires, there are also poorly made ones. In an effort to play the "who has the lowest resistance" game (as well as to save money), some manufacturers will put too few spiral wraps on the core which greatly reduces the suppression characteristics of the wire. It is also worth noting that some vehicles will not tolerate the increased RFI due to noise sensitivity of the control electronics.
A common situation is the customer who has put a set of these wires on a late model computer controlled vehicle. The increased interference is enough to cause the vehicle to run erratically. However, the customer is unwilling to remove the wires because he wants the "performance". At which time it's nearly impossible to resist asking whether the performance was better before or after installing the wires.
Just like speaker cables, spark plug wires have undergone the same marketing performance enhancements where big and fat is better. Now if there was some way to carry that over to other aspects of life...Anyway, what's up with these jumbo wires?
When only 7mm was available, life was simple. Then 8mm came along for "high performance", and that was ok. Then 8.8mm showed up, which is about the size of the pencil they give you in first grade, and that was what you needed for even better performance. Recently, some 12mm showed up, which is about the size of one of Clinton's cigars and looks like it must be used to light off the space shuttle.
If you're unfamiliar with the terminology, 7mm, 8mm, etc. refers to the diameter of the plug wire insulation. The larger the diameter the thicker the insulation, and hence the greater ability to contain the high voltage on the center conductor. This naturally leads to the argument that these fat wires will prevent arcing or leakage and deliver more power to you plugs. The fact is the break-over voltage of the spark plug is lower than the breakdown voltage of most average 7mm wires. When you see leakage, it usually is because the wire is old, cracked, ugly, and basically used up. Going to 8mm may increase the service interval since the thicker insulation will take longer to break down.
So should you put on a set of these wires? Absolutely - that's what keeps our economy going. Besides, they look really cool. If for nothing else, sometimes they have nice molded boots that don't fall off when you pull on the wire.
Bottom Line - as with any performance part, be conscious of the
new-parts effect. This is where the motor runs better not because the
new part is so great, but because the old part was so bad.