This article has been reproduced with the permission of Ross Farnham. It was first published by Ross in 1998 http//sdsefi.com/techcomb.htm
Inductive Discharge Coils – Ignition spark for motorcycle is accomplished by the iginiton coil, coils have 2 sets of windings, a primary and a secondary. A typical coil will have around 250 turns of wire on the primary and about 25,000 on the secondary for a ratio of 100 to 1. The secondary section often uses an iron core to increase its inductance. Coil resistance on the primary will be from .3 to .5 ohms usually and on the secondary, between 5000 and 12,000 ohms. The inductance and resistance of the coil will determine how quickly a coil can be charged and discharged.
A transistor is used to switch the current flow off and on in the primary coil. When the transistor is switched on, current rapidly builds from 0 to a maximum value determined by the coil inductance and resistance. This current flow induces a magnetic field within the primary. When the current is turned off, this magnetic field collapses which cuts the windings of the secondary coil and induces a high voltage surge.
Output voltage is determined by the rate of field collapse and the windings ratio between primary and secondary. Because the path to ground for the current involves the spark gap, initial resistance is extremely high. This allows the voltage to build to a high value until it gets high enough to jump the plug gap. The difference must be high enough to first ionize the gas between the electrodes. The ionized gas creates a conductive path for the current to flow, at this point the arc jumps and current flow is established.
If only 10,000 volts are required to jump a plug gap under a given condition, that will be the maximum delivered. It is also important to note that the spark duration is determined by coil inductance and total resistance of the circuit, plus spark plug gap. Most inductive discharge systems have a spark duration of between 1 and 2 milliseconds.
When cylinder pressure increases, the voltage required to jump the plug gap increases. The second problem on high performance engines with high rev limits, is that there is less time to charge the coil with increasing rpm, high rpm and high output puts greater demands on the ignition system.
Coil Charge Time and Saturation – The time it takes to charge the coil or bring the current to maximum in the primary windings is called charge time. Input voltage and coil resistance are the main parameters relating to charge time, when the current has reached its maximum value in the primary, it is said to be fully saturated.
If current is applied longer than the time needed to fully saturate the primary, energy is wasted and there is nothing to be gained. If the current is cut off before saturation is achieved, the maximum spark energy available will be reduced.
Coils require charge times of between 2.1 and 6 milliseconds. Obviously, a coil requiring 6 milliseconds to saturate would be unsuitable on a high revving engine as there is not 6 milliseconds available to charge it between discharges at high rpm. For this reason, most racing coils have low primary resistances between .3 and .7 ohms and are fully saturated in less than 3 milliseconds permiting full coil output at very high rpms.
Capacitive Discharge Ignition – On very high output engines, an inductive discharge coil is inadequate to supply spark at high rpm and high cylinder pressures. A CD ignition or CDI is used to reduce charge times. The MSD system is very popular worldwide.
In normal inductive discharge coils, only 12-14 volts is available from the battery to charge the primary. The CDI charges capacitors to store a high voltage kick to fire to the primary side, putting between 30 and 500 volts onto the primary windings which reduces the charge time substantially. A coil that would take 3 milliseconds to become fully saturated with 12 volts is now fully saturated in less than 1 with a CDI. The same engine now will be able to turn twice the RPM and experience a major increase in cylinder pressure before encountering misfire.
Some CDIs also include a multispark function where more than 1 spark is generated after the first spark. This improves ignition probability besides the high rpm coil saturation advantages and a greater resistance to plug fouling.
Ignition Wires (Spark Leads) – The purpose of the ignition wires is to conduct the maximum coil output energy to the spark plugs with a minimum amount of radiated electromagnetic interference (EMI) and radio frequency interference (RFI). There are 3 basic types of conductors used in racing applications: carbon string, solid and spiral wound. Most production engines come equipped with carbon string. The solid core types are used exclusively for racing, mainly with carbureted or non-computer controlled engines because they offer no EMI or RFI suppression. They generally have a low resistance stainless steel conductor. These types are rapidly losing favor, even in racing circles.
The carbon string type is the most common and work just fine in racing applications. The conductor is usually a carbon impregnated fiberglass multistrand. Suppression qualities are fine with resistances in the 5K to 10K ohms per foot. They are cheap and reliable for 2 to 5 years usually, then they may start to break down and should be replaced. High voltage racing ignitions will likely hasten their demise. Dynatech makes low priced wire set which works well in performance applications.
Ignition Wires (Spark Leads) – The spiral wound type is probably the best type for any application. The better brands offer excellent suppression, relatively low resistance and don’t really wear out. Construction quality and choice of material vary widely between brands. NGK makes low priced wire sets which work well in performance applications.
Some amount of resistance is required along with proper construction to achieve high suppression levels. Resistance is also important to avoid damaging some types of coils and amplifiers due to flyback and coil harmonics. Beware of wires claiming to have very low resistance. These CANNOT have good suppression qualities.
Beware of any ignition wires claiming to increase hp. Ignition wires CANNOT increase hp. As long as the wires that you have are allowing the spark to jump the gap properly, installing a set of $200 wires is strictly a waste of money.
Lately, some truly “magic” wires have come onto the market claiming to not only increase power but also to shorten the spark duration from milliseconds to nanoseconds. As previously mentioned, spark duration is determined primarily by coil inductance and coil resistance so these wires CANNOT shorten the spark duration by the amount claimed. The wire resistance has a minimal effect on discharge time because of the high voltage involved. A very short duration spark is in fact detrimental to ignition because of lower probability.
These same wires claim to increase flame front propagation rates and the ability to ignite over-rich fuel mixtures for more power. Once ignited, the mixture undergoes a flagregation process and that the progression rate of the flame front is totally independent of the spark. As previously mentioned most gasolines will not ignite nor burn at air fuel ratios richer than 10 to 1, period, and that maximum power is actually achieved at around 12 / 13 to 1 AFR so the claim also has no basis in fact.
These wires use a braided metal shield over the main conductor which is grounded to the chassis. This arrangement offers poor suppression because it does not cover the entire conductor. Any energy leaking out of the main conductor by induction is actually wasted to ground and will not make it to the spark plug. These wires also have very low resistance which as mentioned can have a detrimental effect on coils and ignition amplifiers due to severe flyback effects which are normally damped by circuit resistance.
Other claims for these wires include current flows of up to 1000 amps. The current flow in the ignition circuit is determined by the coil construction and drive circuits, not by the ignition wires. Most ignition systems are current limited to between 5 and 15 amps. The most powerful race systems rarely exceed 30 amps. To flow current at 1000 amps, you would require #0 welding cable for the ignition system!
Spark Plugs – The final part in the ignition system is the spark plug itself. The average plug consists of steel shell which threads into the cylinder head, a ceramic insulator, an iron or copper core leading to a nickel or platinum center electrode and a ground electrode of similar material. The spark jumps between the center and ground electrode. Certain special application plugs may have multiple ground electrodes. Different heat ranges are available depending on application. For constant high power applications, a colder than stock plug is usually selected to keep internal temperatures within limits.
Again, many “trick” plugs come onto the market from time to time expounding the virtues of their incredible new design, usually offering more hp of course. Split electrode plugs are a waste of money because the spark will only jump to one of the electrodes at a time in any case.
You will find that most reputable engine builders for racing use standard NGK, Bosch or Champion plugs with a standard electrode setup. A properly selected, standard plug will easily last 25,000 miles of hard use in most engines. A platinum tipped plug will easily last twice as long on most engines. There is no rocket science here, modern spark plugs coupled to modern ignition systems in a modern engine are extremely cheap and reliable, even on race engines, a $2, off the shelf, NGK plug will work just fine.