The ignition system is one of those entities that is overlooked, taken for granted and passed by until something goes wrong with it. Consequently, many enthusiasts don't fully appreciate the down-in-the-trenches job it does day in and day out. Further, many do not realize when it is time to step up the power of the ignition system.
Ignition: The Prime ObjectiveThe prime objective of the ignition system is to energize the spark plug to the point that a spark jumps the gap between the electrode and the strap. It must also do this at the correct moment in the cycling of the engine. Typically the plug fires near the end of the compression stroke just before TDC. The timing of the spark event is a critical piece of the tuning puzzle and the exact moment depends on many variables.
The ignition system can be divided into two sectors. The transition point of this division is the coil and the focus is on the type of energy in the system. From the coil back to the battery the system is low voltage. This portion, called the primary side, is made up of the ignition key circuit, the battery and the coil input wire.
The coil is where all the magic happens. The 12 volts or so input by the battery are supercharged to 20,000 volts or more then released. The hot wire on the coil is where the secondary side of the system starts.
From the coil the next link in the chain is the distributor, which does what its name implies; sees that the 20,000 volts are shot from the coil to the correct spark plug. The distributor houses a triggering device, which rotates via a shaft in the distributor that is driven by gears on the camshaft. The distributor shaft is how the distributor gauges the cycling of the engine. The triggering device itself can be mechanical points, magnetic pickups or a Hall-effect device.
Points complete the electronic circuit (firing the 20,000 volts) by physical contact. Magnetic pickups use magnetic fields to initiate the firing of the system. However, in many cases the magnetic pickup array is incorporated into the crankshaft and these setups are known as external triggers because they are not contained within the distributor body. The external magnetic pickup uses crankshaft rotation to determine the cycling of the engine. The Hall-effect device is the least common approach however they can be used in a number of different ways.
We have explained the "how" of trigger but it is the "when" that is most critical; especially when engine speed, load and power modifications come into play. If the spark hits too early during the compression stroke the air/fuel mixture may not be properly atomized, which can result in detonation (also called pre-ignition) because the mixture burns unevenly and too soon before TDC. This early burn can lead to engine failure by putting too much pressure on the piston and rod while the piston is still moving upward in the cylinder
Firing the plug too late means there is less time for combustion and a partially combusted mixture means only partially realized power. Also, the timing demands of the ignition change as engine speeds increase so distributors use a mechanical advance that cleverly uses centrifugal force to change the timing dynamics of the system.
The Distributor Is Dead; Long Live Distributorless Ignition SystemsThe modern ignition is a distributorless ignition system (DIS) that was pioneered in a performance application on the 1986-87 Buick Grand National.
The distributor was ditched for a number of reasons both economical and performance related. The distributor shaft rotates via helical cut gears on the camshaft, which takes its cues from chain driven gears on the crankshaft. The meshing of these gear teeth is not conducive to precise timing. Factor in the exceedingly high redlines of modern performance engines and any shortcomings are further magnified.