An ignition coil is like a bank account: You can only take out what you put in. Section (3) represents the coil/condenser oscillations and is the energy that still needs to be dissipated once the spark plug extinguishes. For example, if it takes 10 kV to light the plug and 1 kV to keep the plug lit, once the ignition coil is depleted to 999 volts the plug stops arcing. But there are still almost 1,000 volts in the coil that are looking to go to ground. The energy then races back and forth between the spark plug and the open primary circuit and eventually wears itself out, as represented by the diminishing peaks. A good ignition coil should be able to create three to five coil and condenser oscillations. Section (4) represents the leftover electricity that does not have enough energy to look for a ground path and remains in the coil. It is known as the coil build-up period.
Remember, the spark plug fires when either the ignition points break open or the module shuts off in an electronic system. Section (5) is the points-closed signal (or module-on signal) that is the beginning of the dwell period.
Ignition dwell describes a time period in distributor-cam degrees that the primary circuit is turned on and the ignition coil is being charged in anticipation of the next spark plug firing event.
Myth: Coils Don't Wear OutSince a coil has no moving parts, many think it cannot wear out, but that is false. Coils degrade over time and should be considered a consumable, especially on a high-horsepower engine with increased cylinder pressure. After enduring millions of charging and discharging cycles, a degraded coil will respond slower, often causing a high-speed misfire when there is not enough energy to keep the spark plug arcing. An aftermarket performance coil is designed to charge or store energy quicker than an OE design. For this reason a good performance coil is a worthwhile addition to any car crafter's ride. But choose the proper coil. A drag-race-style design is intended to charge very quickly for high engine rpm but will overheat and burn out if asked to run for more than a few minutes.
Plug Indexing: Need It Or Don't?Is indexing, or identifying the position of the spark plug electrode in the bore, worth any horsepower? Yes and no. Older, less efficient combustion-chamber designs with poor spark-plug locations and slow flame-expansion rates often reaped very good power gains from indexing the plugs. It created a more uniform ignition event on every cylinder. Modern, high-efficiency combustion chambers with a centralized spark-plug location seem to see less-if any-benefit from indexing, though it couldn't hurt if you have the inclination to take the time to do it.
The ideal spark-plug location in a combustion chamber is the center of the bore. It is not only the hottest but also the most turbulent region and allows a uniform flame propagation. Many newer-style cylinder heads, such as the GM Vortec casting, have the plug enter through the side, but a long reach places the ionization event almost in the center of the combustion chamber.
Myth: Vacuum Advance Is A Smog DeviceMany street machiners believe that a vacuum advance has no place on a modified ride. But each engine should be looked at individually. The purpose of a vacuum-advance unit is to increase the ignition lead under part-throttle, light-load conditions when the engine's volumetric efficiency (VE), or cylinder fill, is low. VE impacts cylinder turbulence and thus the flame or burn speed. Low levels of VE mean slower burn speeds and a need to advance the ignition timing for the best throttle response, power, and fuel economy. At WOT there is minimal vacuum in the intake, so the vacuum advance has no impact on the timing curve.