The firing line is important since it would reveal a break in an ignition wire that is smaller than the gap of the spark plug. For example, if a plug wire has a 0.035-inch break and the spark plug gap is 0.045 inch, the ionization voltage may not be impacted, but the firing line would. A normal firing-line height is 0.9-1.2 kV (900-1,200 volts) at idle and light load. The shape of the firing line indicates the air/fuel ratio of the cylinder. Ideally, the line should remain flat. One that tilts upward shows a lean mixture and the additional energy required to keep the plug arcing; a firing line that tilts down is the result of an overly rich mixture.
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 ground. The energy then races back and forth between the spark plug and the open primary circuit, eventually wearing itself out as represented by the diminishing peaks. A good ignition coil should be able to create 3-5 coil and condenser oscillations. Section 4 represents the left over electricity that doesn't have enough energy to look for a ground path and remains in the coil. It is known as the coil build-up period.
It must be remembered that 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. Talking about dwell, as the rubbing block wears on a point-style ignition, the dwell period increases, the gap closes up, and the ignition timing retards.
Myth 3:Coils Don't Wear OutReality:Yes, They DoSince it has no moving parts, many think an ignition coil can't 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. From the millions of charging and discharging cycles they endure, a degraded coil will respond slower, often causing a high-speed misfire when there isn't enough energy to keep the spark plug arcing. An aftermarket performance coil is designed to charge or store energy quicker than an original-equipment (OE) design. For this reason, a good performance coil is a worthwhile addition to any HPP reader's ride, but choose the proper coil. A drag-race-only 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.
Myth 4:Indexing Spark Plugs Increases Power All The Time Or Indexing Spark Plugs Never Increases Power Reality:Indexing Spark Plugs Increases Power In Certain SituationsIs 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 that had unidealized spark-plug locations and slow flame-expansion often reaped 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, although it is certainly a good effort if you have the inclination.
In a combustion chamber, the ideal spark plug location is the center of the bore. The hottest and most turbulent region, it 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.
Here is the firing of a spark plug as a function of voltage and time as viewed on an oscilloscope. |
Spark plug electrode gap impacts the necessary voltage to create the ionization event. If the coil has the ability to bridge it, a larger gap creates an improved ionization window and allows for a leaner air/fuel ratio with excellent idle quality and driveability. |
An often overlooked but effective tuning tool is an adjustable vacuum advance. These units can be bolted onto a stock Pontiac distributor and allow a fine degree of tuning for better throttle response, fuel economy, and idle quality. |
The term misfire describes any combustion event that doesn't burn the fuel and air mixture completely. As the flame expands across the bore, a reaction zone is created. In this region, the heat from the burned mixture travels into the cooler unburned charge and aids combustion. If the ignition isn't strong enough for this to occur, the engine will only burn a small amount of the fuel in the cylinder. The rest will exit the tailpipe as unburned hydrocarbons, wasting gasoline and not producing any power. |