Of the many different aspects of car crafting, ignition-system function is often the most misunderstood. Since it deals with an intangible-electricity-it is very easy to assume one thing is happening while another actually is. But don't worry. Sit back and let Car Craft show you how to dazzle your buddies with ignition-system knowledge.
Scope It To MeIf ignition systems are hard to understand, it's probably because electricity flow cannot be seen. The common voltmeter doesn't do much good as it only assigns a value to the electrical "pressure" in a circuit. To truly understand an ignition system and how the spark plug fires, an oscilloscope (or scope) is required. This device displays an electrical picture of the ignition event as a function of voltage and time.
The scope connects to the coil's primary circuit (the negative post of the coil), the coil wire for secondary output, and the No. 1 spark-plug wire as a synchronization signal. In addition, most automotive scopes require a 12-volt positive and negative connection.
A scope can be thought of as a high-speed visual voltmeter that shows not only the intensity of the voltage, but when the signal occurs. Instead of a bouncing voltmeter needle, the height of the scope pattern indicates the intensity of the voltage. The horizontal axis of a scope indicates the time period in milliseconds (ms), or one thousandths of a second.
With a scope, we can not only see how many volts were required to light the spark plug, but how long it stayed lit, which is very important in diagnosing performance problems with the ignition, engine, and fuel system.
How A Spark Plug FiresOn any ignition system that uses a single coil there are five distinct components of a scope pattern. Refer to the accompanying illustration (below) while reading this explanation to keep it all clear.
The secondary spike (1) is read in kilovolts (kV) and is the energy required to overcome all secondary resistance and bridge the gap of the spark plug under compression. It is impacted by many factors, including the condition of the secondary wires, cap, and rotor; the gap of the spark plug; the cylinder pressure in the bore; and the air/fuel ratio. The proper name for the secondary spike is the ionization voltage. It normally takes 5 to 10 kV (5,000 to 10,000 volts) to get the arc going at idle in a typical engine.
More energy is always required to light the spark plug than to maintain the arc, in much the same manner that it is harder to get a car rolling from a stop than to keep it moving. Section (2) is identified as the firing line. It represents the amount of energy removed from the coil to keep the spark plug arcing and also shows how long the spark plug remained lit in milliseconds. A good electronic ignition system should have the capability to keep the plug burning at idle for at least 1.5 ms. When designing an ignition system, engineers do not reference this reading but instead look to maintain an arc for 20 degrees of crankshaft rotation.
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. 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 be. A normal firing-line height would be between 0.9 and 1.2 kV. 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 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.
Engines with lower compression ratios (10.0:1 and below) usually benefit the most from a functional and properly tuned vacuum-advance unit. The first step is to connect a manifold-vacuum gauge. Drive around with the gauge duct-taped to the windshield and mentally record the readings. If there are at least 5 inches of mercury present under most driving conditions, the engine will probably respond to a vacuum-advance unit. Try connecting to a full-time (manifold) vacuum source and a ported (timed) outlet. The engine will tell you which one it likes better. A manifold-vacuum source will provide advance at idle and then ramp off slowly as load is applied to the engine.
Myth: All HEIs Are Created EqualHEI is a brand name for High Energy Ignition, which General Motors pioneered for the '74 model year. Though other makers had electronic ignition prior to this, namely Chrysler in 1972, HEI stood alone as a brilliant new theory. Designed with transistors that would survive on full battery voltage, HEI did not use resistors to drop the system running voltage. It also incorporated a unique concept called expanding dwell. Unlike the other early electronic-ignition systems that used a fixed dwell time just as breaker points did, HEI would increase the dwell period as engine speed increased.
On a V-8 at idle, the module dwell (on time) would be 5 to 10 degrees and expand to 30 to 35 degrees by 2,500 rpm. The higher input voltage, along with the increased coil saturation (dwell), allowed HEI to fire the spark plug longer and with a larger gap. The increased gap opened a larger ionization window and, since the ignition was very powerful, allowed a longer burn time. Higher ionization voltage with longer burn times equals more power, better fuel economy, and a smoother idle.
Just because an HEI-equipped engine runs does not mean the module is operating properly. To check, simply hook your old breaker points-style dwell meter to the tach terminal on the distributor cap and read the dwell scale at idle and as rpm increases. Cheapo import modules often do not have the proper dwell period, which impacts performance. This simple test should be part of a performance diagnostic routine and performed with every tune-up. Note that if the engine has a CD box, the current draw will cause the HEI module to assume a fixed dwell of around 35 degrees, making this test invalid.
Diagnosing MisfireThe term misfire describes an ignition event in which either the spark has not jumped the gap of the plug but instead went to ground, or a very short burn cycle of the ignition. Many factors can cause a non-ignition-related misfire, especially an excessively lean mixture. When there is not enough fuel present in the chamber, the flame cannot travel across the bore and extinguishes, even though the spark plug is still arcing. This causes a rough idle, lack of power, and poor driveability. When diagnosing a misfire, determine if it is an ignition or a mechanical issue that is not allowing the ignition to work properly.
Myth: Thicker Plug Wires Have Thicker CoresCommon sense suggests that the thicker the spark-plug-wire designation, the more electricity it can transport. That would be true if the dimension used to describe the wire were a measure of the core size. But contrary to what many think, it actually represents the thickness of the insulation. A 7mm wire has thinner insulation than a 10mm design. The core and resistance are manufacturer-specific, so all plug wires are not created equal. The benefit of thicker insulation is less radio frequency interference (RFI) and electromotive interference (EMI) and greater thermal protection.
Interference can be thought of as stray voltage leaking from the wire. Some may argue that since electricity takes the path of least resistance, thicker insulation is required to keep the electrons flowing to the spark plugs on high-cylinder-pressure applications, and that's true. But don't look for any gain on your 350hp ride by switching to a wire with a thicker insulation unless the conductor resistance is substantially less.
The Best RateAlmost every enthusiast leaves a good deal of performance on the table by not paying attention to the rate of ignition advance. The rate at which the advance comes in is just as important as the total and is crucial on engines operating under transient conditions, such as a street/strip car. The best place to start this task is to have a shop with a distributor machine set up a basic curve and then do your fine tuning at the track or on a chassis dyno. Most major ignition companies offer spring, weight, and adjustable vacuum-advance canisters for most American-produced engines.
Do not forget to also check the distributor bushing for wear along with the distributor gear. The bushing is best checked on a scope or a distributor machine, but it can be diagnosed by grabbing the rotor and trying to move the distributor shaft side to side. Any movement will alter the point-open (or ignition-model on) time and create a different amount of ignition timing on each bore. Many engines are detuned to eliminate detonation that is only occurring on a few bores due to a worn distributor bushing.
Magic BoxesThere are many companies that manufacture ignition boxes or amplifiers, more commonly known as capacitive discharge (CD) ignitions. Most if not all CD systems offer a higher coil output since they convert ignition from inductance to capacitance. In addition, they usually offer multiple ionizations (firings) of the spark plug below 3,000 rpm. Since electricity is passive, an ignition system will use only what is required to get the job done. At idle the task is very easy, but as engine speed and cylinder pressure increase, the ignition system becomes strained. Simply put, if the cylinder pressure your engine creates is within the capability of the stock ignition, no power will be found with a CD box. If not, then the longer burn time and higher current (amperage) flow will really wake up your ride. A side benefit of the multistrike function is it masks fuel-distribution problems at low speeds for a better idle and helps keep the spark plugs clean when used with a cam ground with excessive overlap.
