It only takes a spark to light a fire. But if the fire needs to be lit in a turbulent environment, as is the case with a combustion chamber, you’ll need the right equipment to reliably convert ignition voltage into a spark. During the power stroke of an internal combustion engine, cylinder pressures can range anywhere from 300 psi at a light load up to 1,500 psi in some racing applications. To ignite this compressed mixture of air and fuel reliably, you need to have the right ignition components for the job.

By Richard Fong


What’s in a Spark?

A vehicle’s ignition system produces the spark energy to ignite the compressed intake charge in the combustion chamber. Generating a spark starts at the vehicle’s battery, where 12 volts of DC current flow through an ignition coil that typically steps the voltage up to anywhere from 12,000 to 20,000 volts depending on the driving conditions, engine load and temperatures. Once triggered by the ECU or a mechanical signal, the increased voltage is released through each of the spark plugs to ignite the air/fuel mixture.

Used and Abused

The spark plugs are the maintenance item in the ignition system that must endure the most abuse. They thread into the engine (usually the cylinder head), protrude into the combustion chamber and must endure extreme cylinder pressures and the heat of combustion. The tip of the spark plug features a center electrode and a ground electrode. As the ignition coil releases electricity through the spark plug, a spark jumps across a gap between the electrode of the spark plug and the ground electrode, igniting the compressed air and fuel mixture.

Spark plug designs feature important traits such as conductivity, low resistance and durability. The center shaft is typically composed of steel, while the center electrode is typically made with a copper core (for thermal conductivity) and sheathed in an alloy for reduced wear. The part that can be seen (and is usually marketed most aggressively) is the center electrode tip. The typical spark plug center electrode tip can be made up of a combination of materials including copper, nickel-iron, chromium or noble metals (yttrium, iridium, tungsten or palladium.) Modern spark plugs marketed with exotic tip materials such as platinum and iridium feature high melting points and improved durability. These traits permit the use of a smaller center wire that will absorb less heat. These materials do not necessarily improve conduction or lower resistance, however they usually offer superior longevity and facilitate air/fuel mixture ignition. In the case of a racing or performance application, the tip material becomes less of a priority since the plugs are replaced more frequently than most modern street applications that have very long spark plug service intervals.

SPARK PLUGS:Telltale Signs

Normal Spark Plug

READING THE SIGNS: While new technology provides consumers with tools to monitor combustion chamber performance, technicians have been observing or “reading” spark plugs for decades to get an idea of what’s going on inside a motor. Seen here are spark plugs indicating a number of problems within the cylinder.

Dry Carbon Fouled

Dry Carbon Fouled

DRY FOULING: This is a common result in use of a colder heat range spark plug in an engine that doesn’t operate frequently at higher temperatures. If this was a spark plug rated at the factory heat range, other problems could be an overly- rich AFR, electrical problem causing misfires or extended operation of little to no throttle load.

Wet Oil Fouled

Wet Oil Fouled

WET FOULING: While this is possible with an excessively cold plug or rich AFR, a wet-fouled plug like this is more likely the result of oil or some other contaminant being introduced into the combustion chamber. The smell and appearance of the liquid on the plug would be a key indication at what the problem is.

Deposits+Cracked Insulator

DEPOSITS: Spark plugs peppered with deposits indicates oil or some other contaminant entering the combustion chamber. Instead of fouling out the plug, the liquid material evaporates and leaves behind crystal-like particles capable of deteriorating the plug and engine internals quickly.

Overheated Metal Speckling

Overheated Metal Speckling

SPECKLING PLUG: Seeing this condition is the first sign of too much heat on the spark plug. Before assuming a step colder plug is the only solution needed, make sure the engine itself isn’t overheating, the AFR isn’t too lean and that the timing isn’t over-advanced.

Melted Electrode

Melted Electrode

MELTED ELECTRODE/CRACKED INSULATOR: This is what eventually happens to plugs that face either extreme or extended conditions that cause a plug to speckle. The excessive heat literally melts the metal and the violent detonation breaks away the ceramic insulator. If a spark plug is pulled out of an engine looking like this, expect to have more engine parts needing replacement.

Spark Plug Tech – Selecting and Reading Spark Plugs

Spark plugs are not a one-size-fits-all component. While each engine requires a spark plug of specific dimension, the proper spark plug must still be chosen based on the performance demands of the engine and the spark plug’s corresponding optimal operating temperature or heat range. The heat range of a spark plug describes its ability to dissipate heat. Spark plugs that are “hot” do not dissipate heat very quickly, resulting in a higher operating temperature. On the other hand, “cold” plugs dissipate heat more efficiently, resulting in a cooler operating temperature. Typically, manufacturers control the heat range by changing the shape and size of the ceramic insulator.

Selecting the right spark plug depends on the engine’s expected operating temperatures. Too hot of a plug could melt the tip or the ground strap which could potentially damage the engine. Too cold of a plug will not self clean (burn off) and can become fouled with oil and fuel residue.

How do you know which heat range is right for your engine? A good starting point is to consider what the vehicle manufacturer recommends for a stock engine. Cooler plugs have higher numbers while hotter plugs have lower numbers. For example, a stock Subaru WRX STI (EJ257) comes standard with NGK LFR6A spark plugs (heat range 6). Going one step hotter (from the same plug family) would be an LFR5A (heat range 5), while one step colder would be an LFR7A (heat range 7). If a plug is too “hot” it will retain too much heat, potentially igniting the air-fuel mixture before a spark is generated. This results in a condition called pre-ignition, which can lead to severe engine damage. By contrast, too cold a spark plug will not retain the heat necessary to burn o deposits that form on the electrode, resulting in plug fouling that can inhibit spark production. Knowing this, you can choose the appropriate heat range based on your level of tuning and from reading the spark plugs. In addition to determining if the plugs are of the appropriate heat range, reading spark plugs can also diagnose other potential issues that might arise including the presence of contaminants, bad rings or a blown head gasket.

More With Distributor-Less

Technology drives performance, and simplifying ignition systems by eliminating the cap, rotor, distributor and plug wires of mechanically timed ignition components proved inevitable with the advent of reliable distributor-less ignition systems. Most modern engines employ ECU-controlled distributor-less ignition systems that include multiple coils. These multi-coil arrangements include wasted spark systems with a single coil serving two spark plugs as well as individual coil-on-plug (COP) ignition systems. Wasted spark ignition systems feature one coil for every pair of cylinders. This system fires both plugs per cycle, igniting the air/fuel mixture in one cylinder while the other is “wasted” on the cylinder that is near the end of its exhaust stroke. COP systems gained favor with manufacturers because of their compact packaging, higher performance, lower emissions and reduced maintenance. COP systems, also referred to as “direct ignition”, feature one coil per spark plug.

To enhance distributor-less ignition systems, some aftermarket solutions include higher performing coils and capacitive discharge ignition (CDI) systems. COP systems typically cost manufacturers more than distributor and wasted spark ignitions. Therefore, developing coils that perform adequately for a given application helps to keep the cost down while delivering the performance that OEMs intended. When raising the performance bar, these factory coils could fall short, resulting in misfiring or missed horsepower targets. Bolt-on aftermarket coil solutions designed for increased output could remedy these instances if they are available for your application. Another option is to supplement a COP system with a capacitive discharge ignition (CDI) system. CDI systems store spark energy in reserve to ensure reliable firing of the spark plugs even at higher RPM ranges and at higher boost levels. For older vehicles equipped with a distributor, some aftermarket manufacturers have developed solutions for retrofitting COP systems that include coils, coil drivers, an engine timing device and wiring harness.

(L) A spark plug gapping tool or a feeler gauge (R) can be used to verify plug gaps before installation.

Mechanical Firing

The old standard for ignition systems is the distributor ignition system. This ignition type generally includes a distributor, cap, rotor, plug wires, a coil and an ignitor. The distributor mounts directly to the engine, relying on a gear or camshaft to drive its rotor shaft. It not only delivers spark energy to the spark plugs, it can also regulate the ignition timing. Just rotating the distributor’s position on the axis of its rotor shaft can advance or retard the ignition timing in certain applications. The ignition timing can affect numerous aspects of engine function, including idle quality, drivability and overall performance. However, even if the timing is right, worn components can hamper normal function as well as performance.

Sorting and Sending

The distributor’s main wear items include the rotor and the distributor cap. The rotor mounts to the end of a shaft driven by a gear or camshaft . At the center of the rotor, a contact makes the connection to the ignition coil by way of the distributor cap. As the rotor spins on the shaft , it makes contact with each plug wire terminal, releasing spark energy from the coil through the spark plug wires to the spark plugs.

Typical service intervals for a cap and rotor vary by manufacturer and especially by driving conditions. For example, Honda recommends inspection every 15,000 miles and replacement every 30,000 miles. Even so, in the case of a performance build, it’s best to inspect the cap and rotor more frequently to ensure optimal engine performance. If the contacts are corroded or burnt, it’s a good practice to replace both the rotor and cap.

A specialized distributor cap (L) permits the use of an external coil (R) and an ignition amplifier (Below)

Delivering the Juice

The plug wires join the distributor cap to the spark plugs and deliver the spark energy for ignition. Plug wires should be inspected regularly for damage or increased resistance, potential causes for reduced performance in the form of reduced or lost spark energy. A damaged or failing spark plug wire can also cause damage to the ignition coil (which can cost considerably more than a set of wires). Recommended replacement intervals vary, but upgrading to an aftermarket offering could improve voltage delivery and subsequently increase performance in the form of a smoother idle, reduced or eliminated misfiring and mitigated or eliminated electromagnetic interference (EMI).

Bump Up the Voltage

Ignition systems rely on the coil to step up the ignition output to anywhere between 12,000-25,000 volts (under typical conditions) in order to ensure that the air/fuel mixture ignites. While reasonably sturdy, an ignition coil can experience damage due to a variety of reasons ranging from vibration and excessive heat to shorts caused by worn or damaged plug wires or spark plugs. If you experience misfires even a er replacing the wires and plugs, the coil could be damaged.

When upgrading an engine (both naturally aspirated and forced induction) or tuning more aggressively, increased cylinder pressures demand more of the ignition system. A factory coil could deliver 50,000 volts or more to the spark plugs for a brief moment, but will likely lead to coil failure. Aftermarket coils o er a broad operating range capable of sustained outputs over 40,000 volts, with some racing applications capable of 100,000 volts. However, if you upgrade to a higher-output aftermarket coil, expect to upgrade to an aftermarket ignition amplifier as well. The factory ignitor wasn’t designed to work with a higher voltage coil and will likely result in accelerated wear and premature failure. Many of today’s aftermarket ignition amplifiers employ sophisticated electronics and microprocessors that will ensure reliable spark delivery, control redline and improve startup, throttle response and acceleration.

Keep the Fires Burning

You can’t start a fire without a spark, just as your engine won’t function without a solid ignition system in place. Always keep a watchful eye on maintenance items like spark plugs and wires. They can not only damage other ignition components if they’re worn or damaged, but can also be indicators of your engine’s health. Maintaining your ignition system will contribute to your engine’s efficiency and mileage, but most importantly, performance.


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