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Spark Plugs and Ignition Performance

Platinum has become a buzzword for ignition durability. Platinum is a good conductor of heat and electricity, but it is best known for its ability to resist chemical corrosion and electrical erosion. That is why platinum is used on the electrodes of many spark plugs today.

Some plugs have a solid platinum center electrode, while others have a small button of platinum welded onto the tip of center electrode or both electrodes (single platinum vs. double platinum).

Platinum is used because it minimizes electrode wear. Every time a plug fires, a tiny amount of metal is vaporized and erodes from the surface of both electrodes. The center electrode typically suffers the most wear because it runs hotter than the side electrode.

As the electrodes wear, the air gap across which the spark must jump becomes wider and wider. The gap on a standard spark plug grows about 0.00063 to 0.000126 inch for every 1,000 miles of normal driving. And the wider the gap, the greater the voltage needed to jump the gap.

On standard plugs with conventional electrodes, the firing voltage requirements typically creep up about 500 volts for every 10,000 to 15,000 miles of driving. Eventually, the plugs may need more volts to fire than the coil(s) can produce, resulting in ignition misfire. On OBD II-equipped vehicles, too many misfires will cause the Check Engine light to come on.

Using platinum almost eliminates electrode wear. Platinum is expensive, but it can double or even triple a spark plug's normal service life, from 30,000 to 45,000 miles for a standard plug, up to 80,000 to 100,000 miles or more with platinum. Most aftermarket plug suppliers do not make specific mileage claims for their platinum plugs, but say to follow the OEM replacement intervals which, in most cases, is 100,000 miles for OEM platinum plugs.

Though long-life platinum plugs cost more than standard spark plugs, OEMs use them because they reduce the risk of misfire (which helps protect the catalytic converter) and they reduce the need for maintenance, which allows them to offer 100,000-mile "tuneup" intervals.

One important point to keep in mind about platinum plugs is that all platinum plugs are not the same. There are differences in electrode configurations, design and durability. Some provide better fouling resistance than others, and some are not recommended for use in turbocharged or supercharged engines.

Plug manufacturers are also using other exotic materials in spark plug electrodes to extend plug life. Bosch uses a nickel-yttrium alloy for the side electrodes in its "Platinum+4" and "Platinum2" spark plugs. In Europe, Bosch has a new plug that uses yttrium for both the center and ground electrodes. For years, Champion manufactured a premium plug with a gold-palladium center electrode and copper-filled side electrode. That plug has now been replaced by one with a platinum-tipped center electrode. Autolite uses a chromium-nickel alloy for the ground electrode with its platinum-tipped center electrode plug, while ACDelco uses a silver-nickel alloy side electrode with its platinum-tipped plugs. NGK and Denso have both introduced premium spark plugs with iridium alloy electrodes.

Iridium spark plugs are now replacing platinum spark plugs in many OEM engines, not just Japanese engines. Platinum plugs are great, but are also expensive compared to iridium. So iridium plugs are now being used for more long-life (100,000 mile) applications.

Iridium plugs typically have a fine wire center electrode that forms a spark easily and reduces misfires. Iridium actually has a higher melting point than platinum, and is six times harder and eight times stronger,making it very wear resistant. The small fine wire (0.4 to 0.7 mm) center electrode concentrates the spark as well as heat. This reduces the voltage required to create a spark and fire the plug. Because of this, and the high wear resistance of the metal, iridium plugs are often recommended for "waste spark" distributorless ignition systems, and coil-on-plug ignition systems.

Familiar brands of iridium plugs include NGK Iridium and Iridium IX, and Denso Iridium Power. These are now joined by AC Delco Professional Iridium, Autolite XP Iridium, Champion Iridium, and Bosch Fusion (uses a platinum-iridium alloy in the center electrode).

Regardless of the type of alloy used in the electrodes,
all spark plugs must be able to resist fouling. The trick here is to design the plug so that the electrodes run hot enough to burn off any deposits but not so hot that they cause preignition or detonation. To burn off carbon deposits, the electrodes need to reach about 700 degrees F quickly. But if the electrodes get too hot (above 1,500 degres F), they can ignite the fuel before the spark occurs, causing preignition and detonation. For most plugs, the ideal operating temperature is around 1200 degrees F.

The temperature of the electrodes is controlled by the length of the ceramic insulator that surrounds the center electrode and the design of the electrode itself. Ceramics do not conduct heat very well, so an insulator with a relatively long nose will conduct heat away from the electrode more slowly than one with a relatively short nose. The longer the path between the electrode and the surrounding plug shell, the slower the rate of cooling and the hotter the plug.

Many plugs have a copper core center electrode. Copper is an excellent conductor of heat, and allows the plug to dissipate heat quickly under load yet remain hot enough at low speed and idle to burn off fouling deposits.

A spark plug's "heat range" (heat rating) therefore depends on the length of the ceramic insulator and the design of the center electrode. The heat range must be carefully matched to the engine application, otherwise the plugs may experience fouling problems or run too hot and cause preignition/detonation problems. Most plugs today have a relatively broad heat range, which means they reach a self-cleaning temperature quickly but do not get too hot under load. This allows plug manufacturers to consolidate applications and use fewer plugs to cover a wider range of engines.

spark plug misfiring
spark plug normal spark
When replacing spark plugs, the heat range must be correct for the engine application. Always follow the vehicle or spark plug manufacturers recommendations. If the plugs are too cold, fouling may occur if the vehicle spends a lot of time idling or is used only for short trips (especially during cold weather). If the plugs are too hot, the engine may experience preignition and detonation under load or during hot weather.

In some situations, a slightly hotter or colder plug may be installed than the one normally recommended. Switching to a slightly hotter plug can help reduce fouling in an older engine that uses oil. A hotter plug can also reduce fouling in vehicles that spend a lot of time idling or are used only for short-trip, stop-and-go city driving. But a hotter plug should not be used unless an engine is experiencing a fouling problem because of the increased risk of preignition and detonation. Switching to a slightly colder plug can reduce the risk of preignition and detonation in performance applications (especially turbocharged and supercharged engines), in vehicles used for towing or in those that are driven primarily on the highway.

Many spark plugs today have unique electrode designs such as V-split, grooved or clipped ground electrodes, multiple ground electrodes, fluted center electrodes,

V-notched center electrodes, etc. Though each plug manufacturer takes a slightly different approach and claims various benefits for their design, the basic idea is to make it as easy as possible for the spark to jump the gap and ignite the fuel mixture. A spark jumps more easily from a sharp edge than a rounded blunt edge. That is one reason why new plugs require less firing voltage than old ones with worn electrodes.

The electrodes on some spark plugs are also designed to "unshroud" the spark for easier ignition. This allows the flame kernel to expand more rapidly and reduces the quenching effect that could cause a misfire. SplitFires V-shaped ground electrode as well as Boschs surface gap Platinum+4 and Platinum2 plugs are all designs that claim to expose more of the spark to the fuel mixture.

Spark plugs should be replaced at the vehicle manufacturers recommended intervals, but may have to be replaced sooner if they are fouled. On older vehicles, the replacement interval is typically 30,000 to 45,000 miles. On newer vehicles with platinum plugs, it may be as high as 100,000 miles. But short trip stop-and-go city driving as well as extended idling can shorten the life of any spark plug.

Most spark plugs are pregapped. Even so, the gap may have to be reset for some engines because of consolidations. One exception is Bosch Platinum+4 and Platinum2 spark plugs, which are all gapped the same (1.6 mm) and must not be changed from the original setting.

Good plug wires are just as important to ignition performance as the spark plugs. Three basic types of wires may be used:

* Distributed Resistance wire. This type of wire has a fiberglass core impregnated with latex graphite. It provides the maximum amount of radio frequency interference (RFI) suppression. RFI occurs when high voltage passes through the plug wires. Creating a controlled amount of resistance in the wire (3,000 to 12,000 ohms per foot) suppresses RFI and prevents sensitive onboard electronics from picking up false signals that could cause driveability problems.

One of the drawbacks of carbon core suppression wires is that internal resistance creates internal heat. Over time, this ages the carbon core, causing resistance to increase. And, as resistance goes up, so does the chance for misfire.

Prior to 1980, 95 percent of all vehicles were equipped with carbon core suppression wires. But concerns over emissions and long-term reliability led many of the Japanese OEMs to switch to "mag"-style spark plug wires.

* Inductance (mag) wire. This type of wire has a spiral wound core of copper/nickel alloy wire. RFI is suppressed primarily by the magnetic field formed by the loops of wire wrapped around the core rather than the resistance of the wire itself. Mag wire has less total resistance (only about 500 ohms/foot) than suppression wire, so it reduces the current needed to fire the plugs. But its main advantage is improved durability over the long run.

Mag-style spark plug wires have been used on Honda and Acura engines since 1971, most Nissan and Infiniti applications since 1980, and many Toyota and Lexus applications since 1984.

* Fixed Resistor wire. This type of wire has a steel or copper metallic core with a fixed resistor in the plug boot to control RFI. This wire is used on many European imports.

The type of insulation used in a plug wire is also important for long-term durability and performance. Premium wire sets typically use silicone or EPDM (Ethylene Propylene Diene Monomer). One thing to keep in mind about silicone is that there are different grades of silicone. The "good stuff" can withstand temperatures from

-90 degrees F up to 600 degrees F. But according to one manufacturer, some so-called silicone wire sets contain only 3 percent silicone, which does not provide the same degree of temperature and chemical resistance as pure silicone. Other premium wire sets may add a protective outer covering of EVA (Ethylene Vinyl Acetate) for added temperature resistance and tensile strength.

Under the tough outer silicone jacket is fiberglass braiding for strength and flexibility, and under that is a layer of EPDM insulation that prevents arcing and voltage leaks. Mag core wire is surrounded by a latex silicone bonding layer that provides additional stability and support to hold the wire in place.

Plug wires should always be inspected when the spark plugs are changed and any time there is a misfire complaint. Start with a visual inspection for obvious damage such as burned or cracked insulation, chaffing, contact with the exhaust manifold, loose plug boots or terminals, etc. Any wires that are burned or damaged must be replaced. The same goes for wires with loose or damaged boots or terminals.

Next, start the engine, then look and listen for arcing while the engine idles. A snapping or cracking noise would tell you secondary voltage is finding a shortcut to ground. Observing the engine in the dark may help you see where the voltage is leaking. Any fireworks that are visible along the length of the cables or at the ends would tell you new wires are needed.

Still can't find a bad wire? Look at the secondary firing pattern on an oscilloscope. A bad plug wire with excessive internal resistance may cause an intermittent or steady misfire that is usually most noticeable under load. This will cause an increase in the affected cylinder's firing voltage. An open plug wire or spark plug will cause the firing voltage for that cylinder to spike to the coils maximum output.

If you find that the firing voltage is high in a cylinder, turn the engine off and measure the plug wire's resistance end to end with an ohmmeter. Refer to the manufacturers specifications. If resistance exceeds specifications, the wire needs to be replaced.

A shorted ignition cable or grounded spark plug will cause a drop in the firing voltage. Rubbing a grounded probe along the length of each plug wire while the engine is idling may help you find any weak spots in the insulation.

When one cylinder in the superimposed display has a firing line higher than the rest and a shorter spark duration, high secondary resistance is indicated. High secondary resistance may be caused by bad plug wires or worn spark plugs, but also a lean fuel condition.

To further isolate the cause, the KV demand for the affected cylinder should be compared to the other cylinders. If the required firing voltage is 20 percent or higher than the rest, the problem is either too wide a plug gap or a lean fuel condition. But if the firing voltage is roughly the same as the other cylinders, the likely cause is high resistance in the plug wire or spark plug.


  • If more than one plug wire has excessive resistance, replace the entire set.
  • Handle cables with care during installation. Do not jerk, force, twist or bend sharply.
  • Start with the longest or shortest wire first, and change one wire at a time to avoid mixing up the firing order.
  • Listen for the "click" to make sure the plug boots and end terminals are fully seated.
  • Follow the original cable routing to avoid crossfire problems. Cables for cylinders that fire consecutively should not be routed parallel or be in close proximity to one another. Keep them separated by several inches or cross them.
  • All cables should be supported by wire looms, and kept away from exhaust manifolds and sharp edges.
You will need the following items to diagnose and replace spark plugs and wires:

Plug gap tool;

Plug boot puller tool;

Digital Multi Meter for measuring plug wire resistance;

Digital Storage Oscilloscope (DSO) for observing secondary ignition patterns; and a

Scan tool for OBD II vehicles if the Check Engine light is on due to a misfire code.

Adapted from an article written by Larry Carley for Import Car magazine


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