Information
- Overview
- Evolution
- Did you know?
Spark plugs have been around as long as internal combustion engines have, and are often a misunderstood component. This basic guide is designed to assist the technician, hobbyist, or race mechanic in understanding, using, and troubleshooting spark plugs.
Spark plugs are a "window" into the engine, and can be a valuable diagnostic tool. The spark plug displays the condition inside the combustion chambers of the engine. The experienced tuner can use spark plugs to find the root cause of problems, determine air-fuel ratios, and to increase vehicle performance.
Spark Plug Basics
The primary function of the spark plug is to ignite the air/fuel mixture within the combustion chamber under any operating condition.
Spark plugs must provide a path and a location for electrical energy from the ignition coil to create a spark used to ignite the air-fuel mixture. A sufficient amount of voltage must be supplied by the ignition system to spark across the spark plug gap. This is called “Electrical Performance.”
The spark plug firing end temperature must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called “Thermal Performance”, and is determined by the heat range selected.
Heat Rating
The spark plug heat range has no relationship to the electrical energy transferred through the spark plug. The heat range of a spark plug is the range in which the plug works well thermally.
The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type.
Some basic structural factors affecting the heat range of a spark plug are:
- Surface area and/or length of the insulator nose
- Thermal conductivity of the insulator, center electrode, etc.
- Structure of the center electrode such as a copper core, etc.
- Relative position of the insulator tip to the end of the shell (projection)
The major structural difference affecting the heat rating is the length of the insulator nose. A hot type spark plug has a longer insulator nose. The insulator nose of a hotter spark plug has a longer distance between the firing tip of the insulator, and the point where insulator meets the metal shell. Therefore, the path for the dissipation of heat from the insulator nose to the cylinder head is longer and the firing end stays hotter. The insulator nose of a hotter spark plug also has a greater surface area that is exposed to more of the ignited gases and is easily heated to higher temperatures. A colder spark plug functions in an opposite manner.
The heat range must be carefully selected for proper spark plug thermal performance. If the heat range is not optimal, then serious trouble can be the result. The optimal firing end temperature is approximately between 500°C (932°F) and 800°C (1472°F). The two most common causes of spark plug problems are carbon fouling (lower 450°C) and overheating (over 800°C).
Causes of Carbon Fouling:
- Continuous low speed driving and/or short trips
- Spark plug heat range too cold
- Air-fuel mixture too rich
- Reduced compression and oil usage due to worn piston rings / cylinder walls
- Over-retarded ignition timing
- Ignition system deterioration
Pre-delivery fouling
Carbon fouling occurs when the spark plug firing end does not reach the self-cleaning temperature of approximately 450°C (842°F). Carbon deposits will begin to burn off from the insulator nose when the self-cleaning temperature is reached. When the heat range is too cold for the engine speed, the firing end temperature will stay below 450°C and carbon deposits will accumulate on the insulator nose. This is called carbon fouling. When enough carbon accumulates, the spark will travel the path of least resistance over the insulator nose to the metal shell instead of jumping across the gap. This usually results in a misfire and further fouling.
If the selected spark plug heat range is too cold, the spark plug may begin to foul when the engine speed is low or when operating in cold conditions with rich air-fuel mixtures. In some cases, the insulator nose can usually be cleaned by operating the engine at higher speeds in order to reach the self-cleaning temperature. If the spark plug has completely fouled, and the engine will not operate correctly, the spark plug may need to be cleaned / replaced and the fouling cause identified.
Causes of Overheating:
- Spark plug heat range too hot
- Insufficient tightening torque and/or no gasket
- Over-advanced ignition timing
- Fuel octane rating too low (knock is present)
- Excessively lean air-fuel mixture
- Excessive combustion chamber deposits
- Continuous driving under excessively heavy load
- Insufficient engine cooling or lubrication
The most serious result of selecting a heat range that is too hot is overheating. Overheating will cause the electrodes to wear quickly and can lead to pre-ignition. Pre-ignition occurs when the air-fuel mixture is ignited by a hot object/area in the combustion chamber before the timed spark event occurs. When the spark plug firing end (tip) temperature exceeds 800°C, pre-ignition originating from the overheated insulator ceramic can occur. Pre-ignition will dramatically raise the cylinder temperature and pressure and can cause serious and expensive engine damage. When inspecting a spark plug that has experienced overheating or pre-ignition, blistering on the ceramic insulator and/or melted electrodes can sometimes be found.
As a general guideline, among identical spark plug types, the difference in tip temperature from one heat range to the next is approximately 70°C to 100°C.
Tip Temperature and Firing End Appearance
Some factors to consider in selecting the proper heat range spark plug
There are many external influences that can affect the operating temperature of a spark plug. The following is a brief list to consider in avoiding reduced performance and/or expensive engine damage.
Engine Speed and Load
- If the engine is to be operated at high RPM, under a heavy load, or at high temperatures for long periods a colder heat range may be needed.
- Conversely, if the engine is to be operated at low speeds or at low temperatures for long periods, a hotter heat range might be needed to prevent fouling.
Air-Fuel Mixture
- Excessively rich air-fuel mixtures can cause the plug tip temperatures to decrease and carbon deposits to accumulate, possibly causing fouling and misfires.
- Excessively lean air-fuel mixtures can cause the cylinder and plug temperatures to increase, possibly resulting in knock and/or pre-ignition. This may cause damage to the spark plug and/or seriously damage the engine.
- If an air-fuel ratio meter or gas analyzer is not available, it will be necessary to visually inspect the spark plugs frequently during the tuning process to determine the proper air-fuel mixture.
Fuel Type / Quality
- Low quality and/or low octane fuel can cause knock which will elevate cylinder temperatures. The increased cylinder temperature will cause the temperature of the combustion chamber components (spark plug, valves, piston, etc.) to rise, and will lead to pre-ignition if the knock is uncontrolled.
- When using an ethanol blend fuel with high ethanol content in high performance applications, a colder heat range may be necessary. The spark timing can be advanced further because ethanol blend fuel has a higher resistance to knock (higher octane). Due to the decreased knock, there will be less audible “warning” from knock before the spark plug overheats and pre-ignites. Some types of fuel additives in lower quality fuels can cause spark plug deposits that can lead to misfires, pre-ignition, etc.
Ignition Timing
- Advancing ignition timing by 10° will cause the spark plug tip temperature to increase by approximately 70° to 100°C.
- A colder heat range spark plug may be necessary if the ignition timing has been advanced to near the knock level. Higher cylinder temperatures near the knock level will bring the spark plug firing end temperature closer to the pre-ignition range.
Compression Ratio
- Significantly increasing the static/dynamic compression ratio will increase cylinder pressures and the octane requirement of the engine. Knock may occur more easily. If the engine is operated near the knock level, a colder heat range spark plug may be necessary due to the resulting increased cylinder temperatures.
Forced Induction (Turbocharging, Supercharging)
- A colder heat range spark plug may be necessary due to the increased cylinder temperature as boost pressure (manifold pressure) and subsequent cylinder pressure and temperature increase.
Ambient Air Temperature / Humidity
- As the air temperature or humidity decreases, the air density increases, requiring a richer air-fuel mixture. If the air-fuel mixture is not properly richened, and the mixture is too lean, higher cylinder pressures / temperatures, knocking, and the subsequent increase in the spark plug tip temperatures can result.
- As the air temperature or humidity increases, the air density decreases, requiring a leaner air-fuel mixture. If the air-fuel mixture is too rich, decreased performance and/or carbon fouling can result.
Barometric Pressure / Altitude
- Air (atmospheric) pressure and cylinder pressure decrease as altitude increases. As a result, spark plug tip temperature will also decrease.
- Fouling can occur more easily if the air-fuel mixture is not adjusted to compensate for the altitude. Higher altitude = less air = less fuel.
Types of Abnormal Combustion
Pre-ignition
- Pre-ignition occurs when the air-fuel mixture is ignited by a hot object / area in the combustion chamber before the timed spark event occurs.
- When the spark plug firing end (tip) temperature exceeds 800°C, pre-ignition originating from the overheated insulator ceramic can occur.
- Is most often caused by the wrong (too hot) heat range spark plug, and/or over-advanced ignition timing. An improperly installed (insufficient torque) spark plug can also result in pre-ignition due to inadequate heat transfer.
- Pre-ignition will dramatically raise the cylinder temperature and pressure and can melt and hole pistons, burn valves, etc.
Knock
- Occurs when part of the air-fuel mixture in the combustion chamber away from the spark plug is spontaneously ignited by the pressure from a flame front originating from the spark plug. The two colliding flame fronts contribute to the “knocking” sound.
- Knock occurs more frequently when using low octane fuel. Low octane fuel has a low resistance to knock (low resistance to ignition)
- Knock is related to ignition timing. (Knock is sometimes referred to as “Spark-knock”.) Retarding the ignition timing will reduce knock.
- Heavy knock often leads to pre-ignition.
- Heavy knock can cause breakage and/or erosion of combustion chamber components.
- Knock is sometimes referred to as “ping” or “detonation”.
Misfires
A misfire occurs when the spark travels the path of least resistance instead of jumping across the gap. Misfires can be caused by the following:- Carbon fouling
- Worn or deteriorated ignition system components
- Too large of gap size
- Spark timing excessively advanced or retarded
- Damaged spark plugs (cracked insulator, melted electrodes, etc)
- Mismatched ignition system components (plug resistance / wire resistance, ignition coils / igniter modules, etc.)
- Insufficient coil primary and/or secondary voltage – voltage required to jump the spark plug gap higher than coil output
- Spark Plug Evolution
As technology advances, the efficiency of the spark plug has increased, resulting in higher combustion efficiencies. The design evolution has reduced the quenching effect on the flame kernel. The quenching effect is the heat absorption by the spark plug ground electrode and center electrode, which hinders the flame kernel efficiency. The NGK spark plug has seen many changes to its design over the last 75 years, with most occurring in the last 30 years.
V-Power Spark Plugs
NGK introduced the V-Power spark plug in the 1970s. This design has a V-cut groove in the center electrode, which focuses the spark to occur between the outside edges of the center electrode and the ground electrode; this relocated the flame kernel to the outside edge of the spark plug, which reduced the quenching effect.
The V-Powerspark plug was adopted by many OE manufacturers and is still very popular today.
Platinum / Iridium Finewire Spark Plugs
With the introduction of precious metals, first Platinum in the 1980s and Iridium in the 1990s, the spark plug center electrode has reduced in size. Both Platinum and Iridium are extremely dense materials with high melting points, making them ideal for center electrode construction. Where a traditional nickel-alloy spark plug had a center electrode diameter of 2.5mm, the newer precious metal spark plugs have a center electrode diameter of less than 1.0mm.
This smaller diameter center electrode provides more area for flame kernel expansion, further reducing the quenching effect. In 2000, NGK introduced the Iridium IX spark plug, which has a taper-cut ground electrode. This taper-cut shape provides even more area for flame expansion, further reducing the quenching effect.
Dual Fine Electrode Spark Plugs
Recently, as emissions and fuel economy requirements have been made tougher to meet by automotive manufacturers, increased use of turbocharging and gasoline direct injection has become common on modern engines. To even further increase spark plug ignitibility, new tip designs such as Dual Fine Electrode (DFE) spark plugs has been introduced.
DFE spark plugs feature opposed fine-wire electrodes from both the center electrode and ground electrode to maximize ignitibility under these extreme conditions.
- Five Things You Need to Know About Spark Plugs
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1. Anti-Seize
NGK spark plugs feature what is known as Trivalent plating. This silver or chrome colored finish on the threads is designed to provide corrosion resistance against moisture and chemicals. This coating also acts as a release agent during spark plug removal. NGK spark plugs are installed at the factory dry, without the use of anti-seize. NGK tech support has received a number of tech calls from installers whom have over-tightened spark plugs because of the use of anti-seize. Anti-seize compound can act as alubricant altering torque values up to 20 percent, increasing the risk of spark plug thread breakage.
2. Corona Stain
Corona stain is observed as a light brown or tan discoloration above the hex (located on the ceramic body of the spark plug). Corona stain is created by oil or dirt particles surrounding the spark plug. Spark plugs create a high amount of static electricity as they fire, attracting these particles to the exposed ceramic between the plug boot and the hex. Corona stain is completely normal and should not be mistaken for exhaust gas blow-by or broken seals inside the spark plug.
3. Gapping fine-wire spark plugs
In the late 1980’s, when fine-wire spark plugs first appeared, installers used incorrect gap tools and procedures resulting in broken-off firing electrodes. As a result, many people assumed that one cannot adjust the gap on a precious metal plug. While most NGK spark plugs are pre-gapped, there are instances where the gap requires modification. NGK recommends a wire-style or feeler gauge gap tool, which can adjust the gap without prying against the center electrode. NGK also recommends adjusting the gap no more than +/-0.008” from the preset gap.
4. Torque
Torque is critical in the plug’s ability to dissipate heat and perform properly. Always follow the manufacturer’s recommended torque specification. An under-torqued spark plug can lead to excessive vibration and improper heat dissipation, causing spark plug and/or engine damage. An over-torqued spark plug may cause thread damage or breakage, or compromise internal seals within the spark plug, leading to improper heat dissipation or exhaust gas blow-by.
5. “Copper plugs”
“Copper spark plugs” is a term mistakenly used for a standard material spark plug. A standard material spark plug traditionally uses a nickel-alloy outer material fused to a copper core. Almost all spark plugs use a copper core center to conduct the electricity, jump the gap, and promote heat dissipation. However, as another electrode material, copper would not be a good choice, as it is soft and has a low melting point (resulting in a plug that would last minutes, not miles). All NGK spark plugs,including precious metal Iridium and Platinum, have a copper core.
When one talks in terms of nickel alloys, platinum and iridium, one is referring to its durability, or how long a spark plug will last before it needs to be replaced. However, when one talks about copper, he or she is referring to its ability to conduct electricity that is needed to fire across the gap and ignite the air/fuel mixture.
- Multiple Ground Electrodes
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Spark plugs come in many different ground electrode configurations. Based on engine design and OEmanufacturers’ specifications, there are several different ground electrode spark plug configurations used today. Some manufacturers have elected to use spark plugs with more than one ground electrode - why? Multiple-ground electrode spark plugs can have two and up to four ground electrodes. Multiple-groundelectrodes can maximize the life ofa traditional material (nickel-alloy) spark plug. As the spark plug fires, plugwear is distributed more evenly among the ground electrodes.
Another added benefit to multiple-groundelectrodes is that of reduced cold-start fouling, in that should the plugstart to become fouled, the spark cantravel across the insulator more easily.With these benefits, why doesn’t every manufacturer use multiple-ground electrode plugs? As the spark plugfires and the initial flame kernel is created, more quenching is experienced, as there is more plug mass in the way of flame travel. The quenching effect refers to an area during the combustion process where heat (flame) is inhibited by an interfering mass (ground electrode).
Traditionally, single-ground electrode and especially precious metal, platinum and iridium plugs have superior ignitability.A common misconception of multiple-ground electrode spark plugs is that they can fire more than one sparkat a time. For example, a plug with four ground electrodes must fire four sparks at once; the laws of electricalphysics tell us this just isn’t possible. Electricity will take the path of least resistance, meaning one spark will occur between the center electrode and which ever ground electrode is easiest to ground to.
- Waste Spark Ignitions
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Like most automotive systems, the ignition system has undergone many changes. One example is the distributorless ignition system, or “DIS” for short. This system was first introduced in the 1980’s. The distributor, cap and rotor in a conventional system were replaced by coil packs. The new DIS system proved to be more reliable and required less maintenance.
The earliest DIS ignition featured a bank of coils; one coil for every two cylinders. Each pair of coils would provide power to two spark plugs. Each of the two paired coils would fire the paired spark plugs simultaneously, one on the compression stroke and the other on the exhaust stroke. Since one plug is fired on the exhaust stroke, serving no real purpose, the system was known as a “waste spark system.” This style of ignition has negative and positive polarity sides to the coil; meaning the spark plugs also has positive or negative polarity.
It is important to know that in a waste spark ignition, both the center firing electrode and ground electrode will see gap erosion. In plugs with a positive polarity, most wear will occur at the center-firing electrode. Plugs with negative polarity will experience more wear at the ground electrode. As a result of this, vehicles equipped with waste spark ignitions often use double precious metal spark plugs, such as the Laser Iridium or Laser Platinum. These dual precious metal plugs feature either iridium or platinum on the center firing electrode and platinum at the ground electrode. Because these electrode materials are denser, gap erosion is reduced, allowing a longer service interval. Being that these materials are denser, this will reduce gap erosion, thus leading to a much longer service interval. The use of nickel-alloy or single precious metal spark plugs in vehicles equipped with waste spark ignitions will result inmore aggressive gap erosion and shorter spark plug life.
- Tune - UPs
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What comes to mind when you think of a tune-up? In older model vehicles, a tune-up could have referred to replacing spark plugs, spark plug wires, the distributor cap and rotor, or adjusting the timing, or carburetor. With modern vehicles, fuel delivery and timing are controlled electronically via the vehicle’s Engine Control Unit or ECU. This brings one question to mind, what does a proper tune-up consist of on today’s modern vehicles?
For many modern vehicles, a tune-up consists of nothing more than replacing spark plugs and checking the condition of other critical ignition and emission components.
Conventional Spark Plugs
Older vehicles used conventional, non-precious metal spark plugs thatrequired replacement every 30,000 miles or less. However, in today’s modern vehicles, most manufacturers have switched to long-life precious metal platinum and iridium spark plugs. Not only do these spark plugs provide a longer service life, they also provide additional benefits, such as improved fuel economy and reduced emissions.
Coil On Plug Boots
Ignition systems have also undergone advancements from the days of the distributor cap and rotor. Modern ignition systems have eliminated spark plug wires and distributor cap and rotor with coil-on-plug or COP technology. COP ignition coils are typically non-wearing items, which are replaced only if one has failed. Often COP boots connect the ignition coil to the spark plug. COP boots can become brittle when exposed to years of heat and chemical damage. Cracked or damaged coil on plug boots should be replaced at the time of spark plug replacement.
Oxygen Sensor
Oxygen sensors also play an important role in fuel economy. The condition of oxygen sensors can also be checked at the time of spark plug maintenance. A professional shop can check oxygen sensor performance by checking sensor response time and heater performance. A slow or non-responding oxygen sensor should illuminate a check engine light.
Please refer to the vehicle service schedule for a complete list of maintenance items to maximize engine life and performance.
- Misfires
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When one thinks of misfires, whichautomotive component of the ignition system comes to mind?
Most people would agreeon spark plugs. However, there are many elements that can cause a misfire. A misfiring spark plug is often a symptom of another issue with the vehicle and not always the cause of the trouble. Replacing the spark plugs may temporarily improve the vehicle’s performance, but ultimately, the problem will return unless the root issue is repaired. Problems with the ignition system, fuel system, emissions system and overall engine condition, may lead to misfire.
Ignition system components including the distributor, distributor cap, rotor, spark plug wire set, coil, or coil on plug boot, can create misfires if they have failed. A worn set of spark plug wires may allow voltage to escape before reaching the spark plug, causing a misfire or no-spark condition and may eventually result in a fouled spark plug, requiring replacement. Coil failures may be intermittent and may only occur under high temperature conditions. Spark plugs which have exceeded their recommended service interval and have expanded gaps, may also contribute to misfires.
Fuel system problems may cause a rich or lean condition, which can contribute to spark plug misfires. Leaky, or clogged, fuel injectors or poorly tuned carburetors are often the cause. Routine replacement of fuel filters can help prevent fuel system problems. Engines which are used for seasonal use, such as lawnmowers or snow blowers, can often suffer from build-up of varnish on fuel system components if proper fuel storage methods are not followed.
The emissions system, specifically the oxygen sensors, is designed to provide feedback to the ECU, which can strengthen or weaken the fuel delivery. If an oxygen sensor has failed, or is nolonger communicating with the ECU, the vehicle will default to a safer rich fuel condition. This is known as open loop operation. Prolonged driving in open loop mode not only wastes fuel, but can cause damage to the catalytic converters and may cause fuel-fouled spark plugs.
Finally, we must not overlook the overall condition of the engine. Spark plugs can serve as a window to the engine’s health. Spark plugs which are oil-soaked, or have large amounts of deposits, may show the engine has poor oil seals or low compression. If these conditions are severe enough, the spark plugs may become oil-fouled and result in a no-spark condition. These mechanical problems must be repaired or the problem may continue to repeat itself.
If any of these conditions appear outside of routine spark plug maintenance, it is a good idea to have the engine inspected by a reputable repair facility and to have any service issues repaired. Correcting the root problem will save time, money and headaches in the long run.
- Heat Ranges
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One of the most misunderstood aspects of spark plugs is its heat range. It is believed by many that the heat range measures spark temperature or intensity. This is incorrect as the heat range is actually a measurement of the plug’s ability to transfer heat away from the tip of the spark plug. One cannot change the temperature of how hot a fuel burns.
A hot spark plug has an insulator design that will be slower to draw heat away from the plug tip (thinner insulator mass), whereas a cold plug has an insulator design that will be faster to draw heat away from the plug tip (thicker insulator mass). For a spark plug to function properly it must have a tip temperature hot enough to invoke self-cleaning, while remaining cool enough to avoid pre-ignition.
For most vehicles, the factory recommended heat range is sufficient; however,on some modified or special use engines alternative heat ranges may be necessary. Often hotter heat-ranges have been used to attempt to correctan underlying fuel or oil consumption problem; this is merely a cover-up fix and the underlying issues will ultimately have to be addressed.
The image below is a representation of the difference between a hot and cold heat range. NGK spark plug’s heat range goes from 2 (hottest) to 11 (coldest).
- NGK Racing Spark Plugs
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Did you know that all of NGK’s racing-series plugs have their part number engraved on the metal body and not on the ceramic insulator? The ceramic insulator on racing spark plugs will only show the heat range number. In order to find the full part number, one will need to look under the hex and above the seating surface — there one will find the full part number (this number will always begin with an ‘R’ and will end with the heat range number, e.g. R5671A-8).The pictures below are good illustrations of both the heat range stamping on the ceramic as well as the full part number engraved in the metal body.
- Power Equipment Spark Plugs
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Did you know there are several different spark plugs for power equipment applications? With so many spark plug options, it is very important to use the correct spark plugs for each application. Grouping all power equipment spark plugs into “long” or “short” spark plugs is not specific enough to meet the necessary spark plug design demands for each different engine. When NGK spark plugs are used as original equipment, the NGK plug number is stamped into the metal around the base of the spark plug.
Critical design specifications:
- Heat Range
- Thread size/reach
- Projection
- Seat style
Heat Range
Critical to keep spark plugs from pre-ignition and fouling
Thread Size / Reach - c ritical to spark plug functions including heat dissipation
Projection
A plug with more projection than specified may cause piston interference.
Seat Style
Spark plugs are available in both “tapered” and “gasket” - seat styles, which must match the cylinder head specifications.
Caution
Use caution with cross references, as cross references should be used as reference only and may not provide a perfect apples-to-apples replacement. Whenever possible, note the specific engine model information, in order to look up the correct spark plug in a catalog.
Spark plug manufacturers’ print and online catalogs should be checked whenever looking up spark plug part numbers. Incorrect spark plugs can result in damaged equipment and increased down time. To a professional landscaper, down equipment means lost opportunity, i.e., productivity and profits. Taking the time touse the correct spark plugs can help prevent this aggravation.
Keep the machine’s owner’s manual handy. It is a good idea to keep a list of frequently replaced service parts fore ach piece of equipment owned, as a folder or notebook containing the equipment model information and part number information for items such as spark plugs, filters, and blades — this will help ensure the correct parts are ordered. It is also a good idea to make a note of the date each machine has been serviced, to see to it that recommended maintenance intervals are met.