What are the primary functions of a spark plug?
The spark plug performs four main functions:
It is important to remember that spark plugs do not create heat, they help remove it. Anything that prevents a spark plug from removing the proper amount of heat can lead to pre-ignition, detonation, premature spark plug failure and even internal engine damage, especially in two stroke engine or modified engines.
In the simplest of terms, the spark plug acts as the thermometer of the engine... much like a doctor examining a patient, this “thermometer” can be used by mechanics to effectively diagnose the amount of heat present in each combustion chamber.
The experienced technician will then be able to interpret the plugs’ visual cues and accurately determine the engine’s overall operating condition, get a feel for air/fuel ratios, and even diagnose driveability problems, provided the technician has received proper training.
Spark plugs are valuable tuning tools, when interpreted correctly. They will show symptoms of other problems and can reveal a great deal about the engine’s overall condition. The experienced technician will evaluate the appearance of the spark plug’s firing tip and will begin to make a diagnosis or some basic assumptions based on the physical appearance of the spark plugs.
Why do we need to change spark plugs in the first place?
As spark plugs grow older, they lose their sharp edges and material from the center and ground electrodes is slowly eroded away. As the gap between these two points grows, the voltage required to bridge this gap increases proportionately. The ignition system must work harder to compensate for this higher voltage requirement and hence there are a greater rate of misfires, or incomplete combustion cycles.
Each misfire means lost horsepower, reduced fuel economy and higher emissions. Replacing worn out spark plugs with new ones (with sharp new edges) effectively restores the ignition system’s efficiency and reduces the percentage of misfires, restoring power, economy, and reducing emissions.
How long spark plugs last will depend on a variety of factors, including engine compression, fuel used, gap, center/ground electrode material, how the vehicle is used, etc.
How much power can I expect from a spark plug change?
A common misconception is that changing spark plugs will sometimes result in large power gains. In most cases, removing even seriously worn out spark plugs will only result in very modest power gains, typically about 1%-2% of total engine output, perhaps less for computer controlled vehicles, primarily because most newer vehicles have more powerful ignition systems and the vehicle’s computer can make adjustments to make vehicle operation seem smoother and more seamless .
Many people think that simply supplying more spark to the firing tip can/will combust more fuel. What most people don’t understand is that most newer cars engine’s are so efficient that they are already burning all of the available fuel...simply adding more spark or voltage can’t burn any more fuel, because there is no more fuel to burn!!
But, as time goes on, the spark plugs lose much of their electrode surface area to natural erosion and even the best ignition systems will be strained to supply enough voltage to bridge the gap...some fuel is left unburned and it is at this point that a spark plug change becomes necessary.
In most cases, not until the engine is modified or compression is raised significantly that stock ignition systems and standard spark plugs begin to show signs of inadequacies.
At this point, a variety of factors will determine which spark plug would be best suited for a particular application. In these modified engines, specific electrode/tip configurations, electrode materials, and colder heat ranges can indeed provide measurable gains in power, but only in highly modified engines.
When a stock or near stock engine is given a fresh set of spark plugs, peak efficiency is restored, and the power gains that come from this restored state of tune are usually minimal. Any company that tells you that their spark plug will provide significant gains in power in a stock or near stock engine is making blanket statements that may be insupportable.
What is a heat range? When should I change heat ranges?
A heat range refers to how much heat a spark plug is capable of removing from the combustion chamber...remember, spark plugs DO NOT create heat.
Selecting a spark plug with the proper heat range will ensure that the tip will maintain a temperature high enough to prevent fouling, yet be cool enough to prevent pre-ignition.
While there are many other things that can cause pre-ignition, selecting the proper heat range spark plug will ensure that the spark plug itself is not a hot-spot source.
For NGK Spark Plugs, every heat range colder translates into the ability to remove 70-100 degrees Celsius more heat.
As a rule of thumb, every time you increase compression by one point, or for every 75-100 hp you add, go at least one step colder. Remember, it is better to go colder than is thought necessary...the worst that can happenin this scenario is that the plugs will foul out...if you inadvertently select a heat range that is too hot, the resulting pre-ignition and detonation can cause serious engine damage.
What is a “fouled” spark plug?
A spark plug is said to be fouled when the insulator nose at the firing tip becomes coated with a foreign substance, such as fuel, oil, or carbon.
This coating makes it easier for the voltage to follow along the insulator nose and leach back down into the metal shell, grounding out, rather than bridging the gap normally.
Fuel, oil, and carbon fouling can all be caused by different things, but in any case, once a spark plug is fouled, it will not provide voltage to the firing tip and that cylinder will not fire properly.
In many cases, the spark plug cannot be cleaned sufficiently to restore normal operation. It is therefore recommended that fouled plugs be replaced.
Do I need to use a resistor spark plug? Does a resistor hurt power output?
It takes only 1,000 volts to pass through the resistor and even the weakest and most primitive ignition systems are easily capable to pushing voltage through a resistor.
Today’s ignition systems are easily capable of sustaining well over 20,000 volts.
In fact, using a non-resistor plug in certain applications can actually cause the engine to suffer some strange side effects, such as an erratic idle, high rpm misfire, engine run-on, power drop-off at certain rpm levels, and abnormal combustion. This is because resistor spark plugs reduce electromagnetic interference with on-board electronics.
In English, this means stray voltage can “talk back” to on-board electronic systems, confusing them. In some cases, engine damage can occur.
NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance, or on any vehicle that has other on-board electronic systems such as Engine Management computers, two way radios, GPS systems, depth finders, etc., or whenever recommended by the engine manufacturer.
What is “detonation” and how does it affect the spark plug?
Detonation refers to a type of abnormal combustion that is usually preceded by pre-ignition. It is most often caused by a hot spot formed in the combustion chamber. As air and fuel is drawn into the combustion chamber during the intake stroke, this hot spot will “pre-ignite” the air fuel mixture without any spark from the spark plugs.
Detonation exerts a great deal of downward force on the pistons as they are being forced upward by the mechanical action of the connecting rods. When this occurs, the resulting concussion, shock waves and heat can be severe...spark plug tips can be broken, or melted, and other internal engine components such as the pistons or connecting rods themselves can be damaged.
Left unresolved, engine damage is almost certain to occur, with the spark plug usually suffering the first signs of damage.
Remember, when you observe signs of detonation or pre-ignition by examining the spark plugs’ firing tip, you are observing a symptom of another problem.
The trick is to figure out what caused the hot spot to form in the first place. You should seek professional guidance to determine the root cause of the problem.
Please review the reference chart below to examine the symptoms, causes, and results of knock and pre-ignition.
Are plugs pre-gapped from the factory?
Yes and no. A spark plug part number might fit hundreds of engines, and although the factory will typically set gap to a pre-selected setting, this gap may not be the right one for your particular engine, and may not take into account modifications that you may have performed to the engine.
Insufficient spark plug gap can cause pre-ignition, detonation, even engine damage. Too much gap can result in a higher rate of misfires, noticeable loss of power, plug fouling, and poor economy.
When you raise compression or add forced induction (a turbo system, nitrous, or supercharger kit), you must lower the gap (reduce gap about .004 inches for every 50 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology,) you can open the gap back up about .002 -.005 inches.
As an example, let’s use a hypothetical ‘96 350 Chevrolet LT1 engine build-up...standard gap is .050" for an un-modified LT1. We’ll add 150hp Nitrous, so we must lower the gap about .012 to .038 inches. We then decide to add that killer MSD 6A/Crane Hi-6 box and, using our guidelines as outlined above, we can now open the gap up .002 -.005 inches to about .040 - .043 inches.....following this basic guideline should get you very close.
Further experimentation may be necessary, but by always starting with a larger gap than it thought necessary to reduce the risk of detonation, you should be safe.
What is the purpose of multiple ground electrode plugs? Will they give me multiple sparks?
No, a spark plug with more than one ground electrode will NOT give you more than one spark at a time. Electricity is lazy, so it will always follow the path of least resistance. In the case of spark plugs with multiple ground electrodes, one ground electrode surface will always be just a little closer than the other, and hence it will receive the spark. As this surface begins to wear, the next closest ground electrode will then start receiving the spark.
Then why use them in the first place? NGK offers many spark plugs with multiple electrode configurations. Each type is designed for a specific application, such as high voltage ignition systems with extremely high cylinder pressures, including some outboard engines, rotary engines and some import applications. The extra ground electrodes serve only to keep the ground electrodes cooler, extend the life of the plug, remove more heat, and to fire effectively in certain combustion chamber configurations where single electrode spark plugs would foul.
They will usually not deliver more power in a non-specified application, and the fact that some of these are a much colder heat range than average spark plugs makes them very susceptible to fouling when used outside of their recommended application.
To suggest that multiple ground electrode spark plugs hold some performance advantage over single electrode spark plugs would be citing an exception rather than the rule, be wary of such claims.
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