how come there is very little information on spark plugs here....
I found some very useful info...
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.
Spark Plugs 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 spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type.
Heat rating and heat flow path of Spark Plugs
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 (< 450°C) and overheating (> 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
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.
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.
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.
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.
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.
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.
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 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.
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”.
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:
- 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
A. Installing Spark Plugs
Torque is one of the most critical aspects of spark plug installation. Torque directly affects the spark plugs' ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat transfer will be slowed. This will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage is not far behind.
An over-torqued spark plug can suffer from severe stress to the Metal Shell which in turn can distort the spark plug's inner gas seals or even cause a hairline fracture to the spark plug's insulator...in either case, heat transfer can again be slowed and the above mentioned conditions can occur.
The spark plug holes must always be cleaned prior to installation, otherwise you may be torquing against dirt or debris and the spark plug may actually end up under-torqued, even though your torque wrench says otherwise. Of course, you should only install spark plugs in a cool engine, because metal expands when its hot and installation may prove difficult. Proper torque specs for both aluminum and cast iron cylinder heads are listed below.
Spark Plug Type Thread Diameter Cast Iron Cylinder Head (lb-ft.) Aluminum Clyinder Head (lb-ft.)
Flat seat type (with gasket) 18 ø mm 25.3~32.5 25.3~32.5
" 14 ø mm 18.0~25.3 18.0~21.6
" 12 ø mm 10.8~18.0 10.8~14.5
" 10 ø mm 7.2~10.8 7.2~8.7
" 8 ø mm -- 5.8~7.2
Conical seat type
(without gasket) 18 ø mm 14.5~21.6 14.5~21.6
Conical seat type
(without gasket) 14 ø mm 10.8~18.0 7.2~14.5
B. Installing Spark Plugs - Lawn & Garden Equipment
Confirm that the thread reach of the spark plug is the right one for your engine.
Remove the dirt at the gasket seal of the cylinder head.
Tighten the spark plug finger tight until the gasket reaches the cylinder head, then tighten about 1/2-2/3 turn more with a spark plug wrench.
(Taper seat: About 1/16 turn more).
Since the gap size has a direct affect on the spark plug's tip temperature
and on the voltage necessary to ionize (light) the air/fuel mixture, careful attention is required. While it is a popular misconception that plugs are pre-gapped from the factory, the fact remains that the gap must be adjusted for the vehicle that the spark plug is intended for. Those with modified engines must remember that a modified engine with higher compression or forced induction will typically require a smaller gap settings (to ensure ignitability
in these denser air/fuel mixtures). As a rule, the more power you are making, the smaller the gap you will need.
A spark plug's voltage requirement is directly proportionate to the gap size. The larger the gap, the more voltage is needed to bridge the gap. Most experienced tuners know that opening gaps up to present a larger spark to the air/fuel mixture maximizes burn efficiency. It is for this reason that most racers add high power ignition systems. The added power allows them to open the gap yet still provide a strong spark.
With this mind, many think the larger the gap the better. In fact, some aftermarket ignition systems boast that their systems can tolerate gaps that are extreme. Be wary of such claims. In most cases, the largest gap you can run may still be smaller than you think.
This is for racers only !!
Indexing refers to a process whereby auxiliary washers of varying thickness are placed under the spark plug's shoulder so that when the spark plug is tightened, the gap will point in the desired direction.
However, without running an engine on a dyno, it is impossible to gauge which type of indexing works best in your engine. While most engines like the spark plug's gap open to the intake valve, there are still other combinations that make more power with the gap pointed toward the exhaust valve.
In any case, engines with indexed spark plugs will typically make only a few more horsepower, typically less than 1% of total engine output. For a 500hp engine, you'd be lucky to get 5hp. While there are exceptions, the bottom line is that without a dyno, gauging success will be difficult.
E. Heat Range Selection
Let's make this really simple: when you need your engine to run a little cooler, run a colder plug. When you need your engine to run a little hotter, run a hotter spark plug. However, NGK strongly cautions people that going
to a hotter spark plug can sometimes mask a serious symptom of another problem that can lead to engine damage. Be very careful with heat ranges. Seek professional guidance if you are unsure.
With modified engines (those engines that have increased their compression) more heat is a by-product of the added power that normally comes with increased compression. In short, select one heat range colder for every 75-100 hp you add, or when you significantly raise compression. Also remember to retard the timing a little and to increase fuel enrichment and octane. These tips are critical when adding forced induction (turbos, superchargers or nitrous kits), and failure to address ALL of these areas
will virtually guarantee engine damage.
An engine that has poor oil control can sometimes mask the symptom temporarily by running a slightly hotter spark plug. While this is a "Band-Aid" approach, it is one of the only examples of when and why one would select
a hotter spark plug.
F. Using "Racing" Spark Plugs
Be cautious! In reality, most "racing" spark plugs are just colder heat ranges of the street versions of the spark plug. They don't provide any more voltage to the spark plug tip! Their internal construction is no different (in NGK's case, as all of our spark plugs must conform to the same level of quality controls) than most standard spark plugs.
There are some exceptions, though. Extremely high compression cars or those running exotic fuels will have different spark plug requirements and hence spark plugs that are well-suited for these requirements. They are classified as "specialized spark plugs for racing applications". Some are built with precious metal alloy tips for greater durability and the ability to fire in denser or leaner air/fuel mixtures. However, installing the same spark plugs Kenny Bernstein uses in his 300+ mph Top Fuel car (running Nitromethane at a 2:1 air/fuel ratio and over 20:1 dynamic compression) in your basically stock Honda Civic (running 15:1 a/f ratios with roughly 9.5:1 compression) will do nothing for you! In fact, since
Kenny's plugs are fully 4 heat ranges colder, they'd foul out in your Honda
in just a few minutes.
A racing spark plug (or ANY spark plug) will NOT give you large gains in horsepower. While certain
spark plugs are better suited to certain applications (and we're happy to counsel you in the right direction) we try to tell people that are looking to "screw in" some cheap horsepower that, in most cases, spark plugs are
not the answer.
To be blunt, when experienced tuners build race motors, they select their spark plugs for different reasons: to remove heat more efficiently, provide sufficient spark to completely light all the air/fuel mixture, to survive the added stresses placed upon a high performance engine's spark plugs,
and to achieve optimum piston-to-plug clearance.
Some of these "specialized racing plugs" are made with precious metal alloy center/ground electrodes or fine wire tips or retracted-nose insulators. Again, these features do not necessarily mean that the spark plug will allow the engine to make more power, but these features are what allow the spark plug to survive in these tortuous conditions. Most racers know screwing in a new set of spark plugs will not magically "unlock" hidden horsepower.
G. Using High Power Ignition Systems
Many of the more popular aftermarket ignition systems are of the capacitive discharge type. They store voltage, or accumulate it, until a point at which a trigger signal allows release of this more powerful spark. Companies like Mallory, MSD, Crane and Accel, to name a few, offer such systems.
They affect spark plugs in that they allow the gaps to be opened up to take advantage of the increased capacity. The theory is that the larger and the more intense the spark you are able to present to the air/fuel mixture, the more likely you will be to burn more fuel, and hence the more power you will make.
We encourage the use of such systems, but only on modified or older non-computer controlled vehicles.
In reality, computer controlled vehicles do such a good job of lighting off the air/fuel mixture (as evidenced by the ultra-low emissions), added ignition capacity would do little to burn more fuel since the stock configuration is doing such a good job. Older non-computer controlled vehicles or those
that have been modified with higher compression or boosted (nitrous, turbo, supercharged) engines can certainly take advantage of a more powerful ignition system.