The Ultimate Guide to Spark Plugs
The BasicsGeneral Description A spark plug (sometimes in British English a sparking plug, colloquially a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil or magneto. The spark plug’s metal shell is screwed into the engine’s cylinder head and thus electrically grounded. The central electrode protrudes through the porcelain insulator into the combustion chamber, forming one or more spark gaps between the inner end of the central electrode and usually one or more protuberances or structures attached to the inner end of the threaded shell and designated the “side”, “earth”, or “ground” electrode(s). Spark plugs may also be used for other purposes; in Saab Direct Ignition when they are not firing, spark plugs are used to measure ionization in the cylinders – this ionic current measurement is used to replace the ordinary cam phase sensor, knock sensor and misfire measurement function. Spark plugs may also be used in other applications such as furnaces wherein a combustible fuel/air mixture must be ignited. In this case, they are sometimes referred to as flame igniters. Operation The plug is connected to the high voltage generated by an ignition coil or magneto. As the electrons flow from the coil, a voltage difference develops between the central electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows electrons to flow across the gap. Spark plugs usually require voltage of 12,000–25,000 volts or more to ‘fire’ properly, although it can go up to 45,000 volts. They supply higher current during the discharge process resulting in a hotter and longer-duration spark. As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K. The intense heat in the spark channel causes the ionized gas to expand very quickly, like a small explosion. This is the ‘click’ heard when observing a spark, similar to lightning and thunder. The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. A small kernel will make the engine run as though the ignition timing was retarded, and a large one as though the timing was advanced.
ConstructionTerminal The top of the spark plug contains a terminal to connect to the ignition system. The exact terminal construction varies depending on the use of the spark plug. Most passenger car spark plug wires snap onto the terminal of the plug, but some wires have spade connectors which are fastened onto the plug under a nut. Plugs which are used for these applications often have the end of the terminal serve a double purpose as the nut on a thin threaded shaft so that they can be used for either type of connection. Insulator The main part of the insulator is typically made from sintered alumina,), a very hard ceramic material with high dielectric strength, printed with the manufacturer’s name and identifying marks, then glazed to improve resistance to surface spark tracking. Its major function is to provide mechanical support and electrical insulation for the central electrode, while also providing an extended spark path for flash-over protection. This extended portion, particularly in engines with deeply recessed plugs, helps extend the terminal above the cylinder head so as to make it more readily accessible. Ribs By lengthening the surface between the high voltage terminal and the grounded metal case of the spark plug, the physical shape of the ribs functions to improve the electrical insulation and prevent electrical energy from leaking along the insulator surface from the terminal to the metal case. The disrupted and longer path makes the electricity encounter more resistance along the surface of the spark plug even in the presence of dirt and moisture. Some spark plugs are manufactured without ribs; improvements in the dielectric strength of the insulator make them less important. Insulator tip On modern (post 1930’s) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminium oxide (alumina) ceramic as the upper portion, merely un-glazed. It is designed to withstand 650 °C (1,200 °F) and 60,000 volts. The dimensions of the insulator and the metal conductor core determine the heat range of the plug. Short insulators are usually “cooler” plugs, while “hotter” plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core. Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of mica, compressed by tension in the centre electrode.
With the development of leaded petrol in the 1930’s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by Siemens in Germany to counteract this. Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and (thermal) shock resistance at higher temperatures, and this ability to run hot allows it to be run at “self cleaning” temperatures without rapid degradation. It also allows a simple single piece construction at low cost but high mechanical reliability. Seals Because the spark plug also seals the combustion chamber or the engine when installed, seals are required to ensure there is no leakage from the combustion chamber. The internal seals of modern plugs are made of compressed glass/metal powder, but old style seals were typically made by the use of a multi-layer braze. The external seal is usually a crush washer, but some manufacturers use the cheaper method of a taper interface and simple compression to attempt sealing. Metal case The metal case (or the “jacket” as many people call it) of the spark plug withstands the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the central electrode to the side electrode. Spark plug threads are cold rolled to prevent thermal cycle fatigue. Also, a marine spark plug’s shell is double-dipped, zinc-chromate coated metal. Central electrode
Variations on the basic designOver the years variations on the basic spark plug design have attempted to provide either better ignition, longer life, or both. Such variations include the use of two, three, or four equally spaced ground electrodes surrounding the central electrode. Other variations include using a recessed central electrode surrounded by the spark plug thread, which effectively becomes the ground electrode (see “surface-discharge spark plug”, below). Also there is the use of a V-shaped notch in the tip of the ground electrode. Multiple ground electrodes generally provide longer life, as when the spark gap widens due to electric discharge wear, the spark moves to another closer ground electrode. The disadvantage of multiple ground electrodes is that a shielding effect can occur in the engine combustion chamber inhibiting the flame face as the fuel air mixture burns. This can result in a less efficient burn and increased fuel consumption Surface-discharge spark plug A piston engine has a part of the combustion chamber that is always out of reach of the piston; and this zone is where the conventional spark plug is located. A Wankel engine has a permanently varying combustion area; and the spark plug is inevitably swept by the tip seals. Clearly, if a spark plug were to protrude into the Wankel’s combustion chamber it would foul the rotating tip; and if the plug were recessed to avoid this, the sunken spark might lead to poor combustion. So a new type of “surface discharge” plug was developed for the Wankel. Such a plug presents an almost flat face to the combustion chamber. A stubby centre electrode projects only very slightly; and the entire earthed body of the plug acts as the side electrode. The advantage is that the plug sits just beneath the tip-seal that sweeps over it, keeping the spark accessible to the fuel/air mixture. The “plug gap” remains constant throughout its life; and the spark path will continually vary (instead of darting from the center to the side electrode as in a conventional plug). Whereas a conventional side electrode will (admittedly, rarely) come adrift in use and potentially cause engine damage, this is impossible with a surface discharge plug, as there is nothing to break off. Surface-discharge spark plugs have been produced by Inter Alia, Denso, Champion and Bosch.
Plug GapThe spark plug gap, along with the combustion chamber pressure and the ignition timing has a direct bearing on the amount of voltage you require from the ignition system. The bigger the spark plug gap, the more air/fuel mixture will come into contact with the spark and the easier it will be to ignite the air/fuel mixture. However, a bigger spark plug gap requires more voltage from the ignition coil to create a spark that can arc across the gap between the central electrode, that is connected to the coil via the HT leads, and the ground electrode. Similarly, when the combustion chamber pressure is increased more voltage is required to arc the spark plug gap. Insufficient voltage will result in a failure to create a spark across the gap and may be noticeable as a misfire. However, misfires are not always noticeable, especially at high rpm but it will have an adverse effect fuel consumption and on engine power and performance. A narrow spark plug gap would require less voltage to spark, but the spark might be too small and weak to ignite and consume the fuel-air mixture in the time available for the ignition phase of the Otto cycle. The result is a failure to effectively convert the chemical energy in the fuel-mixture to mechanical energy, which is how engine power is produced. Thus, engine power and engine performance will not be optimized. However, it’s not simply a matter of increasing the spark plug gap and the output voltage from the ignition coil to improve power as firstly, there is a limit to the amount of voltage the ignition system can handle and, secondly, there is an optimal spark plug gap that will best suite the performance of your engine and your driving style. As a rule, a properly gapped spark plug will burn hot without being too wide at high rpm to cause a misfire. Ironically, the car manufacturer’s recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010″ for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010″ for better performance. The spark plug itself, and the residue that forms on it, would indicate whether the gap is too big or too small. A light brownish discoloration of the tip of to porcelain insulator indicates the proper operation of the spark plugs with the gap being ideal or close to ideal for the most recentengine speeds. Thus, to check the spark plug gap at high engine speeds, you’d need to run at full throttle and immediately turn the ignition off without allowing the engine to idle. But ultimately, you’d need to run your car on a dynamometer to find the best spark plug gap, and the right ignition timing for your engine. Remember that when you increase the spark plug gap you need more voltage from the ignition coil to create a spark across the spark plug gap. We’ll discuss ignition voltage at a later stage. When a greater voltage is required to create a spark, cold starting and firing fouled spark plugs become more difficult. Therefore you should ensure that your secondary, high-tension ignition wiring is at least 8 mm in diameter, and that it is always clean, dry and in peak condition. Also note that it is not advised to adjust the gap on a multi-electrode spark plug as this will affect the proper operation of the spark plug.
Different spark plug sizes. The left and right plug are identical in threading, electrodes, tip protrusion, and heat range. The center plug is a compact variant, with smaller hex and porcelain portions outside the head, to be used where space is limited. The rightmost plug has a longer threaded portion, to be used in a thicker cylinder head.
Heat RangeThe 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 some spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type, while other brands index theirs in the opposite manner. Make sure to always verify with the plug manufacturer. 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)
- 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.
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.
- 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.
- 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.
- 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.
- 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.
Fouling and CausesCauses 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
- 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
Types of Abnormal CombustionPre-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.
- 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:
- 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
Reading Your PlugsReading For Air Fuel Mixture The porcelain around the plug’s center electrode can be divided into three areas for reading. The area that is closest to the tip is affected by the idle and transition circuits carburetor circuits and is of no real concern to a racer. If this area is gray then you drove the car back to the pits and you cannot correctly read the plugs. The middle area is only colored when you drive down the road at around a steady 30-40 mph and is normally affected by the primary circuit jetting with the power valve closed and this is really of no concern to the racer. The area you are interested in is that third that is all the way up inside the plug where the sun don’t shine. This area is colored when all is wide open under full power because the combustion chamber heat totally cleans off the other two areas. It will take a special plug reading flashlight with the magnifying glass to view it correctly. Plugs cannot be correctly read by just quickly looking at them with the naked eye. You see people doing it all the time because they do not know how to read plugs. Normally aspirated cars should have a light gray or tan hydrocarbon ring or as some call it a “fuel ring” all the way up inside around the third area closest to the point where the porcelain is attached to the metal jacket of the plug. The actual color may depend on type of fuel you use. This fuel ring should appear like a light shadow. Most VP C-15, C-16 or C23+ fuels will show as a light gray when correct. This fuel ring starts to color on the porcelain side that is below the ground strap and works its way around either side of the center electrode until it completely joins. Sometimes it may take two or three runs to see a good coloring. Note: New engines or engines that pump a little oil may show a thin oily line way down inside on the porcelain where the porcelain meets the metal wall of the plug. This oil line has nothing to do with the air/fuel mixture but may be confused with the fuel ring you are looking for.If you are having a hard time figuring out if what you are reading is correct or because you are not sure if the plug heat range is correct then tow the car back to the pits and drop the headers and look inside the pipes. If they are black then you are too rich, if they are light gray or white then you are too lean. The pipes should be a medium to dark gray or tan color. Normally the white area of the porcelain has a chalky appearance. If you see the porcelain take on a shine then it is time to change the plugs because the glass that is in the porcelain has been melted and has glazed the surface. If the car has been running rich (due to lots of idling or incorrect fuel mixture) then it is possible to glaze the plugs and short them out during a run because of the sudden heating of the plug with the soot on the porcelain. This glazing appears to be a glossy coating on the porcelain with a splotches of color of greenish yellow or brown. These two different glazings will cause the plug to short out and misfire and raise ring lands or make a popping through the exhaust when going down the track. Reading For Ignition Timing Ignition timing is directly responsible for the heat in the combustion chamber and therefore the color of the plug’s ground strap and the color of the first few threads on the outside of the plug. The ignition timing can be checked by looking at the color of the plug’s ground strap and the position of the “blue line” on the strap. The blue line really indicates the point at which the strap has reached annealing temperature of the metal. To help to understand this think of a bar of steel (ground strap) on a table that is being super heated with a acetylene torch at one of the tip ends. As the end heats up and the heat starts moving down the bar you will see a blue line across the bar at some point down the bar away from tip with the torch. This blue line reflects the temperature that is the annealing point of the metal. As the temperature increases the blue line moves further down the bar away from the torch. Similarly, the blue line moves down the spark plug ground strap as you put more heat in the engine. If you are using a gold colored ground strap like with an NGK spark plug then not enough timing will show the ground strap as still gold or going light gray maybe with a few bubbles on it after a run. As you advance the ignition and put heat in the engine the plug ground strap will turn darker gray as well as the metal at the end of the threaded area. As the metal turns medium to dark gray you should start looking for the blue line (band) around the ground strap. Ideally, you want this blue line to be just below where the ground strap makes the sharp bend and above the weld. If you advance the ignition too far the blue will disappear off the strap and the strap will pick up rainbow colors (blues and greens). The next step beyond that is to start melting the strap from the tip end and detonation. When you are close to the correct timing then only change the timing by one degree at a time. If you ignition system has the capability of adjusting the timing of each cylinder independently (ICT) then you can use that feature to have the blue line in the same position on all the plugs. First, adjust the basic timing to get as many of the plugs to have the blue line just at the sharp bend in the strap. Now adjust the ICT to move the blue line to the same point on the remaining plugs. Once all the plugs read the same you can advance the ignition a little at a time to put the blue line just above the weld on the strap or whatever point gives you the best performance. Other Things To Look For The round flat circular area of the plug at the end the threads should be dark gray or flat black and should not be sooty. If it is sooty then it can mean that your plug has not been tightened enough and you are sucking and blowing fuel and air past the threads of the plug. Detonation shows up on the plugs as spotting on the porcelain. There are two different types of spotting seen. One type appears as just black spots and the other appears as little bright spots like diamonds. The black spots (look like pepper sprinkled on the plug) indicate a little too much heat on the plug which causes detonation by having the heated plug fire off the mixture prior to the spark firing. This creates two flame fronts that collide and can cause great amounts of damage. If you see black spots on the porcelain and you know the tune-up is correct then you may need a colder plug. If you are not sure then increase the carburetor jet size slightly, take out some timing, or go to a colder plug. If you hold the plug in the sun and you see what appears to be small diamonds on the porcelain then your detonation is severe enough to be blowing off the aluminum from your piston and you need to add fuel and/or take out timing now. Spark Plug Heat Range If you keep on adding timing until your finish MPH falls off but you still have no color on the plug’s ground strap but the porcelain has good color then your plug is too cold. If you have lots of color on the ground strap but the porcelain is clean and white then the plug heat range is too hot. The heat from the plug is cleaning of the fuel ring from the porcelain. Tip Temperature and Firing End Appearance
|1 Oil Fouled||2 Oil Fouled||3 Carbon Fouled||4 Too Cold|
|5 Too Cold||6 Cold or Rich But OK||7 Cold or Rich But OK||8 Cold or Rich But OK|
|9 Good||10 Good||11 Good||12 Good|
|13 Real Good||14 The Best||15 Best||16 Best|
|17 Best||18 Good||19 Good||20 Good|
|21 Kinda Hot But OK||22 Hot or Lean But OK (?)||23 Hot or Lean But OK (?)||24 Hot or Lean But OK (?)|
|25 Too Hot or Lean Pre-Ignition Range||26 Too Hot or Lean Pre-Ignition Range||27 Too Hot or Lean Pre-Ignition Range||28 Too Hot or Lean Pre-Ignition Range|
|29 Too Hot or Lean Pre-Ignition Range|
Blown Alcohol Motor Spark Plug ReadingReading a blown alcohol tune-up using spark plugs is a very different than reading a gasoline tune-up on spark plugs. First major difference for alcohol is that you do not read the color off the porcelain around the center electrode. Air to fuel ratios within the combustion chamber are going to be read by the appearance of the metal base ring at the end of the threaded area of the plugs and the color of the first three threads of the spark plugs. The amount of ignition advance is still read as with gasoline plugs by the blue line on the plugs ground strap or sometimes referred to as the ground electrode. An important note is that the spark plugs only reflect the tune-up that was in the motor just prior to finish line engine shutdown. In a ¼ mile this is normally just the last 300 feet in high gear but it takes 1000 feet to develop the correct appearance. It is possible for the plugs to indicate a good tune-up at this point but to have run too rich or too lean in first or second gear or at a lower rpm. This makes it possible to damage the engine due to incorrect fuel mixture in the first part of the run and actually be correct at the finish line. It is highly recommended that the initial tune-up runs be limited to 1/8th mile plug readings before proceeding on to the ¼ mile run. Most the damage to an engine is done in the last 300 feet. The fuel mixture and the ignition timing are totally intertwined as to the effecting the appearance of the spark plug ring at the of the threads and the EGT values. Changing the fuel to air ratio and changing the ignition will both change the combustion chamber temperature. The leaner the mixture or the more advanced the ignition the higher the combustion chamber temperature and the richer the mixture or the more retarded the ignition the lower the combustion chamber temperature. Remember the EGT sensor is outside the combustion chamber so it is only reading the exhaust gas/flame temperature. The more the ignition advance the lower the EGT and more the ignition is retarded the higher the EGT. This opposite effect is caused moving the heat or the flame out of the chamber into the exhaust with a retarded ignition and raising the temperature of the EGT. So having a high EGT because of retarded ignition can and will show less heat in the spark plugs. It is highly recommended to pick a maximum ignition timing point that is known to be good for your particular engine setup and tune the mixture for that point. This way the tune-up is safe and you can retard the ignition to pull out power without drastically changing your fuel tune-up. You can always go back to the maximum power ignition point without damaging the engine. Reading For Air Fuel Mixture An important step first is to degrease the plugs by spraying the threaded end with brake cleaner to remove any deposits of oil that may have been put on the plug during shutdown or when the plug was removed from the head. Most of the spark plug manufactures that make plugs used for racing plate the spark plug shell with cadmium or zinc which oxidizes at a temperatures that corresponds to the correct operating range of the temperatures within the combustion chamber of a racing engine. As one is tuning the motor and leaning the fuel system out the first part of the plating to start oxidizing will be the ground strap. This oxidation (burning) is uneven in progression around the ring at the end of the threaded part of the plug because the side of the plug ring that was closest to the exhaust valve seat gets hotter faster than the side of the plug ring that is closest to the intake valve seat. The result is a crescent of unburned cadmium that gets smaller as the engine is leaned out. When the cadmium is oxidized and has turned white across the entire face of the plug ring or countersink area just inside the ring the increased temperature then progresses down the side of the plug into the threaded area. The peak performance is at the point where the cadmium or zinc plating oxidizes and turns white over about 90% of the plug ring and a small crescent of unburned plating is left on the ring. Burning 100% of the plating off the ring all the way down to the first thread will not result in any damage but will also not result in any increase in engine performance. There is a fairly large tuning range between the burned area being at 90% and being burned all the way down to the first thread. Using this large area will ensure that no damage is done to the engine. The next stage from this safe appearance is when the cadmium is burned down to the second thread and the ring loses its white appearance and picks up a greenish tint with small visible bubbles and the ground strap picks up rainbow colors (blues and green when held in the bright sunlight). The strap getting hot enough to exhibit rainbow colors is hot enough to start igniting the fuel mixture too soon and causes pre-ignition/detonation. As the plug gets hotter then sooner the mixture will ignite and this will result in the melting of the ground strap and possible breaking of the plugs porcelain and damage to the upper rod bearings. By keeping good records of actual performance this peak performance point should be readily seen and matched to the indication on the spark plug ring. When the 90% white ring is obtained with the fastest MPH noted you are now ready to move on to adjusting the ignition timing. Reading for Ignition Timing Once the fuel mixture has been adjusted so that 90% of the plug ring is white and all the cylinders have been adjusted so that the white area are the same on all plugs the ignition timing can be now checked by reading the blue line on the ground strap of the plug. Ignition timing is also directly responsible for the heat in the combustion chamber and therefore the color of the plug’s ground strap is a tattletale sign of this temperature because it is thinner than anything else on the plugs and sits right out in the combustion chamber. The ignition timing can be checked by looking at the color of the plug’s ground strap and the position of the “blue line” on the strap. The blue line really indicates the point at which the strap has reached annealing temperature of the metal. To help to understand this think of a bar of steel (ground strap) on a table that is being super heated with an acetylene torch at one of the tip ends. As the end heats up and the heat starts moving down the bar you will see a blue line across the bar at some point down the bar away from tip with the torch. This blue line reflects the temperature that is the annealing point of the metal. As the temperature increases the blue line moves further down the bar away from the torch. Similarly, the blue line moves down the spark plug ground strap as you put more heat in the engine. Assuming that you have adjusted the alcohol fuel mixture correctly and if you are using gold colored ground strap like with an NGK spark plug then not enough timing will show the ground strap as still gold or going light gray maybe with a few bubbles on it after a run. As you advance the ignition and put heat in the engine the plug ground strap will turn darker gray. As the metal turns medium to dark gray you should start looking for the blue line (band) around the ground strap. Ideally, you want this blue line to be just above where the ground strap makes the sharp bend and above the weld. If you advance the ignition too far the blue will disappear off the strap and the strap will pick up rainbow colors (blues and greens). The next step beyond that is to start melting the strap from the tip end and detonation. When you are close to the correct timing then only change the timing by half a degree at a time. If you ignition system has the capability of adjusting the timing of each cylinder independently (ICT) then you can use that feature to have the blue line in the same position on all the plugs. First, adjust the basic timing to get as many of the plugs to have the blue line just at the sharp bend in the strap. Now adjust the ICT to move the blue line to the same point on the remaining plugs. Once all the plugs read the same you can advance the ignition a little at a time to put the blue line just above the weld on the strap or whatever point gives you the best performance. If your timing is too far retarded then it maybe necessary as you adjust the timing to add a little more fuel to keep the crescent on the end of the plugs white for 90% of the area. Be very careful on adjusting timing because it does not take much change to make a lot of difference. I recommend limiting the changes to half a degree at a time. It is easier to set the timing at a known good degree for the type of engine and adjust and individual cylinder timing (MSD ICT) to balance out all the cylinders and then adjust the mixture to show the correct amount of white area on the metal ring of the plugs as explained above.
Nitrous Motor Plug Reading
|Too lean and too hot, time to back some timing out of it and add some fuel. This plug has a blue tint to it due to the camara flash used, it didn’t look that blue in regular light.|
|The above plug from a nice clean mid 9 second pass, this one was a little bit lean, added 1 psi fuel to it and picked up 3 mph. Note: Might need to take a bit of timing out of this tune up or use a colder plug, some deterioration on the center electrode however ground strap looked about right.|
|“Pepper” AKA Detonation. This was in a car that was running so rich that it blew black smoke out of it, but had too much timing. Note the center electrode is getting ate up a little and the ground strap has the look of metal that was heated up with a torch till it was bright red. The specks on the porcelain are supposed to be due to oil getting into the combustion chamber during detonation. This plug has been washed with brake cleaner, prior to that it had black soot everywhere.|
|Same car, next pass with 2 lbs LESS fuel, and two degrees less timing. The plug is running cooler now from less timing and is now coloring the porcelain. Now it’s time to start backing down the fuel a bit more. This plug has been washed with brake cleaner, prior to that it had black soot everywhere.|
|Same car, next pass with 1/2 lbs LESS fuel, and same timing. Still see some peppering of the plug, probably getting some detonation. Still rich. Note: The rings were going away in this motor on this pass. We also burnt the clutches out of the TH400.|
|Here’s an interesting one.. The above plug was in a cylinder that went “out”, notice the gray down inside. The plug was working, then quit – fuel washed the soot off the rest of the plug. This is an Autolite racing plug, several plugs in this motor did this, the rest all looked gray on the base ring – perhaps a bit on the rich side but motor was running very good.|
|Ryan’s Plugs AKA 66 283. Almost perfect fuel and timing, after this pull added 1-2 degrees more timing to take the heat in the ground strap just a bit more towards the corner bend and picked up 40rwhp.|
|Terry Wise sent these in, they are from his BBC, AR3933’s a little rich, but real close.|