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Spark Plug Reading 101 for Gasoline Motors


Other than installing thermo-coupler sensors in the combustion chambers the only real way that you know what is happening inside your engine is to read the spark plugs. Both the fuel mixture and the ignition timing result in coloring of the spark plug’s porcelain and ground strap. The trick is to how to get the correct coloring without going into detonation and destroying the engine or by going too rich and raising the ring lands.
Spark plugs can only be correctly read if the car has been shut down immediately at the end of a run without driving it back to the pits. Get off the track and coast to a place you are safely out of the way and either read the plug there or change one or two with some you have in your pocket so you can read these uncontaminated plugs when back to the pits. Or you should tow the car back to the pits. It may take a couple of runs to get see the sparkplug color.
Reading 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 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.
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Blown Alcohol Motor Spark Plug Reading 101

Reading a blown alcohol tune-up using spark plugs is a very different than reading a gasoline tune-up on spark plugs. The 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 by the color of the first three threads of the spark plugs. The amount of ignition advance is still read as it is with gasoline plugs: by the blue line on the plug's ground strap, 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 quarter 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 eighth mile plug readings before proceeding on to the quarter mile run. Most of the damage to an engine is done in the last 300 feet.
The fuel mixture and the ignition timing are totally intertwined as to affecting 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. 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 by 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 first step 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 manufacturers that make plugs used for racing will 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 out the fuel system, 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 also will 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. 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 greens when held in the bright sunlight). When the strap gets hot enough to exhibit rainbow colors, it is hot enough to start igniting the fuel mixture too soon and causes pre-ignition/detonation. As the plug gets hotter, the sooner the mixture will ignite. This will result in the melting of the ground strap, and possible breaking of the plug's 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 is 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. Therefore, the color of the plug’s ground strap is a telltale 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 the 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 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 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 your 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 at whatever point gives you the best performance.
If your timing is too far retarded, then it may be 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 of a 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, adjust 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.
 

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