Reading Spark Plugs
(This applies to Naturally Aspirated as well as Nitrous Engines)
It's best to view plugs with a 10x power illuminated magnifying type glass.
What each part of a plug signifies:
The ground strap indicates the heat range of the spark plug. If the "color" of the ground strap "changes" too close to the ground strap's end, (located above the center electrode), then the heat range is "too cold", meaning that the strap is loosing heat too quickly to the base ring. It is not able to burn off deposits until near its end.
If the "color" of the strap changes near where it is welded/attached to the base ring (last thread ring), then the plug heat-range is "too hot", because heat is not being transferred or cooled from the strap to the base ring quickly enough. The strap might begin to act like a "glow-plug". Eventually causing pre-ignition and/or detonation later on. The proper heat range is when the "color" is at the half-way point on the strap. This means the plug is neither too cold or too hot.
* Do not look at the porcelain to read jetting. *
The porcelain shows up pre-ignition and detonation (timing/octane). It will not accurately reveal
jetting/air/fuel ratios. To look for the first/beginning signs of detonation, search the white porcelain for tiny black specks or shiny
specks of aluminum that have fused to the porcelain. When detonation occurs, part of the air/fuel mixture explodes instead of burning, the
explosion is heard as a "metallic knock", this audible knock is the result of a sound shock-wave, this shock wave travels back and forth
across the clearance volume "disrupting" the cooler boundary layer gases that cover the entire clearance volume area. This disruption allows
"more" heat to be transferred into parts, especially, domes/piston tops,...along with the very rapid rise in pressure like a hammer blow,
pistons can get torched with melted sides and holes !!! With the early signs of detonation, the shock-wave will also rattle rings causing
the tiny amounts of oil that now gets by rings, to be fused to the white porcelain as tiny black specks, also fused as specks are soot that
was clinging to clearance volume surfaces in the relatively "still-air" of the boundary layer. One step beyond the black specks, will be
tiny specks/balls of aluminum coming off the pistons that will be fused to white porcelain,....the next step to be reached is occasional
pieces of the porcelain being broken-off as detonation gets worse, etc. Soon after that are holes, blown head gaskets, broken connecting
Tuning With Unleaded Gasoline:
When tuning with an unleaded racing gasoline, one needs to pay a little more attention to some details than with leaded gasoline. Spark
plug reading becomes more critical because it is harder to get color on the plugs with unleaded. Getting the air/fuel ratio correct is
tougher because most unleaded gasoline contain oxygen which adds to the oxygen in the air.
Some of this is really tough because we get accustomed to one way of doing things and our world begins to turn upside down as fuel parameters and environmental regulations change. To start the process you need a good light to “read” the spark plugs. It has to be a light with a magnifying glass associated with it.
There are several available on the market. They resemble the instrument that your doctor sticks in your ear to have a look at what ever he is looking at in there. Maybe for us involved in racing, he is simply checking to see if the pathway to the other ear is clear so that he can see daylight out the other side.
Spark Plug Reading:
It is always best to start with a new set of plugs. When reading the plugs, always look at the entire set, and make jet changes where
they are needed. The carburetor does not have to have the same size jets in all four corners.
Reading spark plugs needs to be done with proper light / magnifying glass mentioned previously so you can see all the way down (invert the plug to read it) into the plug where the insulator (white part of the plug) comes through the steel case. What you are looking for here is a slight coloring on the insulator just above the steel case. This is called the “fuel ring” and to have the correct air fuel ratio, the “ring “ should be visible all the way around the insulator.
If it is spotty or very light, the engine either has not run long enough to develop a fuel ring, or it is too lean. A visible ring that just offsets the white color of the ceramic is usually a good sign. In contrast, leaded racing gasoline will color a plug much quicker than unleaded and a correct “fuel ring” will usually be little darker due to the lead in the gasoline. To correctly read a plug with leaded gasoline, it is necessary to have the same good light / magnifying glass identified above. They are not cheap, so don’t try to steal one from your doctor’s examining room.
Besides the fuel ring as an indication of the correct air-fuel ratio, another indication is light shiny spots at or near the tip of the insulator on Autolite plugs only. These are difficult to see and show up on Autolite plugs only. It has to do with the material used in the insulator. Not all insulators are the same.
If the spark plug insulator shows any sign of small dark spots, especially at or very near the tip, detonation is taking place in that particular cylinder. Those dark spots are sometimes referred to as “speckles” and are very small pieces of aluminum oxide that have been cooked off the top of the piston due to the abnormal temperature / pressure that occurs during detonation. One cannot afford much of this before failure. If this condition is observed, a determination must be made as to whether the mixture is too lean, or if there is too much spark timing. Keep reading, we’re not there yet.
Looking at the side electrode of the plug will help to determine the correct spark timing. Ideal timing is indicated when the side electrode shows heat all the way to the case. If there is not an indication of heat all the way to the side electrode / case, then spark timing should be increased. If the heat line stops 1/16th of an inch short of the side wire / case connection, the engine needs an additional 0.5o of timing.
If spark timing appears to be correct, but there are signs of detonation, the air fuel mixture is too lean. The carburetion must be made richer by going to larger jet(s).
Detonation is unlikely to occur in all cylinders at the same time unless the spark timing and/or the air fuel ratio is way off. Normally detonation will show up in one or two cylinders. Richen only the corner of the carburetor(s) that feed the problem cylinders. Be sure to verify that the detonation problem has been resolved.
Jet Changes and Density Altitude (DA):
Racers use “ Density Altitude” (DA) as a measurement when tuning an engine. If, for instance the DA is 1200 feet, this means that the air at that racetrack has a density equal to what air would be at 1200 feet above sea level if standard conditions exist at sea level. As the DA increases there is less oxygen available, and the mixture needs to be leaner. For every 750 feet that DA increases, the jet size needs to be reduced by one size.
In contrast, for every 750 feet that DA goes down, jet size needs to be increased by one size.
Air Fuel Ratio:
In the “old” days, all that we had to be concerned with is getting the correct amount of oxygen from the air mixed with the correct amount of gasoline being used. In tuning with unleaded gasoline, we also need to take into account the amount of oxygen in the gasoline (if there is any). This part can become critical.
Let’s assume that a racer has his engine “dialed in” on Rockett Brand TM 111 Leaded Racing Gasoline and his sanctioning body is switching to Rockett Brand TM 100 Unleaded Racing Gasoline. The 111 has a specific gravity (SG) of 0.721, and the 100 has a SG of 0.744. Normally when changing from a gasoline with a lower SG to one with a higher SG, one must lean the mixture by going to smaller jets in the carburetor. In this case it would be about 2 jet sizes in a Holley carburetor. That is not the correct move in this example because the unleaded contains 2.7% oxygen which in this case will negate the jet change. The engine may like richer jets than it did with the 111, but the jets used with the 111 are a good place to begin.
Just try to remember, “When in doubt, go richer”. This can reduce the need to build (or buy) a new engine.
Using an unleaded gasoline will not color a pipe to the gray that we saw with leaded gasoline, so don’t look for it. Exhaust pipes used with unleaded gasoline will be slightly black in contrast to the gray that was observed with leaded gasoline.
All of this takes experience. The more you do it, the easier it will become. Take your time, pay attention to details, and everything will come together.