Simply put, getting the maximum power from any race engine is a function of the temperature, pressure and timing of combustion gases in the cylinder. Since it would be expensive and impractical for everyone to put a thermometer or pressure transducer into the combustion chamber, another avenue must be found to monitor these elements. A look at the spark plugs will provide pretty reliable information about what is happening in the combustion chamber, but the groundwork must be laid before you look so the information gathered will be credible.
Setting Up
Even before leaving the shop, there are things that can be done to produce better results from the plug test. First, make sure the ignition system is in good condition. A simple way to test this is to take a plug and wire out of the engine. Ground the threads of the plug to the valve cover or exhaust manifold. In low light, and while taking care to avoid getting shocked, have someone crank the engine a few times. Look at the spark. An ignition system in good condition should have a blue spark and make a crisp snapping sound. If the spark is yellow or does not spark in a definite pattern, try to get a better ground and attempt this test again. If the spark does not meet the above criteria, you must then check the condition of the wires, rotor, cap and voltage at the battery in addition to the actual plug. Once these items have been inspected and cleared, it is time for on-track preparation.
Getting Started
A brand-new set of plugs will need to be installed before conducting the test (Champion has a plug tech line that offers information on the correct style and heat-range plug for your application). Before installing the new plugs, make sure you have made any planned adjustments to the engine. These include jet changes, float adjustment, changing the power valve and setting the timing. After completing these changes, gap and install the new plugs. It is essential to keep as much idle time as possible off these plugs. The more idle time the plugs get, the less accurate the readings will be, and it could completely invalidate the test. Even running at slow speed around the bottom of the track to get the temperature right can hurt the data. Doing a test during the second practice while the engine is still warm from the first is recommended because the results of an engine at race temperatures will be more reliable from a cold engine.
When out on the track (up to .75 miles), take 10 to 15 race-condition laps. Larger tracks will not require so many laps. After completing those hot laps, finish with a strong one. At the end of a straight, lift off the gas, push in the clutch and shut off the ignition all at the same time. Missing any of these steps or getting any idle time on the plugs after this point will destroy the readings. Coast into the pits or get a push.
Set up a plug rack on the radiator support or air cleaner and take all the plugs out. Contrary to popular belief, the condition of an engine cannot be told by pulling the one or two most accessible plugs. Put the plugs into the correct positions in the plug rack so patterns, if they exist, can be seen. Start examining these plugs, one at a time, under a plug light. A light allows you to see deep into the plug. Champion suggests a 4x or 6x magnification. Plug lights can be obtained from many sources, such as BSR Products, and cost from $15 to $40.
Three Main Parts
There are three main parts of the plug you should be examining: the side electrode, center electrode and the ceramic.
Starting with the center electrode, it should be the same color as a new one out of the box. If it is blue or purple, it’s been overheated. Its shape should be that of a cylinder with sharp corners at the top. “Round” corners and erosion of the electrode will more often be seen on overheated plugs and those run for a long period of time. Sharp corners are desired because it takes less energy for a spark to jump from a corner than a rounded edge.
The side electrode is more difficult to diagnose than the center electrode. The color of the side electrode should be similar to what it looks like out of the box. Green is slightly hotter than normal, blue is way too hot. The fuel mixture should be adjusted as soon as possible to alleviate this problem. On the side electrode there should be a “heat line,” a ring around the electrode generated at the tip. The hotter the chamber, the farther the ring will travel toward the base of the electrode. If the ring is in the first third, the chamber is too cold. If this ring is in the final third, almost to the base of the electrode, the chamber is too hot. When the ring is found somewhere in the middle of the electrode, the chamber is in the correct temperature range. The final area to examine is the ceramic. The color should be a like-new white, but, to be on the safe side, a light tan will do. If the ceramic is shiny black, the plug was probably fouled by oil. Dark gray or black is a good sign of an over-rich condition. Glazing, shiny white, yellowing or pitting on the ceramic is a sign of overheating. A white fuzz between the center electrode and the ceramic is known as “cement boil,” which is what you get when the material that holds the electrode and ceramic together gets too hot.
Additionally, on the ceramic there may be specks of material. The two most common types are pepper specks and shiny balls. They show detonation, but the pepper specks also can mean a slight oil-control problem. The specks will attach themselves to the ceramic and the outer shell of the plug. The shiny balls are tiny pieces of aluminum from the top of the piston. Similar to the side electrode, the ceramic also should have a ring around it. This is called the fuel ring. It should be a brown-gray color and half-way up the height of the ceramic. If it’s too deep on the ceramic, the chamber is again too hot. If it’s too close to the electrode, the chamber is too cold.
Look for Patterns
Once you have decided what each plug indicates, it’s time to look for patterns in the readings. The simplest patterns affect all plugs in the rack. All are lean, rich or detonating. Other patterns to look for include poor manifold distribution, a malfunctioning carburetor or cross-firing plug wires. Finding and solving these problems weeds out local puzzles and allows for a better understanding of what the whole engine is doing.
Now that you have taken an accurate plug reading and corrected any unusual patterns, it’s now time to analyze and solve the indicated problems. If the chamber is too hot, this is most likely a sign of a lean mixture and/or too much timing. This also could be insufficient cooling of the engine, detonation or too high a heat range on the plugs. To solve this condition, you will need to start adding some fuel or removing some timing. If these remedies do not provide the answer, look into the cooling and plug heat-range options.
If the chamber is too cold, it is likely the inverse of the above: the mixture is too rich and there’s not enough timing. Try removing some jet from the carburetor, or try a plug with a hotter heat range. If you see traces of oil in the chamber, this is an indication of a piston ring that has not sealed or has broken, worn valve guides or excessive crankcase pressure. These should be fixed before you go any further.
You are looking for are plugs that show correct temperature and sufficient oil control in the combustion chamber. The electrodes should be the same color as when they came out of the box and not eroded. The ceramic should be white and have a fuel line half-way up its height. There shouldn’t be any black specks or shiny balls anywhere. Following these instructions may not produce large power gains, but you will gain reliability and possibly avoid major engine damage in the future.
