Many of my posts and comments about tuning, especially spark plugs, get lots of questions. Here is a pretty solid article about reading your plugs. This is a mix of some Wiki articles, my own discoveries and some knowledge gained from many a gray haired old race car mechanic before me. One suggestion is once you start tuning your car, always try to stick with the same brand of spark plug. Different manufacturers use different coatings, plating, etc and that can change or mislead your diagnosis or tuning efforts.
Let’s go to school, kids!
The spark plug’s firing end is affected by the internal environment of the combustion chamber. As the spark plug can be removed for inspection, the effects of combustion on the plug can be examined.
An “reading” of the characteristic colors and markings on the firing end of the spark plug can indicate conditions within the running engine. The spark plug tip will bear witness to what is happening inside the engine. This helps to know what is going on inside an engine running at peak power.
Engine and spark plug manufacturers publish generic charts containing information about the characteristic appearance of spark plugs after being subjected to various normal and abnormal conditions. Such charts are useful for general use but are of almost no use in reading racing engine spark plugs, which is an entirely different matter.
A light brownish discoloration of the tip of the block indicates proper operation; other conditions may indicate malfunction. For example, a sandblasted look to the tip of the spark plug means persistent, light detonation is occurring, often unheard. The damage that is occurring to the tip of the spark plug is also occurring on the inside of the cylinder.
Heavy detonation can cause outright breakage of the spark plug insulator and internal engine parts before appearing as sandblasted erosion but is easily heard. As another example, if the plug is too cold, there will be deposits on the nose of the plug. Conversely if the plug is too hot, the porcelain will be porous looking, almost like sugar. The material which seals the central electrode to the insulator will boil out. Sometimes the end of the plug will appear glazed, as the deposits have melted.
An idling engine will have a different impact on the spark plugs than one running at full throttle. Spark plug readings are only valid for the most recent engine operating conditions and running the engine under different conditions may erase or obscure characteristic marks previously left on the spark plugs. Thus, the most valuable information is gathered by running the engine at high speed and full load, immediately cutting the ignition off and stopping without idling or low speed operation and removing the plugs for reading.
A lighted magnifier is the best tool to have on hand for reading your spark plugs.
Remember when reading plugs…
- The “Ground-Strap” = Heat Range
- The “Plug’s Base-Ring” = Jetting
- The “Porcelain” = signs of preignition/detonation
You can also verify heat range by the threaded area. The threads will show a dull burnt-looking color change, 2-3 threads is ideal.
- To increase the number of burnt threads, increase the heat range of the plug
- If you have 4-5-6 threads burnt you need to get a colder plug
Next, we examine the “fuel ring”. (and the porcelain)
On the air/fuel ring, you’re looking for a complete ‘full turn’ of discoloration. Ideally this should be lightly colored soot.
As the fuel ring soot darkens, this indicates a richer air/fuel ratio. If you’re tuning the engine for maximum horsepower, the ring should be very light and not make a full circle around the ring.
The second place to inspect for the air/fuel ratio is the the porcelain. Being too rich leaves light to dark color rings on the porcelain. Ideal conditions leaves the porcelain like new to shiny white.
Here is a quick sample plug.
- 1 is a timing indicator, you’ll see a definite color change on the ground strap right about at the arrow. Too much timing and the color change will be very close to the body of the plug, too little timing and it’ll be closer to the tip. Ideally it’ll be right at the 90 degree bend on the ground strap, in this image timing and temperature look good.
- 2 indicates the base timing is ideal.
- 3 shows another timing indicator; usually there’s a brown ring right at the tip of the porcelain. It should be a sharp defined ring about 0.020″ wide. A wider ring indicates not enough timing. If the ring is smaller, incomplete or nonexistent (like shown here), that could indicate too much timing.
- 4 is a heat range indicator seen at the threaded portion of the plug. Looking at the threads you’ll see that a few toward the end are a dull burnt looking color, and the rest are black and shiny. You want about 2 threads showing the heat on the end of the plug and the rest of the threads to be shiny. If you’re using a longer reach plug, 2.5 to 3 threads is optimum. Too few colored threads indicates a too-cold plug; too many colored threads indicates a too-hot heat range.
Next time, I will cover some basic carb tuning for the Edelbrock.