When tuning a performance engine the spark plug is an important tuning tool. Being that there are many different tuning techniques, I will keep this article about the basics of spark plug reading on a Quarter-Mile Drag Strip (or wide-open-throttle tuning).
When tuning by the spark plug, you must always use a NEW plug to get an accurate reading. To get a clean plug reading the engine should be shut down after the run and towed back to the pits. Any running of the engine after that will skew the plug reading.
When tuning by the spark plug, you must always remember there are two major variables at play (air-fuel ratio and ignition timing). To make it easier to determine you're on the right track when tuning, make small incremental changes, and only one or two changes at a time. Engine tuning is a learned skill or art, with many different outcomes of getting a good plug reading. These tips are to identifying a safe engine tune, horsepower or torque numbers may not be peak. For example, if I was looking a spark plug showing to much heat. I could pull some timing out or add more fuel, both changes would result in less heat. As I have already stated, air-fuel ratios and ignition timing play a huge part in the outcome of a good engine tune.
The following tips are just a baseline of what to inspect on the spark plug. When reading a spark plug there are four different places to look. The first place I inspect is the plug strap, this is the ignition timing. The strap will show a color change in two spots on the 90-degree bend. The first mark shows the base timing and the second indicates total timing. IF THE SECOND MARK IS SHOWING AT THE BASE WHERE THE STRAP IS WELDED ON, THIS IS A SIGN OF TOO MUCH HEAT. An ideal ‘Safe’ Timing range is when the total timing discoloring is located just passed the bend.
Next, I’ll inspect the threaded area for the heat range. The threads will show 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.
Inspecting for Air/Fuel can be found in two spots, one spot known as the Fuel Ring. On the Fuel Ring, your look for a complete ‘Full Turn’ of discoloring. Ideally, this should be lightly colored soot. As the Fuel Ring colored 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. BUT, BEWARE THIS IS ON THE EDGE OF BEING TOO LEAN.
The second place to inspect for the air/fuel ratio is the porcelain. Excessive fuel leaves light to dark color rings on the porcelain. Ideal conditions show the porcelain as new to shinny white. But, beware of grayish specials as it is most-likely piston material.
Common Spark Plug Readings and Common Causes
