Reading spark plugs is a essential part to engine tuning. Selecting the right plug will reduce tendency to knock and be sure that your engine is running in a healthy safe zone.
Reading Spark Plugs
Reading spark plugs is a task most people overlook when finishing the tune on a vehicle but really it is one of the more important things to handle before finishing with the car. A good plug read can tell you what the mixture is like, if the heat range is appropriate, and if there is too much timing (or to little). It is definitely not a replacement for good scanning/datalogging but a better result will be had if you do pair good plug reading and datalogging together.
Reading the ground strap
Reading the ground strap is a easy way to tell if the plug you are using is too hot or cold of a heat range. It will also give you an idea if the timing is too low or high. For timing you'll also want to check the section about reading the porcelain. The general rule of thumb is you want the annealing mark, that is the spot where the ground strap changes color, to be in the center of the bend in the spark plug. Don't go crazy chasing this! I would say if your somewhere between the center plug example and the third example you are in a good area. Note that timing can change the location of the anneal mark but generally its a function of plug heat range first, timing second.
Heat range is too hot! The annealing mark on the ground strap has gone past the bend to the base and has even taken up the first thread or two. Step plug range down at least one step. Timing may also be too high, check section on reading porcelain also.
Heat range is too cold. Annealing mark is just at the tip. Sometimes if the plug is too cold the anneal mark will not even show up on the ground strap. Sometimes in this situation more timing can be used.
Heat range checks perfect. No change needed.
Reading the spark plug porcelain
Reading the spark plug porcelain is a very important step and requires good eyes, magnifying glass, or otoscope. This is to be done with a brand new plug and a full throttle pass or dyno run. It cannot be checked accurately on a plug that has idled or drove around too much. For best results kill the ignition at the top of the dyno pull or track pass. If your spark plug has little black specs all over it that typically is an indication of a slightly too hot plug or a engine that consumes a little oil. The black specs are fuel and oil flash frying on the porcelain. It is not a reason to get too excited unless they are silver or metallic. If you are able to see little silver or metallic dots/specs on the porcelain be aware, that is a indication of detonation. Often you will see signs here before you can ever hear it. Back off timing and also check the ground strap to make sure the heat range is not too high.
Reading the mixture
The last item you can check by reading the plugs is the mixture. This is a tricky thing to do sometimes. Different types of fuel show up differently for example E85 just about not show up at all while some racing fuels will turn orange or rusty looking. The best way to do is is to make sure all forced induction or nitrous engines have a good medium dark shade with normal street gas. A naturally aspirated engine will have a every so slight shade of gray with normal street gas. Typically 93 octane and a high 12 to low 13:1 AFR will just barely start to take a shade of gray. The best way to check your plugs mixture is with a otoscope or by cutting the threads off the plug with a lathe.
The amount of shading and the color are going to depend on the engine and fuel used. Benchmark what you have and work from there.
Note that we are looking at the shading at the very base of the porcelain. The fuel that can be seen with the naked eye is the mixture at part throttle and idle. The fueling that happens at wide open throttle is at the base of the plug only. Also be aware that this is to be done with a brand new plug and a full throttle pass or dyno run. It cannot be checked accurately on a plug that has idled or drove around too much. For best results kill the ignition at the top of the dyno pull or track pass.
Example of a good running spark plug
In the example you will notice that the porcelain is clean and without metallic specs - this indicates that detonation is not present. You will also notice a light shade of gray at the bottom part of the porcelain - this indicates a good mixture. The last thing to notice is that the anneal mark happens exactly at the bend in the ground strap - this indicates that the plugs are not too hot or cold.