With all of the reported problems of detonation, jetting questions, and so on, I thought it might be of value to some to have some info on what your spark plugs can tell you about what's going on inside your engine.

The following are some of the things that will give you some input on plug reading .Keep in mind that most of the info will be valid only if you do a plug chop or, if on the dyno, when doing a full load acceleration or steady state test. Also, plugs are best read with a 10 -15x illuminated viewer. The absence of lead in fuel today will make plug reading more difficult if not impossible as compared to the older leaded fuels where you could tune by the color of the plugs.

A lot has been written about how to read plugs, I've boiled it down to three areas of the plug:

The ground strap and center electrode ... shows heat range and timing.

The base ring and the junction of the porcelain to the plug shell ... shows AFR/jetting

The porcelain ... signs of detonation/pre-ignition

Pre ignition/Detonation:

The porcelain shows up pre ignition/detonation, 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 the 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/***** 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. Soon after that are holes, blown head gaskets, broken connecting rods, and so on. It should be noted that in some cases the early signs of detonation may be present without being audibly detected. Also, some black specs may show up on the insulators from oil passing by intake valve guides or seals and oil entering the chambers from crankcase ventilation "re-breathers". These specks will usually be on the portion of the insulator that faces the intake stream as opposed to the entire insulator.

Additional signs of beginning detonation are piston rings. By comparing a ring's "free-diameter" to the" out-of-box" free-diameters of new unused rings it can reveal beginnings of detonation before much harm is caused. This is a result of the above previous explanation about heat being transferred more readily because the of the insulating/cooler boundary layer being disrupted by the shock-wave. A piston ring is a simple spring, when a spring is over-heated it will loose tension.

The very tiny sharply defined porcelain -"ditch" that encircles the center electrode, will also show up early signs of pre ignition/detonation and the wrong heat-range. Look for signs of the ditch beginning to be filled up with melted porcelain, you will need a 10x - 15x illuminated viewer.

Heat Range/Timing:

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, (which is above the center electrode), then the heat-range is "too cold" , meaning