The color of plugs should be somewhere between light brown and grayish tan. However, slightly white may not be too bad unless it is "chalky white."
A little discussion is in order:
The heat range of a spark plug is the range of heat or the physical temperature of the plug itself from as cool as it may be to as hot as it may get when the engine is under load or higher speed. It has nothing to do with the intensity of the spark. Or in other words a "hot plug" does not mean hotter spark.
A plug that runs too cold will not keep deposits (carbon, soot, etc. that form when combustion takes place) off the plug electrode area. The physical temperature of the plug must be hot enough to keep these deposits burned off. Otherwise the plug will not spark properly or possibly not at all because the carbon, soot, etc. covers like insulation on a wire and the spark or "arc" can't overcome it. The color of a plug that's running too cold is "sooty" black or dark.
Keep in mind that my plug descriptions are for engines in good condition, correct carb mixture, good ignition, etc., etc..
A plug that is running too hot can turn chalky white or bright white. This can reduce the life of the plug or even cause the electrodes to burn up. A plug at these temperatures can also get hot enough to glow which can ignite the fuel mixture before the spark occurs (a.k.a. pre-ignition). This can be bad news, FAST.
Unlike detonation (which one can hear) pre-ignition can be and often is completely silent. In other words you won't hear it and the damage is done.
Here's how it works:
An engine can be running fine with a normal chamber temperature and no detonation even though the wrong heat range spark plug is getting hotter and hotter. If the plug gets hot enough to glow and the chamber temperature does not get hot enough to cause detonation there will be no ping or knock sound BUT there can be pre-ignition. The temperature in the chamber gets hot so fast that WELDING TEMPERATURES occur in just a few seconds. The result is a hole can melt right in the middle of an aluminum piston!
Now when detonation (knock sound or pings) occurs for other reasons often discussed by myself and others on this forum, pre-ignition can still be the end result. The excessive heat of detonation can lead into pre-ignition by making even a correct heat range plug glow along with other combustion chamber parts. So here come the melted pistons same as above, not to mention the damage that the previous detonation or just detonation in general can do, i.e., beat the babbitt out, crack the pistons, crack the insulators on the spark plugs, etc..
The reason pre-ignition causes high heat so fast is because the piston coming up on the compression stroke is trying to compress a burning fuel mixture that is expanding against it trying to push back.
The moral of the story is just don't screw in any spark plug just because it fits the hole and sparks. Conduct a heat range test to determine what's best if you are not sure.
Larry B.
