why do spark plugs fail...

[ECeptor]

...when they still look good? I've seen so many times that the plug really doesn't look too bad and is fairly new yet changing make the engine run much better.
What wears out on a spark plug?

[Moneypitt]

Look where they live. Combustion chambers are the hottest part of an engine. The plugs must live thru some really rough conditions. Lean a motor out, and the temp goes to the moon, keep it fat and the plugs soot up, loose the ground conection for the spark. The inside of a spark plug has come a long way in the last 20 years, ignition systems and fuel management systems have also advanced. I think one of the factors that murder plugs is voltage variation. The spark you have at idle vs. the spark you have at WOT will cause one side of the spark part to heat and wear in one area, and then the voltage/heat changes will wear the other part. Or something like that........Having said that, consider using the same spark plugs for 4 or 5 seasons of circle racing. We did just that, using a Mag, which is a constant voltage, 24K, idling, or WOT. The plugs never skipped a beat, never banged or popped, and I couldn't find any reason to change them..........