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interpretation of dark spark plug insulators
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Posted by Dean (more from Dean) on Fri, 10 Nov 2000 14:26:12 Share Post by Email
In Reply to: Oh no. Not plugs again. :-), pmr, Fri, 10 Nov 2000 12:37:01
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I feel that dark insulators are simply a sign of the plug being too cold to allow the carbon deposits on the insulator to burn off. With a cold plug, there are carbon deposits that build up. Until a plug start to track sparks to ground over this fouled surface, the ignition probably occurs normally and combustion is as complete as with a proper heat range of plug.

But when the fouling does get to the point of causing problems, then the combustion can be affected, and the rate of fouling can be a run away situation. So when a plug has gotten to this stage it can very black. If miss firing has occured, then the plugs can be washed in unburn fuel which gets roasted on, but not burnt off.

When a plug gets bad, it can allow for reasonable off boost performance. But when on boost, the increased cylinder charge density presents a higher break down voltage requirement from the plug. So the voltage climbs with pressure until a voltage is reached where the dirty insulator presents a lower path of resistance the spark plug gap. Then you are typically runing full throttle with a rich mixture and missing. Then there is lots of unburnt fuel around and things get really blackened up. Cat converter is not happy either.

At higher levels of boost, spark plug gaps are often reduced to compensate for this change of breakdown voltage with pressure.

With turbo charged engines, when under boost, the AF mixture is rich and soot and CO emissions are expected. Running a hotter plug will allow for deposits to be burnt off while non on boost. So your driving profile will have a significant affect on your plugs. I would expect to see a significant difference in plugs between the highway commuter and the street terrorist!

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