>I just got off the phone an hour ago with a guy who's selling a GT6. He told
>an interesting story about an overheat problem that he just could not
>diagnose, until someone told him to look at the plugs. It turns out that a
>spark plug's heat range is governed, in large part, by the length of the
>insulator, and that the Vitesse/GT6 head is sensitive to that.
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While it sounds good, the fact is that the plugs themselves have nothing to
do with generating or SIGNIFICANTLY transferring heat in an engine, other
than they provide the source for igniting the fuel/air mixture.
The only thing that changing the plug heat range will do is change the
amount of heat the INSULATED portion of the plug sticking into the
combustion chamber will retain. And that AMOUNT of thermal energy would be
very small. The area of the plug That is actually heated by combustion is
VERY small indeed, compared to the REST of the chamber area, and any heat
carried by the insulated portion of the plug would be QUICKLY dissipated
anyway and of almost no consequence as far as combustion chamber heat
transfer. The heat range of any plug is determined by the operating
characteristics of the particular engine it's operating in. Not to control
the temp of an engine. There is an optimal heat that the spark plug
operates in to burn off deposits, yet still retain efficiency. That's what
determines a plugs heat range for a particular engine. In a nutshell, too
hot a (plug) and you get pre-ignition, and possible melting of the center
electrode. Too cold and you would experience fouling by combustion
deposits that don't burn off - While his particular engine might have
required a colder plug because it was running hot to begin with, changing
plugs alone would most certainly not have contributed to it running cooler.
Using a hotter plug (one which retains more heat in it's center
electrode/insulator in an effort to transfer less heat to the head) in an
already overheating or hot running engine would be inviting disaster -
Barry Schwartz (San Diego) bschwart@pacbell.net
72 PI, V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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