[Public-list] Re: spark plug heat ranges

[Gordon White]

The heat range is determined essentially by the distance from the tip of 
the white ceramic center insulator to the metal shell of the plug. The 
longer the distance heat has to travel before it bleeds off into metal, 
the hotter the plug. Hot plugs burn off  oil or other fouling, however 
too hot and the ceramic melts, getting so hot it ignites the incoming 
fuel charge before the spark fires. This can do great damage to an 
engine. (detonation) though I somehow doubt an A-4 is subject to much 
detonation. Same thing with too-low fuel octane: it ignites too soon or 
burns to rapidly (explodes). Ignition in a cylinder is supposed to be 
burning, not an explosion.

    In my  18 mm race engine plugs the insulators are much larger in 
diameter than in modern 14 mm or 10 mm plugs and the distance from the 
tip to the metal is only a couple of mm. MUCH cooler than most of the 
plugs you will see today.

    Generally detonation only occurs at full-throttle. In a 
manual-transmission car it is most likely when you floor it in top gear. 
That puts the most pressure in the cylinder.

Recently I put clean plugs in my Honda - just ordinary plugs recommended 
for the car at the NAPA store. But they were too hot. The car did not 
run well and when I looked at the plugs after a few hundred miles of 
long-distance  high speed Interstate driving the insulators were very 
badly eroded/melted. That can also happen from running too lean a fuel 
mixture.  I put in a range cooler plugs and all was well.

- Gordon White

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