Quote:
Originally Posted by baldlobo
i've got a question do you have a multi-meter; if yes do a continuity test(or resistance test) on the hotter plugs between the electrode and the metal body.
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At first thought, this *should* work. Use a multimeter to apply a small voltage to the insulator, and measure the resistance of the insulator to ground.
*However*, this approach is not valid as the voltage level goes from let's the 0.5 volts that the meter puts out to measure resistance, to the 30,000 volts that the ignition system applies to the spark plug.
The effective resistance values of these two measurement methods will give completely different results.
Some multimeters provide "ranges" for different applied voltages to the test item, and these in turn give different effective ohms readings as well.
One would need specialty equipment to measure the effective resistance of the spark plug at operating voltages.
And then let's not forget, that as the insulator heats up, the resistance will change again.
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Even if one had the proper equipment to do the above, it still does not define the intent of spark plug heat range on a running engine.
The intent of the insulator length is to simply keep the insulator running at a temperature that allows it to stay reasonably clean from contaminants, and thus provide a solid spark to the cylinder.
If the ceramic gets too hot, the shiny surface finish changes to a grainy, lower resistance surface finish, that then allows more contaminants to adhere to the finish, resulting in spark loss to ground.
Too short of an insulator, allows the ceramic to build up more deposits, and again, allows spark to short to ground.
The heat range is carefully selected for the reasons above.
Jim.