HEAT RANGE TEST - Spark Plugs

[Larry Brumfield]

"Reading the plugs" can be used to determine all sorts of engine problems. BUT, it must be known that the correct plug was used to start with before any problems occurred.

A heat range test is the best way to determine the correct spark plug for a particular engine set up and a person's driving habits. When in doubt always choose a colder plug first until you determine what works best. The worst thing that can happen is the plug can foul out whereas too hot a plug can cause pre-ignition and possible piston damage, i.e., a hole melted right through the piston! Some of you may think it's baloney but it's the truth.

Conduct a heat range test as follows:

Drive out someplace where you can run the car at a sustained speed for a few miles without having to slow down or stop; very important. This allows the heat to build.

Run the car up to the maximum speed in high gear that you would typically drive if you decided to drive fast, and I don't mean dangerously high speed. Hold the car at that sustained speed and without letting off the throttle, reach over and turn off the ignition. Let off the throttle, push in the clutch right quick and coast over to the side of the road. Open the hood and allow the engine to cool off a bit. Remove the spark plugs and examine the color and condition of the electrodes and insulators. The color will be a true indication of how hot or cold the plugs are running.

Now remember this .... if you run the engine and then let it slow down or idle and then check the plugs, you will not get a true indication of the heat!

Of course this is not all set in stone. A person who putts around at say 35 or 40 MPH and never faster may need a hotter plug. The lower the combustion chamber temperature, the hotter the plug should be. On the other hand, as conditions cause the combustion chamber temperature to rise, a colder plug may be necessary. Also, a fuel mixture that is too rich or too lean can confuse the readings; plus a poor ignition system, poor compression, an oil burner, etc., etc.; so the accuracy of the above depends on an engine with proper function.

In conclusion, the plugs should run hot enough to keep the deposits burned off and that's it.... This means the color should range anywhere from light brown to grayish tan. Sooty black is too cold and chalky white is too hot.

Larry B.

[d.j. moordigian]

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Larry...Excellent, I have had my customers doing that for 35+ years. The only thing
I would add to what you have written, unless I didn't see it, a FRESH set of plugs and
a 10 power loop. Dudley