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Spark Plug Debate at the MINSU Meeting

The Puzzler


RAY: I should mention that many newer cars come through with platinum plugs, which will last longer. If they weren't platinum, change your plugs every 30,000 miles. But his argument was...

TOM: Does it have to do with rotations?

RAY: Yes.

TOM: Does it have to do with number of sparks per mile?

RAY: Yes.

TOM: Ha! Ha! Ha!

RAY: But not the way you think. Actually, engines run more slowly now than they used to. But what happens is many cars have distributor-less ignitions. And let's look at a typical 4-cylinder car, if you will.

TOM: If you will.

RAY: A typical 4-cylinder car has not 1 ignition coil, not 4 ignition coils, but 2 ignition coils. And when number 1 spark plug fires on its power stroke on a conventional engine, number one would fire, then number 3, then number 4, then number 2. So every 2 revolutions of the crank shaft each one of the spark plugs is fired one time.

TOM: One time. And only one spark has gone to it.

RAY: But with distributor-less ignition, when number 1 piston is up on its compression stroke, number 4 is up not on its compression stroke, but up on its exhaust stroke. But because there is one coil for those two cylinders, a wasted spark goes to number 4. Of course, it doesn't do anything because there is nothing for it to combust except wear out the spark plug. That's what causes the electrodes to wear out: the spark jumping from one electrode to another. So in 2 revolutions of the crank shaft, each spark plug doesn't fire once, each fires twice. So they should wear out twice as fast.

TOM: So every 10,000 miles you should need new spark plugs.

RAY: Every weekend.

TOM: Instead you don't need them for 50,000 miles.

RAY: Explain that paradox.

TOM: So actually the puzzler question, however, was: What was the rationale that he used that made you agree with him?

RAY: And that was the rationale.

TOM: And that was the end of the puzzler. Doesn't mean we can't continue to discuss it ad nauseum. Because why is he wrong? It certainly does sound right to me.

RAY: And the reason he's wrong is because modern ignition systems generate much higher voltages and are capable of jumping a much bigger gap. So as the plug wears out, it is no longer rendered ineffective as the plugs of yesteryear were.

TOM: Sure, but you'd think the 50,000 volts is gonna make it wear out even faster.

RAY: Well, evidently it doesn't. And when it does wear out, it can still jump the bigger gap, and still ignite the gas, and whatever. It was an interesting observation, wouldn't you say so?

[ Car Talk Puzzler ]

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