The Crusty Phenomenon

May 28, 2022

A few weeks ago we had a puzzler about spark plugs and the large air gap between the end of the plug wire and the spark plug. My mechanic, Crusty, had a flooded engine, and he pulled the plugs almost all the way out, creating an unstable connection, and it started right up. Remember this?

The question was about, "How come this works?"
And the Carl Sagan worthy answer was something about how the highest voltage appears across the highest resistance. This is true.

Now, here is this week's question. 

After I explained it to our friend Stanley, he said, "Well, if that's the case, why wouldn't you want the hottest spark all the time? Why wouldn't you leave the wires dangling like this all the time? How about if you just made the spark plug gap bigger?"

He had an old jalopy, like a '63 Dodge Dart, whose spark plug gap was supposed to be 32,000-35,000th of an inch. If you were supposed to have it at 35,000, what if you increased it to like 90,000?
Then, it would really be the highest voltage appearing across the highest resistance, right?

Then you could take advantage of what we are now calling the Crusty Phenomenon!

Then you would have a really strong, heavy spark all the time. 

Your car would start all the time, rain or shine, flooding, non-flooding... And you get obviously improved combustion because you'd have this more intense spark. 

This is a good question. The Crusty Phenomenon could, in fact, be extended to the Stanley version of the Crusty Phenomenon... 

So the question is, why don't we do that?

To be clear, we currently have bigger spark plug gaps on some modern cars, but why couldn't you increase the spark plug gap on this 1963 Dodge Dart?



Okay, puzzler answer time. Crusty mechanic. Spark plug gaps. 1963 Dodge Dart.

I think we have milked this puzzler as far as we can at this point. The biggest gap creates the biggest spark, something like that. Everyone remembers...

And our friend Stanley asked, "Why wouldn't you want a spark like that all the time? Why don't you leave the wires half falling off like this? Why don't we just increase a spark plug gap instead of having the gap at 35,000ths of an inch? Let's make the spark plug gap a half an inch so that the spark has to jump across and we'll get a nice, strong voltage every time. Why couldn't we do that?"

And here is the answer. You could do that. And your car would start, sure. But the issue is, you can't drive a car like that. 

If you make the spark plug gap larger, the answer is the car will indeed start but it won't run, because the turbulence of the gas coming in will actually blow the spark out. There's a lot of turbulence, a lot of turbulence you got a long distance for this spark to travel and much more chance of it getting blown out.

So, if you do that, you can certainly start the car, but you won't make it out of the driveway!

Now, modern cars have the largest spark plug gaps, but they also have higher spark plug voltages to jump those big gaps, because the bigger the spark, the better the explosion you get.

But in that 1963 Dodge Dart, not a chance!


If you love your old car and love public radio, when the time comes, donate that old car to your favorite station. They’ll turn the proceeds into great news and information programming. Here's how. 


Get the Car Talk Newsletter