How important is the spark plug gap recommended by car manufacturers?

Dear Car Talk

Dear Car Talk | Nov 01, 1999

Dear Tom and Ray:

I have a '97 Nissan Sentra. I changed the spark plugs at 30,000 miles and just
did it again at 60,000. The owner's manual says the spark plug gap should be
between .039 and .043 inches. I set them at .040 and the car runs fine. I checked
the gaps on the old plugs I pulled out, and they were between .050 and .060 --
WAY off! But the car had been running fine. How important is the gap? -- Daniel

RAY: Well, as you've discovered, Daniel, not that important! Most cars will run
fine, even with the gap pretty far off.

TOM: And in your case, what probably happened was that the plugs burned down to
.050 or .060 from their original settings. That's what happens to old plugs --
the metal in the electrode and the tip wears down from all that firing. And
that's one of the reasons you replace old plugs.

RAY: At some point -- it may have been at .070 or .080 in your car -- the engine
would have developed a miss because the gap would have gotten so big that the
spark could NOT jump it. And if you had other problems -- like a weak coil or bad
spark plug wires -- that miss would have developed sooner rather than later.

TOM: The gap recommended by the owner's manual is the optimal gap. That's where
the engine runs most efficiently. But in modern cars with otherwise healthy high-
energy ignition systems, within 10 or even 20 thousandths of an inch either way,
most spark plugs will usually fire well enough so that you won't notice any
difference in performance.

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