Yesterday on this SAABCentral thread the question was raised about spark plug gap size and fuel mileage. I offered to re-gap my plugs and see what happens.

My spark plugs were gapped at 1.1mm and I was getting 25mpg before I started.

I re-gapped them to .88mm and put everything back together.

Moments after tightening down the direct ignition cassette I drove to dinner. Immediately there was an improvement in the car’s idle. The idle was smoother and quieter; I had forgotten how quiet the car could run. I don’t have the proper tools to gauge performance on the car, although my butt-dyno did not feel any decrease in performance. Obviously it is too soon to know if the miles per gallon have changed.

I plan on driving around with the smaller gaps this weekend, see what happens, and report back here and on SAABCentral. I’ve decided, barring any problems, that I’m keeping the gaps small for the improved idling even if there is no improvement in gas mileage.

UPDATE:

I widened the gaps to just under 1mm because my MPG started falling with the small gaps and the car had trouble starting one afternoon.

I’m also in the middle of another spark plug experiment. As I confessed at the end of this post from October, I installed Autolite platinum spark plugs in my SAAB 9-5 instead of the stock NGK brand. I’m up to around 6,000 miles on these plugs and I’ve had no issues. As I stated at the time, I’m willing to put my cassette at risk to help prove that a different brand of plug won’t cause any problems.