Spark Plug Tuning, Pro Stock Style

This is what Yates’s plug should look like if the fuel mixture and timing are dead on. The Yates team pulls the plugs and examines the ground straps (circle) after every run. The discoloring on the ground strap helps determine whether or not timing needs to be advanced or retarded. If the fuel and timing are where they need to be, the ground strap will display a rainbow-effect discoloration at the base of the strap. If the entire ground strap is discolored there’s too much timing; if there isn’t any at all, then timing is too far retarded.

After two or three passes, a cut-off wheel is taken to the plugs to reveal the porcelain core. Here the Yates team can see how rich or lean the fuel mixture is. The higher the ring (arrow) the fatter the mixture. But after about three passes, the plugs get harder to read and tend to look on the rich side regardless. This particular plug reveals an optimum mixture as verified by Yates’ 6.838 at 201-mph qualifying pass during last year’s NHRA Winston Finals at Pomona.

If the plugs read excessively rich like this one...

...the main jets and the air bleeds in the carburetors are changed to lean it out.

Since tolerances are extremely tight inside a Pro Stock motor, the plugs must be indexed with the spark gap down between the 10 and 2 o’clock positions. First the plug is marked on the backside of the ground strap.

Then it’s inserted into a modified socket that has been engraved with a line matching the marked plug.

Based on the plug readings and weather conditions, Yates uses a Holley Annihilator Individual Cylinder Timing ignition system to advance or retard timing. For example, on a relatively humid day, if the plugs read rich, the timing will be bumped up to help burn off the muggy air in the cylinders.

Yates and engine-builder Bob Ingles of J&B Performance run the show and decide what the tune should be without the aid of a crew chief. They have years of experience between them and two championships on the mantle including a Pro Stock runner-up finish in the points standings last year. We say their tune is right on.

There’s a lot of mystery and secrecy when it comes to professional level race car tuning. What’s the procedure for determining timing and jetting, and can a change that would be seemingly insignificant in a street car really make the difference between winning and losing? To get some insight into the black magic of tuning, we roped Mr. SplitFire himself, NHRA Pro Stock racer Jim Yates, into explaining the nitty-gritty on how he tunes his Pontiac Grand Am by reading the plugs. And with Pro Stock being as close and competitive as any class in racing, this is truly a place where tuning is key.