Gapping and Indexing


Since the gap size has a direct effect on the voltage necessary to jump the gap and ionize (light) the air-fuel mixture, careful attention is required. While plugs are pre-gapped from the factory, the gap may need to be adjusted for the vehicle that the spark plug is intended for. Those with modified engines must remember that higher compression or forced induction will typically require a smaller gap setting (to ensure ignitability under higher pressure).

The voltage requirement is directly proportional to the gap size. The larger the gap, the more voltage is needed to jump the gap. Most experienced tuners know that increasing the gap size increases the spark area exposed to the air-fuel mixture, which maximizes burn efficiency. For this reason, most racers add high-energy ignition systems. The added energy allows them to increase the gap, but still have enough voltage to jump the gap.

Many think the larger the gap, the better. However, if the gap is too large, and the ignition system can’t provide the voltage needed to spark across the gap, or turbulence in the combustion chamber blows out the spark, misfires will occur.


Indexing is done by placing (usually copper) washers of varying thickness on the spark plug shoulder, so that when the spark plug is tightened, the plug will rotate a certain amount, and gap will point in the desired direction. In most engines, performance will improve when the spark plug gap opens toward the intake valve(s). However, without testing different index positions in an engine on a dyno, it is nearly impossible to determine which index increases performance. Indexing spark plugs will typically make only a few more horsepower – usually less than 1% of total engine output. For a 500 hp engine, that’s only 5 hp. It is usually not worth the effort.

There are also drawbacks to indexing spark plugs. When washers are added, the spark plug may loosen if the washers don’t crush properly and hold the torque. Also, if too many washers are added, the firing end of the plug will not be as far into the combustion chamber and performance can decrease.