What is "indexing" and how much more power will I get?

Indexing refers to an old racer's trick whereby the spark plugs are installed so that the ground electrode is oriented to face the intake valve (usually) in an effort to "open up" the spark to the incoming air/fuel charge. This is accomplished by placing a washer underneath the spark plug's shoulder area (of a specified thickness) so that after properly torqueing the spark plug, the electrode would be pointed in the desired direction, usually towards the incoming air/fuel charge from the intake valve.

Some racers have discovered that certain engine configurations made more power with the spark plug pointed away from the air fuel mixture.

It is important to note that any power increase will be small, typically 1-2% of total engine output, and the proper indexing orientation can only be discovered after extensive engine dyno testing.

Indexing is a waste of time for racers that do not have the training or the dyno facilities to document the results of their efforts. However, professional race teams racing in classes that have strict engine specifications can gain a slight edge over their competitors when spark plugs are indexed for optimum performance, provided they have performed adequate dyno testing.