What is spark plug indexing? Are they really only for true racers? Thanks for any info

Indexing is usually done only in all out race cars. It has to do with making sure the ground strap points in the direction you want it to. Towards the exhaust valve I think. I could be wrong on that. Basically, you mark the plug/socket where the ground electrode is and put it in/take it out and put different thickness washers in until they all point the way you want. Seems like a pain in the neck and what's it worth? 2 HP or so? It's not something I'd worry about unless my name was Warren Johnson.

Indexing spark plugs used to be a lot more common than it is now, for a couple of reasons. The most common problem that was helped by indexing a spark plug was when the ground strap of the plug would interfere (hit!) the dome of the piston on engines which were built with the highest possible compression ratio. The dome would simply crush and close the gap of the plug if the strap wasn't oriented away from the piston's dome while at top dead center.
The second reason was caused by the design of the cylinder heads of that time. Even the NASCAR Grand National engines were using factory cast-iron heads that were highly ported and therefore were stuck with the stock spark plug location. With a big dome, they hit the plug if the plug wasn't 'indexed' away from the piston.
The process itself is easy.
Draw a line with a SharpieŽ marker on the porcelain that lines up with the ground strap. Now screw the plug into the head and see where it lines up. If it isn't at 12 o'clock you either put it in another cylinder until it does or you change/add/subtract the copper seat gasket 'til it does.
Not really a horsepower issue as much as a little more insurance that the piston won't close the gap on a very high compression engine. Modern cylinder heads have relocated plug positions and much different combustion chamber shapes than earlier 'high-performance' versions.

Hope that may help answer some of the question.

Louie

I do what Louie says, put the cyl # on with a Sharpie oriented with the electrode, but just for kicks sicne I've never bought washers to index the plugs. Many plugs these days have multiple electrodes anyways.

Indexing the plugs will not add any appreciable horsepower in a street-driven vehicle, but it can be one of those little things that adds up in an all-out racing engine. The plugs should be indexed so that the ground strap points toward the exhaust valve. This insures that the spark will be pointed towards the incoming air/fuel mixture.
Have heard some "big-name" engine builders say that you should not use a sharpie to mark the plug body as it can cause the spark to travel the outside of the plug to the base (ground). Don't know of any actual tests to prove of disprove this, but it seems like "splitting hairs" to me.

Thanks to everyone for your info!