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Thread: spark plug indexing

  1. #1

    spark plug indexing

    On page 154, last paragraph, Dec. 2003 Excellence Magazine Jim Pasha mentions "indexing the plugs". Is this a worthwhile tuning consideration on a 2.7 hot rod motor with webers??? TIA for anyones experience in this area.
    Ed Barnett
    Ed Barnett
    RGruppe #124
    Northern California Racing Club
    American Racing Club
    Member, Northwest Hillclimb Association

  2. #2
    Indexing really is intended to increase the efficiency of the ignition process. It involves making certain that the ground electrode of the spark plug is not in the way of the fuel entry. To do this, people add shims or choose plugs so that when the plug is fully seated, they ground electrode is on the opposite side of the intake valve or fuel injector as the case may be.

    Does it help? Well, ignition is a probabalistic event at best - it does not happen every time. And some cycles are better than others. So, indexing increases the likelihood of a good iginition event. It also reduces interference with the flame propagation in the combustion chamber. If you look at some racing plugs you will note two things - they are extremely "cold" heat ranges and many do not have ground electrodes at all. They fire from the center electrode to the shell.

    Multiple ground electrode plugs come closer to assuring ignition on every cycle, and they often have lower dialectric thresholds (required less energy for a hotter spark), but they hinder flame propagation and can shield the air/fuel mixture from the spark.

    How to index? That can be a problem. Most assembly processes do not orient the electrode to the threads. So it is a selection of plugs from a random lot to hope to align the electrode to the desired orientation. Certain plugs are oriented to certain engines, but I am certain you don't want to buy those! Using a shim, like a series of gaskets on a flat seat plug (won't work on a conical seat plug) can work, but is tedious and can change the heat range slightly from cylinder to cylinder. The quality of the seal and the distance the insulator tip extends into the combustion chamber are the key contribitors to heat range (the amount of heat conducted out of the combustion chamber to the heads), so gaskets can impact the threaded depth of the plug and the effiency of thermal conduction.

    These impacts are way past the decimal place, so do not make a big difference. Same with indexing. If you can do it, great. If not, with our engines, there will not be a big impact either way. If you want to try it, though, just buy a couple dozen plugs, mark the location of the ground electrode somehow (probably on the extension of your installation wrench so you can see it once the plug is under the upper valve cover), and keep trying different plugs until you find a set that is oriented when it is torqued down.

    That may be a long answer to a short question, sorry. To establish my credentials to answer, I hold the patents on ground electrode design for the RapidFire spark plug (#5,821,676), and spent way too much of my life studying the d**n things.
    1970 911 T
    1977 911 S

  3. #3

    indexing link

    I got this link from the RennList Racing Forum. Best info so far.
    Ed Barnett
    RGruppe #124
    Northern California Racing Club
    American Racing Club
    Member, Northwest Hillclimb Association

  4. #4

    Spark Plug Indexing

    The latest issue of Excellence answers a reader's question on this - they claim about an extra 4HP on the dyno with indexed plugs.

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