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Thread: Spark Plug Selection

  1. #1

    Default Spark Plug Selection

    I am currently using an MSD Atomic EFI, Digital 6A ignition box and a MSD distributor controlled by the atomic. These are on a GM crate 502 in a 1985 GMC 1 ton pickup. The spec sheet for the crate engine recommends an AC MR43T spark plug which is a marine plug so I'm assuming this engine was originally spec'd for marine use. With all the upgraded fuel and ignition and obviously not being in a boat would you recommend still using the factory recommended plugs or should I be making some adjustments?

  2. #2

    Default

    The main thing that matters to us is that the spark plug is a copper core, resistor style spark plug that you can change the gap on. Which that plug looks like it is so you should be fine with that one even though it isn't in a marine application.

  3. #3

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    Is it typical to increase the gap with a system like mine? I’m familiar with reading plugs and using different heat ranges but how do you know if the gap needs to be adjusted?

  4. #4

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    Increasing or decreasing the gap depends on the motor and how it reacts, in more higher compression motors you more often have to decrease the gap size, since with the higher compression it can blow out the spark easier if you have larger gap, but if the compression is close to stock then increasing the gap will make the plugs fire hotter so it will burn the fuel better for you. I hope this helps you out.

  5. #5
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
    Posts
    1,204

    Default

    The heat range and the plug gap are determined by the compression ratio, and the chamber shape.
    Since those are both stock in your case, run the recommended plug for street use.
    If you do drive it hard some, then drop the gap by 5-10 thou, if hard often, run one range colder.
    The fueling delivery, even by a well set up carburetor, doesn't alter what the spark plug tips see.
    Gary
    Regards, Gary

  6. #6
    Join Date
    Jul 2003
    Location
    Seattle, WA
    Posts
    303

    Default

    Quote Originally Posted by msdtech20 View Post
    Increasing or decreasing the gap depends on the motor and how it reacts, in more higher compression motors you more often have to decrease the gap size, since with the higher compression it can blow out the spark easier if you have larger gap...
    In this case I will have to disagree with the MSD tech. Obviously everything, not just spark plug gap depends upon the individual setup. If the truck is for street use, stick with the recommended heat range. Open up the stock gap .005" at a time, noting how smoothly the engine runs. When you get rough running, back off a few thousandths until smooth again. The whole point of going to a hot multi-spark ignition is to insure reliable and more complete combustion due to faster flame front. Unless you have a very high compression engine, plugs can generally be opened up, resulting in better mileage and better engine performance. Keep an eye on your plugs as they will need to be replaced more often than on a stock system.

    -L

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