Spark plug for a 9.5:1 compression 460?
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Spark plug for a 9.5:1 compression 460?

  1. #1
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    Default Spark plug for a 9.5:1 compression 460?

    I just finished putting my 460 back together after bending a valve (machine shop said I had a tight valve guide - bummer). The motor is a marine 460 from the mid-80's. Lower end is all stock with flat top pistons with duel valve relief grooves. Heads are 75cc Edlebrock Performer RPM CJs, Intake is Edlebrock Performer CJ, 800cfm Edlebrock carb, full MSD ignition/distributor/coil, and lightning through transom wet headers.

    The machine shop (Bailey's in Mesa, AZ) estimated my compression ratio at 9.5:1. I run 91 octane in the motor. The boat is a 21' kachina with a Berkley pump.

    What plug should put in the motor? It had ACDelco #5 plugs in it, but I was told that might be too hot of a plug to run in a jet boat.
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    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by 460Kachina View Post
    I just finished putting my 460 back together after bending a valve (machine shop said I had a tight valve guide - bummer). The motor is a marine 460 from the mid-80's. Lower end is all stock with flat top pistons with duel valve relief grooves. Heads are 75cc Edlebrock Performer RPM CJs, Intake is Edlebrock Performer CJ, 800cfm Edlebrock carb, full MSD ignition/distributor/coil, and lightning through transom wet headers.

    The machine shop (Bailey's in Mesa, AZ) estimated my compression ratio at 9.5:1. I run 91 octane in the motor. The boat is a 21' kachina with a Berkley pump.

    What plug should put in the motor? It had ACDelco #5 plugs in it, but I was told that might be too hot of a plug to run in a jet boat.
    Cant tell if your whole system is MSD but MSD says .040 to .045 up to 10.50 to 1 N/A

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    Quote Originally Posted by jetboatperformance View Post
    Cant tell if your whole system is MSD but MSD says .040 to .045 up to 10.50 to 1 N/A
    I have a MSD 6A ignition controll modual and MSD coil and distributor. Would an Autolite #12 fall into the recomended plug above?

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    Default Petain robian

    Would a Autolite AR135 work well?

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    Quote Originally Posted by 460Kachina View Post
    Would a Autolite AR135 work well?

    Well now I'm even more confused. My marine mechanic told me to use the AR135 plugs. Those plugs have a tapered seat and a .460 reach. But then I checked the recommened plugs on summit for the Edlebrock heads and they suggest an Autolite 3926, which has a compression seat and a .750 reach.

  8. #6
    Senior Member jetboatperformance's Avatar
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    With due respect to your mechanic, I'd go with the Mfg recommendation as a starting point

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    Senior Member Filucky's Avatar
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    I've ran mild to 1000+hp BBF's and Motorcraft plugs work great! I cant remember the numbers off the top of my head but the $1.50-2.00 plugs will get the job done!
    Nice!

  10. #8
    Senior Member Schiada 201's Avatar
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    Autolite 25

  11. #9
    Senior Member Josh@JBP's Avatar
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    The ELD Heads take a 14mm gasketed .750 reach plug, Edelbrock recommends Champion rc12yc but i've always had good luck with autolite 3924s.

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    Senior Member Cowboy_Mike's Avatar
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    Quote Originally Posted by [email protected] View Post
    The ELD Heads take a 14mm gasketed .750 reach plug, Edelbrock recommends Champion rc12yc but i've always had good luck with autolite 3924s.

    Good info here!! Are the Accel plugs any good for his set up, or even a stock set ogf heads?
    I have ran them on all of my other motors with out an issue!!

  13. #11
    About 500' AGL MudPumper's Avatar
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    I ran the Autolite AR 3924's for years in my BBF with Edelbrock Heads and never had an issue with them.

  14. #12
    Senior Member harley222's Avatar
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    I run the autolites and they work great, not exspensive and easy to find.

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