Magnetos
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Magnetos

  1. #1
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    Default Magnetos

    Seems to be a lot of differing opinions on magnetos and plugs, gap etc. I have a BBC with a stack injected ( mechanical ) setup and a vertex mag. What are the best plugs and gap for this set up? Expected max RPM is 7000. Thanks in advance!

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    Village Idiot fc-Pilot's Avatar
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    OK, there are some variables that we need answered. What style of plug are you running? (thread size and length as well as tapered seat or gasket seat). Then what brand do you like. I like Autolite, but others swear by NGK. Also, what are specifics of your engine and what fuel are you running? Those all factor into the right plug.

    Now Vertex mags tend to like smaller plug gaps (like the .016-.020 range), or that is what I have personally seen. Others may say different.

    Get us all the details you can about the engine and we can get you better info. I would hate to give you a plug recommendation for a short thread plug when you need a long one. Or worse yet a long thread plug and then have it get SMASHED by the piston.

    Paul

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    Default Magneto

    Thanks Paul, I should have been more specific. This is a 468 Chevy, 10.5:1 Brodix heads with a Crower stack injection running AV gas, the mag is a vertex. The heads use a long thread with washer and I have used NGK in the past. I have had some fouling problems from the tune that I think we have figured out. I want to start fresh with new plugs and the new tune and wanted to get some insight on plugs, gaps etc. thanks again for your insight, I appreciate it!

    John

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    John, this is my two cents (and my thoughts are not worth much more than that so be prepared). I would run the gap at .020 on the plugs. I would also run what ever plug is comparable to the autolite 3934. That should be plenty cold for what you are trying to do to get in the ball park. As you get it dialed in you may want to try one step hotter plug. Sounds like a fun combo. what is it in? Post up a picture. I am s sucker for crower stacks.

    Paul

  7. #5
    Some guy obnoxious001's Avatar
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    I didn't do the cross reference to check, but I would probably start it at an NGK 7 heat range.

    I always ran a .018" gap with my turbo engine, always started well (by the way, I did not run 7's with the blown or turbo engines I had)

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    Quote Originally Posted by obnoxious001 View Post
    I didn't do the cross reference to check, but I would probably start it at an NGK 7 heat range.

    I always ran a .018" gap with my turbo engine, always started well (by the way, I did not run 7's with the blown or turbo engines I had)
    That sounds like a good call on the plugs. I was just being safe. The AR3934 crosses to a -8 NGK and the AR3935 crosses to a -7. I have not run many pump gas things. I use the ar3933 or 3934 for some of our alky stuff so I figured the 3934 would be safe to get close and maybe step up when it was close. Sounds like I may have been "extra" safe.

    Thanks for the tip Barry.

    Paul

  9. #7
    steelcomp was here
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    I agree with Barry on the 7 heat range (for NGK). I know nothing about magnetos and plug gap so I can't add or offer anything on that.
    If God is your co-pilot, change seats!
    Acts 2:38, the perfect answer to the perfect question.

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    Default Magneto

    Thank you gentlemen, sounds like we are close. I was using heat range 7 plugs and it sounds like 18-20 for the gap is the consensus and that is also where I had them. Someone recommended using the projected plug instead of the non projected, thought it might help with idle speed fouling, I'd be interested in thoughts on that. I was using non projected, and as mentioned had some fouling issues but I think mostly due to the tune, that being said, it seems like the idle is always somewhat fat on the stacks to get a good throttle response, so anything I can do to help with the no wake zones etc. is groovy. I'm hoping to head back out this weekend and give it a whirl. Paul, this is a long deck Mandella, fun little boat, just need to get it running right. I'll take a pic a or two this weekend! Thanks again!

    John

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    Senior Member jockorace's Avatar
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    John, you have received some spot-on input from several knowledgable guys. In regards to gap (.018-.020)......another little trick I was taught ages ago, is to cut the spark plug strap back to the centerline of the plug. Auto lite now actually sells a plug with this already done. This unshrouds the actual spark, particularly at idle. It only takes about 10 minutes to cut 8 of them back. Jocko

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    Thanks Jocko, you are right it is an all star panel and I truly appreciate everyone's input. Have a great weekend.

    John

  13. #11
    someofamitch
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    Surprised no one asked this, but what is your timing set at? Locked out or mechanical advance? What is the total timing set to?

  14. #12
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    Default Timing

    The timing is locked out at 38 degrees, this motor always liked some timing.

    Thanks,

  15. #13
    someofamitch
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    Cool, sounds about right to me.

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