Indexing plugs
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Indexing plugs

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    Distinguished Member David 519's Avatar
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    Default Indexing plugs

    Back in the day, I worked on combos we had to index the plugs just to keep the piston from closing up the plug gap due to piston/head clearance. On our SS motor, the plugs are indexed even though threre's enough piston to head clearance to not need indexing. I'm guessing the plugs are indexed to improve HP. On a couple cylinders, the index washer is big enough that it pulls the plug 2-3 threads up into the head. So my question is indexing plugs really worth much HP? Also, wouldn't any potential HP benefit be off set by having the plug so far up in the head to get the strap vertical.....
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    Senior Member EVILFORCE's Avatar
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    I think in the old days that you remember, cause you are so old you fart dirt, it helped. You did it to put the spark in the chamber so it was not blocked by the plug. It put it in the open side so it was in the biggest area of air fuel mixture. Combustion chamber design has changed some since then. It did help a bit in the old days but anymore I am not so sure. It is one of those things of getting everything out of it you could. Dyno it and see I guess. Doubt it is worth it for most of us. Uncle Bob will straighten me out anytime now.

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    Senior Member wagspe208's Avatar
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    Back in the day all they had was a mag, so pointing the weak ass, barely usable, spark away from the mixture was even more death to power. ROFL.
    Wags

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    Senior Member bp298's Avatar
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    Quote Originally Posted by EVILFORCE View Post
    I think in the old days that you remember, cause you are so old you fart dirt, it helped. You did it to put the spark in the chamber so it was not blocked by the plug. It put it in the open side so it was in the biggest area of air fuel mixture. Combustion chamber design has changed some since then. It did help a bit in the old days but anymore I am not so sure. It is one of those things of getting everything out of it you could. Dyno it and see I guess. Doubt it is worth it for most of us. Uncle Bob will straighten me out anytime now.
    the "spark" is from the center electrode to the ground strap which is also oriented to the middle of the plug. if the spark is in the center of the plug, how is orientation going to make any remote difference to the combustion process?
    the reason to index the ground strap is to keep the piston from jamming it into the electrode. on the deal i worked on, anywhere from 10 to 2 was ok and good enough to avoid the piston.
    not sure why go through this if clearance is ok. if clearance isn't ok, you'll find out fairly soon. not too sure about benefits when a washer backs the plug out three threads though. it probably changes the compression ratio in that cylinder, compared to the others... i'm thinkin waggles would have a calculation for that...

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    Quote Originally Posted by bp298 View Post
    the "spark" is from the center electrode to the ground strap which is also oriented to the middle of the plug. if the spark is in the center of the plug, how is orientation going to make any remote difference to the combustion process?
    the reason to index the ground strap is to keep the piston from jamming it into the electrode. on the deal i worked on, anywhere from 10 to 2 was ok and good enough to avoid the piston.
    not sure why go through this if clearance is ok. if clearance isn't ok, you'll find out fairly soon. not too sure about benefits when a washer backs the plug out three threads though. it probably changes the compression ratio in that cylinder, compared to the others... i'm thinkin waggles would have a calculation for that...
    Your were reading my mind exactly!!!!!!!!!!!

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    "Need For Speed" Gearhead's Avatar
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    I am with y'all. Primary reason for indexing is to clear the dome and 10 to 2 is good. However we have played with indexing on flat top piston engines on the dyno and the spark origination from being indexed up versus indexed down showed to be worth about 5 hp on a 500 HP 8 cylinder engine, which is a little in a max effort application, but probably negligible to most.

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    Senior Member jetjunky's Avatar
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    prolly a dumb question here, but here i goes...what's the relationship between quench and spark quality/orientation? More negligable as seemingly indicated by Gears example or?

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    Gearhead..........I'm not trying to start a fight but what were you guys smoken between pulls?

    I two have time on the dyno with all sorts of of assorted flavors of engines and horse powers and have yet to see 5 HP over spark plug indexing, unless one plug had a closed up gap!! C-mon................
    Last edited by Spudpilot; 03-11-2013 at 06:33 AM.

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    gn7
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    Quote Originally Posted by David 519 View Post
    On a couple cylinders, the index washer is big enough that it pulls the plug 2-3 threads up into the head. .....
    David, this makes no sense at all. There is a circular device in the world that doesn't come back to the beginning in 360*. So why would any plug need to be pull or 2-3 threads to rotate it a maximum of 180* worst case?



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    Senior Member hellnback's Avatar
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    And there you go.......... took long enough .
    It may be OLD ,it may be Slow ... but at least it's MOBILE !!!

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    LP-25.com Infomaniac's Avatar
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    Indexing them sure makes you feel better about your attention to detail.
    If For Some Reason I Do Something Worthy Of Recognition. God Provided The Ability And Deserves The Credit.


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    Quote Originally Posted by Infomaniac View Post
    Indexing them sure makes you feel better about your attention to detail.
    Agreed.........................

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    Led
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    growing up i thought it was a cool go fast thing too, on my cousins PE blown alky hydro, we always got the sharpie out and marked the strap location, of course reasoning was piston clearance not spark location.

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    Senior Member Scapegoat's Avatar
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    Indexing plugs is for clearance only, no up will be gained.
    even I know this, and I don't know much....lol

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