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Author Topic: Plug Gap  (Read 11556 times)

Offline spitzerscott

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Plug Gap
« on: January 03, 2015, 10:58:17 AM »
 Just pondering the best plug gap for my application. I use Autolite 3924 plugs, recommended by my sprint car buddies, I have a 336 SB chevy, injected alcohol, 13.2 cr, aluminum heads, Msd 7al2, msd pro power coil, msd pro billet distributer.  Powerglide, with 4200 stall, in 225" rear engine dragster.   
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Offline dusterdave173

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Re: Plug Gap
« Reply #1 on: January 03, 2015, 11:41:36 AM »
Plug gap generally is determined by type of ignition
Vertex style mag tight at .018 range
MSD's .035-.060 but most like around .040  I have never seen any advantage to more on our dyno--
the stronger your ignition the bigger gap you can run but real world we see little/ no  advantage to bigger( my two cents worth)
Remember--If your lawn mower will not start--tighten up plug gap and it will usually fire :)
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Offline GlennLever

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Re: Plug Gap
« Reply #2 on: January 03, 2015, 09:12:36 PM »
Is blown, if it is you have to be careful about a wide gape as the blow will try and blow the spark out. I have as MSD 6 AL and run .028
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Offline ricardo1967

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Re: Plug Gap
« Reply #3 on: January 03, 2015, 10:35:34 PM »
Quote from: GlennLever on January 03, 2015, 09:12:36 PM
Is blown, if it is you have to be careful about a wide gape as the blow will try and blow the spark out. I have as MSD 6 AL and run .028

Sorry, can't agree with that... (not to mention that the intake valve closes before spark takes place).

The concern around spark plug gap on supercharged (or turbo) applications is because of the higher dynamic compression ratio, which makes more difficult for the spark to jump across the air gap.
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Offline GlennLever

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Re: Plug Gap
« Reply #4 on: January 04, 2015, 03:41:09 AM »
Quote from: ricardo1967 on January 03, 2015, 10:35:34 PM
Quote from: GlennLever on January 03, 2015, 09:12:36 PM
Is blown, if it is you have to be careful about a wide gape as the blow will try and blow the spark out. I have as MSD 6 AL and run .028

Sorry, can't agree with that... (not to mention that the intake valve closes before spark takes place).

The concern around spark plug gap on supercharged (or turbo) applications is because of the higher dynamic compression ratio, which makes more difficult for the spark to jump across the air gap.

I my reasoning is wrong, are you saying the concern about the wide gap is correct on a blown application.
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Offline wideopen231

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Re: Plug Gap
« Reply #5 on: January 04, 2015, 05:18:57 AM »
Gap on blown motor is still based on ignition type. A 3 amp mag and a 44 amp mag are going to have different requirements. Yes that is extreme but making point. Its not blowing the sparkout its more of the spark working to burn more fuel in tighter compression situatuion.More air more fuel in same space needs stronger spark.

Back to op's question. All combos are different and require fine tuning.Plug gap is just another tuning tool. Start in middle. Work in one direction until car slows or you feel its a waist,then try other direction and see whatr works best .As stated ther is very littler gain in most combos after .035 and we normally ran .025 to .030 in alky motors.
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Offline ricardo1967

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Re: Plug Gap
« Reply #6 on: January 04, 2015, 06:33:37 AM »
Quote from: wideopen231 on January 04, 2015, 05:18:57 AM
"... Its not blowing the sparkout its more of the spark working to burn more fuel in tighter compression situatuion.More air more fuel in same space needs stronger spark.
..."

^^ This
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Offline masracingtd1167

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Re: Plug Gap
« Reply #7 on: January 04, 2015, 07:09:17 AM »
I also agree ! .025 to .030 should be just fine . I have never really seen any magic in larger gaps ! 
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dreracecar

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Re: Plug Gap
« Reply #8 on: January 05, 2015, 12:17:40 PM »
I set my plugs @ .020 (Mag III, NGK B9ES) and valve lash @ .020 that way I only have 1 feeler gauge in the tray
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Offline Frontenginedragsters

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Re: Plug Gap
« Reply #9 on: January 06, 2015, 03:54:29 PM »
 SpitzerScott:
A plug gap of .035" should work fine.
My 2 cents is the same as Duster Dave: No power increases on our dyno.
My other 2 cents is to look close at the Autolite spark plugs.
We used them for years but quality has gone down.
NGK number R5671A-8 / 4554 work really well for us.
Big difference between Japan and China. 8)

Matt
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Offline spitzerscott

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Re: Plug Gap
« Reply #10 on: January 07, 2015, 03:10:45 PM »
Matt, I already had them gapped at .035 when I asked this question. I was sure my 7AL2 and pro power coil would jump that gap. My sprint car friends like these Autolite plugs, but I'm with you, I will keep an eye on them, I know most everyone runs NGK.  Thanks guys for all the help.
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