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 Post subject: Spark Plug Gaps?
 Post Posted: Tue Oct 09, 2007 9:38 am 
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I read some stuff put out by MSD concerning plug gaps and it seems that engines with high cylinder pressures need smaller gaps.  Mentioned this to Joe at MSD yesterday and he suggested trying .022 on the plugs in my engine.  Right now that gap is .035

So what's do you guys think and or do you have any data available on this?

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 Post Posted: Tue Oct 09, 2007 9:53 am 
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I always understood that the larger the gap, the more energy is induced from the coil (hotter spark). It's this reason that you would want as wide a gap as possible. Of course the wider gap requires more from all of the secondary ignition as in the wires cap & rotor. I can see where the wider gap would be more likely to cause a problem with interference so it might not be a bad idea to reduce the gap. BTW I run 14 to 1 CR with mine and gap the plugs around .035.

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 Post Posted: Tue Oct 09, 2007 10:10 am 
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My 18º 360 had 15:1 compression and I always set my gap at .040". Kept the same plugs in it for years. Same gap on the 10.8:1 383 bracket motor and ran same plugs 5 years.

The wider the gap, the higher the voltage gets before it sparks.

The large volume of Alcohol may change the preferred gap.

Rick


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 Post Posted: Tue Oct 09, 2007 10:18 am 
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Street guy version:

I was told the same Ken and run mine at .025.  

This is from NGK:
3. Gapping

Since the gap size has a direct affect on the spark plug's tip temperature
and on the voltage necessary to ionize (light) the air/fuel mixture, careful attention is required. Those with modified engines must remember that a modified engine with higher compression or forced induction will typically require a smaller gap settings (to ensure ignitability
in these denser air/fuel mixtures). As a rule, the more power you are making, the smaller the gap you will need.

Just my .02.

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 Post Posted: Tue Oct 09, 2007 10:24 am 
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Makes sense to me. :-k  V-6's with low compression can run a 60 gap so more compression or forced induction wider the gap easier to blow out the flame. :-s

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 Post Posted: Tue Oct 09, 2007 10:25 am 
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wide gaps are for lean mixtures ... period. The more cylinder pressure you have the smaller the gap you'll want so you dont " blow out the flame "



Friends 707" N20 engine runs .012" plug gap with a MSD Dig. 7


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 Post Posted: Tue Oct 09, 2007 11:43 am 
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Friends 14.5 to 1 nos 477 .022 gap.


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 Post Posted: Tue Oct 09, 2007 3:01 pm 
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Talked to a friend this evening and that did refresh my memory on some things I knew but had pretty much forgotten but to be honest, I never thought that this MSD box would be sensitive to plug gaps.  But then again, I've never owned an engine with this much compression before either.  So I guess I'll close that gap up to see how that does before I put that new set of wires on that I ordered yesterday.

We're planning to test at Narutal Bridge this Friday night and temps will likely be down in the 60s so the air should be good.

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Last edited by Ken0069 on Tue Oct 09, 2007 4:28 pm, edited 1 time in total.

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 Post Posted: Tue Oct 09, 2007 3:10 pm 
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Ken, I have 13 1/2 comp and a digital 7 with a 35 gap (on gas) and I guess it was dumb luck that I got the "red" msd wires when I replaced them....No problems..........................................yet.....................Just got to cure the tire spin thingie but with some help from members here we will get it  =;

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 Post Posted: Tue Oct 09, 2007 7:29 pm 
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Ken i always heard them blue MSD wires were for fords anyway :-k


Mark


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 Post subject:
 Post Posted: Tue Oct 09, 2007 8:47 pm 
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Ken,
You don't think the wide gap is causing the spike and dying going downtrack do you?

Might cause a miss, I'm not buying it being the cause of the other.

The wires might cause enough noise in the digital box to do that, but I would say its on the end of the list of things to try.

Rick


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 Post Posted: Tue Oct 09, 2007 10:07 pm 
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Rick we will find out Friday night as I've already regapped the plugs and I'm going to take the new wires with me and install them there if needed.  

Talked to a friend that has had some interesting experiences with plug gaps in high cylinder pressure engines and he agrees with Joe at MSD on the gap.  Anywho, it won't cost me anything but time to try the gap first and then the wires second.  I'll even take another set of plugs with me just in case.  By the time I changed all that I may not get but 2 runs in but at least I'll have stuff to try while I'm there.  

And the alky carb will be back together by then too so who knows.  I may change carbs while I'm there also.  Be a busy night!

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