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Old 09-01-2005, 21:29   #1 (permalink)
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What is everyone running for a spark plug gap?

I'm running a full MSD ignition, Billet dizzy, Digital 6 plus box, and HVC coil. I have about 10.8:1 compression and run an Autolite AR-25 plug. I had the gap set at .060" and it was fine, didn't get extinguished on the dyno, that I know of, but I decided to put it down to .045" after reading about different gaps. Well, it idles a lot worse now with the smaller gap than it did with the bigger gap. Do I go back to the 0.060" or can I go bigger or ????
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Old 09-01-2005, 21:37   #2 (permalink)
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Re: What is everyone running for a spark plug gap?

I have a similar ignition to you and run 45 thou. With that compression, i have been advised to run 45, and if i real low compression that i could run more. Just run what works i say, as it seems yours works fine.
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Old 09-01-2005, 23:50   #3 (permalink)
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Re: What is everyone running for a spark plug gap?

Could be wrong here, but I think this is how it works.....

With the larger gap (.060") the spark will actually be hotter then the smaller gap, as it takes more power to jump the gap. So, it could be that you had a smoother idle due to the "heat" which is a "by-product" of the gap. Have you tried running AR-35 plugs at 0.045" ?
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Old 09-02-2005, 07:18   #4 (permalink)
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Re: What is everyone running for a spark plug gap?

Quote:
Originally Posted by F100cleveland
I'm running a full MSD ignition, Billet dizzy, Digital 6 plus box, and HVC coil. I have about 10.8:1 compression and run an Autolite AR-25 plug. I had the gap set at .060" and it was fine, didn't get extinguished on the dyno, that I know of, but I decided to put it down to .045" after reading about different gaps. Well, it idles a lot worse now with the smaller gap than it did with the bigger gap. Do I go back to the 0.060" or can I go bigger or ????

Here's why your idle is better with the larger gap. At lower RPMs, your ignition system fires multiple times in a very short period. The larger gap slows that down somewhat by taking longer to arc across the poles. That "huge" spark ignites the mixture at lazy RPM and the multiple spark discharges "keep" the spark there for a rather long time. What you didn't say was where you have your timing set for initial advance. By closing the gap, you would likely need to retard your initial somewhat to bring back the idle quality.

You also didn't say where you have your total advance set and at what rate you have it coming in by...so there's really no way to get an idea of how much gap you can live with, however, I'd think that 60 is enough. I usually run 50-55 and I'm using a 7-series box.

Plug gap is just part of ignition timing. It is often overlooked because the time that electricity takes to bridge a gap of .010 versus even 1 inch is so minute that we need exponential notation to even calculate it. However, on MSD systems, the spark "remains" bridged across the gap of the plugs for a very long time (relative to a single spark system, like common points type ignitions with standard coil and no "black box") and that spark "face" greatly aids in low speed combustion by creating and keeping a solid flame front in the chamber.

Every serious racer should be indexing their plugs and gapping for their timing conditions.

...and, "bigger" probably isn't the way to go, especially at higher RPMs where the MSD "effect" deteriorates into a single spark. Too big and you'll have problems, especially unburnt fuel and even possibly predetonation and/or multiple flame fronts in the chamber (pinging under load) that will cause more problems than would make sense with a reasonable gap. Don't get me wrong, there could be something to be learned in experimenting with gaps in your combiation...but, if you do not start with your plugs indexed, then what's the use in even exploring that path? To begin experimenting, you need a reasonable starting point that is consistently reproducible. You can't obtain that without some forethought. You also need some manner of measuring results whether negative or positive...the seat of your pants is probably not going to get it very accurate and it could be possible to introduce some harm to your engine without recognizing it until that all too famous "too late" occurs.


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Old 09-02-2005, 15:44   #5 (permalink)
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Re: What is everyone running for a spark plug gap?

Quote:
Originally Posted by davis
You also didn't say where you have your total advance set and at what rate you have it coming in by...so there's really no way to get an idea of how much gap you can live with, however, I'd think that 60 is enough. I usually run 50-55 and I'm using a 7-series box.


:davis:
davis, I'm running with my timing totally locked out. It is set at 36 degrees total. So how does that affect the plug gap?
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1982 F100 2wd-408c CHI 3V 225cc Heads, 3v Intake, Pro Systems 1000+HP carb, 260/266@0.050 w/ 0.694/0.697" lift solid roller, JPT Rollerized c6 and 5000 rpm converter, 4.86 gears and S&W Ladder Bar kit with QA1's all the way around.
7.976@89.90 mph 1/8 mile with lots of bounce - with old engine and suspension(should be greatly improved)
1982 F100 408c 3v
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Old 09-02-2005, 19:21   #6 (permalink)
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Re: What is everyone running for a spark plug gap?

0.9mm gap
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