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Technical Spark plug gap?

Discussion in 'The Hokey Ass Message Board' started by Boneyard51, Apr 15, 2020.

  1. saltflats
    Joined: Aug 14, 2007
    Posts: 10,231

    from Missouri

    Put an oscilloscope on the engine and run it on a dyno, that should tell you how many kvs you need.
    You can kinda simulate that without a dyno by giving it a good rod check if you know what I mean. ;)
    loudbang likes this.
  2. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,118


    Well , James, our Dyno is in your state and I know of no one close around here, with a scope we could use.
    We did it the old country boy way! Checked the gap on a new set of plugs, put them in and pulled the car out onto the street in front of the shop and let her rip! Oklahoma Dyno!

    saltflats, Gammz and Driver50x like this.
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694


    Keep in mind that my little Chevy II is in the high 9s with a blown 427 with point ignition, stock wires, stock type plugs with stock gap....
  4. G-son
    Joined: Dec 19, 2012
    Posts: 651

    from Sweden

    This may be of interest, spark length vs. voltage. The super high voltages are basically useless, as the spark will find another way to ground than at the spark plug anyway.

    Joined: Aug 1, 2009
    Posts: 547

    from 94577

    Purpose of a larger gap, as stated earlier, is to better ignite the mixture.
    Larger gap exposes the spark kernel to the, ideally, homoginized air fuel mixture. This should allow for an easier and more complete ignition of the gases for the most complete combustion.

    This is what Splitfire plugs tried to do using conventional plug material, but failed at it for use in actual performance applications. Good idea, bad execution. Probably would have been better to simply grind back the J strap and half expose the center electrode at the expense of longevity.

    The larger the gap there is an inverse issue that occurs, spark is more readily exposed but a slightly richer or denser(supercharged) mixture creates a higher resistance between the electrode and ground. Much easier to close the gap(reducing resistance) than it is to add more ignition power, reliably.

    You are not going to get a definitive answer on a power number.
    That is a subject of dyno tuning. The higher the horsepower, the smaller the gap you are more likely going to have. You can get away with larger than ideal gaps up to about 400HP, beyond that the gap is going to need to be closed down to stock spec or smaller. As mentioned a proper oscilloscope monitoring the ignition system is what will show you if your ignition system is up to snuff.

    Narrow gap makes it easier for the spark to jump the gap. Initially the electric discharge must break down the insulation in the air, until the air itself becomes electrically conductive. In air or air/fuel mixture, current breaks down and ionizes the air, then a spark can jump the gap and ignite the surrounding air/fuel mixture.

    The larger the gap, the more current and power are used to break down the insulation of the air/fuel mixture between the spark plug gap before a spark can jump the gap.
    *Since the coil has a finite amount of power it can create, you will only have the same amount of power available no matter what the gap is.*
    With a larger gap more of the available power is used to create the conduit for the spark to jump through and less power in the spark itself.

    With a larger gap it is easier for the turbulent air/fuel mixture to 'blow out' the spark by having a higher resistance than the ignition system can overcome in creating that conduit of ionized air/fuel for the spark to jump the gap.

    Easiest/cheapest solution is to close the gap, reducing the amount of material that needs to be converted into a conduit for the spark to jump, reliably.

    One more critical component of spark plugs is that there is a sharp surface for the spark to jump off/to on the plug. Its why worn plugs don't fire as well, not because of gap creep, but because a rounded center electrode.

    U-groove plugs work well as they offer more corner edges on the ground electrode. Slightly better than typical CU plugs but nothing magical.

    Splitfire plugs are similar, more sharp corners. However the reduced mass of the twin tips can actually lead to a hot spot and detonation. Not ideal on a performance engine. This is where they fail.

    Platinum plugs were more durable as the platinum kept a sharper surface longer, the self cleaning properties also allowed for longer life even with an out of tune engine. Durability also allowed for waste spark systems. It's how early EDIS systems got away with half the coils of a proper COP system.

    Iridium plugs offer the best as the ground and power electrodes can have needle sized tips that require less power saturation to create a spark and will have a long wear life, they also do not cause a hotspot in the combustion chamber. The smaller electrodes offer a more exposed spark for an easier start in combustion. Something the split fire plugs attempted to do, but failed.
  6. Truck64
    Joined: Oct 18, 2015
    Posts: 4,115

    from Ioway

    Thing about a "20,000 volt" or "50,000 volt" coil, is they don't really work that way. There's some advertising wankery involved. The voltage required to fire the plug at idle is maybe something like 8k to 10k volts. The plug only takes what it needs, kinda.

    This doesn't change regardless of the coil rating. The rest of the voltage is held in reserve. Worn, eroded plugs will cause the coil firing voltage to spike, crisp sharp surfaces are easier to fire, high compression or load needs more juice. I remember Smokey Yunick once wrote one thing they couldn't improve on too much from the factory OEM, were coils and HEI spark ignition.

    But that isn't to say you can't make sure your setup is optomized, make damn sure the charging system and battery is squared away, grounds and cables and connections are of sufficient size to carry the amperage and every connection is clean & bright & tight, distributor body and block is free of paint, that little uninsulated braided copper ground wire on the breaker plate isn't corroded.

    Plug gap isn't the only factor in coil firing voltage. Early emissions era rotors have a huge air gap between the cap terminals, this took me a while to figure out they don't play well with high output ignition coils and point distributors. The "one size fits all" inventory system, where they change the part - though not the part number - is Not Cool.

    You can throw all kinds of new parts, imported gee whiz high dollar parts at an ignition system but that doesn't mean it is any better than what shipped from the factory. It might be a lot worse.

    I picked up a Heathkit tube type ignition oscilloscope a while back to troubleshoot with, and it really opened my eyes how tricky ignition can be. I figured "All New Parts = Good Ignition" and that was definitely not the deal at all. If you know someone or have access to a scope check it out sometime, it only takes a few minutes to really see what's going on.
    Boneyard51 likes this.
  7. inthweedz
    Joined: Mar 29, 2011
    Posts: 398


    Agree with you there, 57Fargo..
    Wre I worked a few years back we had a dyno, a guy with an OT oval dirt race car used it to tune the engine..
    One time he bought a new set of ''Splitfire'' plugs to try/compare to the standard ones he was running..
    Ran the engine up on the old plugs and compared its output with the same revs/load with the new plugs, results were, the old plugs were giving more power..
    Boneyard51 likes this.
  8. jimmy six
    Joined: Mar 21, 2006
    Posts: 6,864

    jimmy six

    I use a DUI hopped up HEI. I currently have the plugs at .050”. The tell me I can go to .075”. The carb I use came with 62 and 64 jets. When the spark plug gap started at .035”. It’s currently has 77 and 80’s jets and the plugs read lean. The carb needles/seats are 150’s.

    Their theory is gasoline is power and if you have the ignition to fire the more you have no matter the A/F ratio the more horsepower you will make. I continue to add larger jets and more gap and go faster at El Mirage and in the past Bonneville. I have had a hard time grasping all they tell me and used baby steps. I have no idea the hp of my engine or actually care as long as I continue to go faster an lead the pack which I’ve been doing since 1985.
    Boneyard51 likes this.
  9. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,939


    Big gaps require higher voltage the spark may jump where you don't want it to. I have replaced modules because of bad wires and secondary voltage was looking for a place to go and it fried the module. If you are not getting misfire I would not go bigger.
    Boneyard51 likes this.
  10. On several vehicles, I have been running CD ignition systems with points, stock coils, and platinum plugs in all of them, since 1971. I set the plug gap at .050 in, and have had excellent results. I have run with Packard copper wire, as well as 7mm and 8mm wires, and have never seen any leakage or crossover.
    Many years ago, I did research the issue of improved power, but have long since forgotten the results. At any rate, there are too many other variables that would have to be considered such as, mixture, energy density, octane, flame travel, to mention a few.
    I just look for reliability, and the above combination has always worked fantastic for me.
    Last edited: Apr 16, 2020
    Truck64 and Boneyard51 like this.
  11. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,118


    James, how much difference was there between , say .030 and .038? Also do you use a gapping tool to keep the electrodes parallel?

  12. loudbang
    Joined: Jul 23, 2013
    Posts: 29,878


    Don't forget to index the plug no matter the gap. :rolleyes:
  13. I always "J" gap my plugs. cut or file teh ground electrode so can be bent down to near the center electrode. You want sharp edges on the electrodes so there is little resistance. I will post an image later today. I always found that decreasing the gap produced more power and better response. I did my testing on a chassis dyno and used a distributor machine.
    Note the heat range of the plug has to be correct in order to use them as a turning aide.
    A coil can only supply the NEEDED voltage, it will not give you extra secondary voltage.
    Check that your distributor has a good ground (don't assume). Heavier gauge wire from the ignition to the coil will also help, found out when testing on the distributor machine the primary wire was getting hot.
    Truck64 and Boneyard51 like this.
  14. Truck64
    Joined: Oct 18, 2015
    Posts: 4,115

    from Ioway

    One thing I've noticed is the problem today with sourcing quality ignition condensers.

    Most folks have no way to actually test one, they buy cheap no name &@$ or, take a gamble on old school parts that are getting long in the tooth. They are a critical component. So it's often a guess and by golly whether the ignition system is at 100%. I run a Pertronix now but I'd recommend having a proper tested and passed ignition condenser or two on hand if running points, or get one of Tubman's reproductions.
    Boneyard51 and loudbang like this.
  15. jnaki
    Joined: Jan 1, 2015
    Posts: 4,746



    In our early drag racing days with the 1958 Impala, every little thing had to be done to be successful. All of the cars had pretty much the same weight, motor, horsepower and it was the little things that made the difference. My brother had me doing all of the spark plug changes and "checking the gap" between our practice runs to see which one was getting better results, with everything else being a constant.
    We had the thin metal one to get the closest gap that provided the best results, then narrowed it down to the Craftsman wire gap tool, and finally the “L” bend pick tool. We had all three at one time, but the final tool that was used most often was the “L” shaped tool.
    We had two sets of plugs for the 1958 Impala. One, strictly for the street, daily driving and one set for the drags. Drive to the drags, and run a quick one with the daily driver plugs. Set the gaps on the racing plugs, then run a few more trials to get the best E.T. and speed for each different gap in the racing plugs.

    Finally, check all plugs one final time before the final round. By the time we got to the end of our A/Stock car racing, we knew which plug ran the best. We also knew which gap provided the best E.T. results, with other things being standard, like my brother driving the same in each race. (which gears provided fast times vs best E.T.)

    We actually had a chart to show what was what in our trials for the 58 Impala. There were times, on a heavy duty weekend street cruise, that we ran the racing plugs, when we knew there was going to be a lot of "one to one" action.


    As a teen, I got fairly efficient, "exchanging those plugs" without breaking any tips. But, our local parts store started to give us a huge discount on spark plugs, then started giving us spark plugs when we got the 1940 C/Gas 671 Willys Gas Coupe up and running. They even gave us a spark plug tool that had some kind of rubber inside to get a better grip on the end of the plug tip. They were going to be our sponsors as we progressed in our goals.

    Our plugs were different in the 292 C.I. 671 SBC motor with more power and a Joe Hunt Vertex Magneto. We were given the information on which plugs and which gaps to use by Reath Automotive. That information was backed up by our expert mechanic in Los Angeles. He had called several drag racing friends and got the scoop on the correct plugs and gap. But, we did the same pre-elimination racing, timed runs for the best gap for the best E.T. with all else being constant.
    upload_2020-4-25_4-32-22.png 1960
    upload_2020-4-25_4-32-43.png Possible door lettering in 1964-65...A big IF...
    Boneyard51 likes this.

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