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Technical Spark plug gap?

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

  1. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,346


    I got to thinking about spark plug gap a lot lately as we were having a little trouble with an engine lately that has a moderate ignition system. Back in the day, my boss was having a stock car cutting out and he told me to put new plugs in and gap them at .022. It ran great, won the race.
    Now I know the smaller the gap, the easier it is for the spark to make the jump and you have to have a lot of spark to jump large gaps.
    But my question is what effect on power does a large gap/spark vs a small gap/ spark? Has anyone done or seen a Dyno test on this? Opinions? Facts? Thanks

    1Nimrod, Desoto291Hemi and VANDENPLAS like this.
  2. Interesting you should post this. Dealing with a tip-in stumble, on my '64 394 Olds 98. With all this time on my hands, experimenting with various things to diagnose. Uncle Tony, over at Uncle Tony's Garage/YouTube, suggested increasing gap slightly, as one measure to increase power, if you have a strong ignition system. My engine has a Pertronix module and wires. Coil is stock now(blew up the oil filled MSD unit Saturday), but plan on ordering the #MSD 8222 epoxy filled coil soon. It can be mounted in any direction, without fear of oil leak.
    So, I'm going to try bumping my gap up from .035" to .040" and increasing timing a bit.
    He talks about it, in one of his "Perfect Daily Driver" series. maybe this one?? Don't let this guy's appearance fool you. He has a deep background in Hot Rodding and Drag Racing. Would love to sit with him and drink a few beers sometime. A real character.
    Boneyard51 and VANDENPLAS like this.
  3. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,973


    Big gaps came in with electronic ignition and lean mixtures that were harder it light. If you are not having lean misfire I do not see any advantage and I'm sure it puts more stress on the ignition system forcing higher voltage. Kind of like the old trick of pulling the plug wire and making it jump a big gap from the wire to the plug to see if a dead cylinder will fire.
  4. Most were around ,030 - .035 back with points before the electronic ignition became the norm for new cars - heck aren't they up to about .050 now on plugs ? Always like to use my dwell meter as part of the package - gap - timing - dwell and the old vacuum gauge. I run mostly towards the stock side and usually start at the factory settings.

  5. I don't have an dyno information on spark plug gap but I do know that you want to run the widest gap that your ignition will handle. The wider gaped plugs will more efficiently burn the fuel.

    Another trick while I am typing is to make sure that the electrode and the ground wire are bot as square and flat as possible. That makes for a stringer spark.
    j-jock and Boneyard51 like this.
  6. Rickybop
    Joined: May 23, 2008
    Posts: 6,617

    from Michigan

    Need more sparkage...

    Dual spark plugs
    Split electrodes
    Perimeter type electrodes
    Bigger gap
    Hotter spark

    Sent from my VS835 using The H.A.M.B. mobile app
  7. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,346


    Hey, Paul, you reminded me of my childhood, with the pull the spark plug wire and hold it out a ways! Back when I was about 14 I had to put in some new ground on the ranch by myself due to my Dads injuries. We had , and still have, an old 1934 Allis WC, that as long as you worked it, it ran fine. But you let it loaf it would foul plugs! Well I was dragging this piece of land with a old converted horse drawn grader, just hitting the high spots and sure enough that old Allis would start missing. I would jump off and pull wire till I found the missing plug and hold that wire out about a quarter inch or more, for a while and that cylinder would go back to hitting. That Allis had a Fairbanks/ Morse magneto and would throw a big blue spark a mile! Good memories, thanks.

    Joined: Dec 14, 2009
    Posts: 2,672


    When emissions test was starting here back in the late 90’s it was a idle and driving on a dyno test.
    Had a few cars fail
    Inspect them afterwards and only thing we could find is spark plug gap set wrong ( don’t remember to big or small at this time)
    Reset the gap and the cars would pass.

    this was on older style T.B.I cars and trucks.
  9. Those fancy split electrode are nothing more than marketing gimmicks.

    Sent from my iPhone using H.A.M.B.
    VANDENPLAS, Driver50x, egads and 2 others like this.
  10. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,346


    In our class, they kinda want you to run an HEI, even though we are running a Ford. The “ good” parts, MSD and Pertix , etc advertise 50,000 volts! But the $75 junk distributors advertise 65,000 volts! We have switched so much, not sure exactly what parts we have in the disturber. But pretty sure it’s the high dollar stuff. It’s running good now at .037 gap.

    Just would like some figures on .035 vs .045, for horsepower?

    j-jock likes this.
  11. Truck64
    Joined: Oct 18, 2015
    Posts: 4,144

    from Ioway

    A big fat spark at the plug is "better", but there are limits to what any given ignition system will handle, otherwise the plug gap would be .080".

    It's a package deal notice that the points style distributors gave way to large diameter widely spaced cap terminals - there's a reason for that.

    A super wide plug gap increases the coil firing voltage, it might idle OK, but it also may break down under the stress and misfire under load or acceleration, crossfire between terminals, arc to ground, burn up rotors. Any weakness in materials or construction the spark will find an easier path to ground than the plugs.

    High voltage is really weird stuff, an ignition scope where you can actually see the results of your handiwork is educational, each individual component makes up part of the whole enchilada. Even changing the coil wire alone with a different ohms can change idle characteristics. On the scope you want to see nice even low coil firing voltage at idle across all cylinders. Most of the potential voltage is kept in reserve.

    Those engineers were smart fellows, and if you change just one thing or use junk parts it might actually make things worse. The manufacturers (for example) had non-resistor spark plugs for a long time, solid core wires, and pre-emission style rotors. Vintage versus modern ignition parts isn't necessarily "better" or worse but it is definitely different.
    blowby, Paul and Boneyard51 like this.
  12. oldtom69
    Joined: Dec 6, 2009
    Posts: 513

    from grandin nd

    and just to confuse things,with a magneto they recommend .018 to .022 gap
  13. jimmy six
    Joined: Mar 21, 2006
    Posts: 7,121

    jimmy six

    Tom Landons hot 6cylinder hei ignition conversions for early sixes recommends .060”. I started a new engine at .045” and after breaking moved to .060. So far good. We will see with a load.
  14. G-son
    Joined: Dec 19, 2012
    Posts: 664

    from Sweden

    A bigger spark gap increases the spark voltage. That may be fine as long as you have the system to supply a high enough voltage, but you also need enough insulation and distance between parts to avoid the spark finding a different path to ground - the higher the voltage, the further the spark can jump. Even things as the distributor cap can be an issue, small diameter caps with the posts close together may allow the spark to jump to the neighbouring post, giving you a spark in the wrong cylinder.
    Boneyard51 likes this.
  15. I run NGK plugs in my Dart heads and everything I read is between .035 and .055, I settled on .040 and I have HEI. So between .035 and .040 is good for my engine.
    Boneyard51 likes this.
  16. When I was doing tune-ups around 1978, the Champion and AC plugs we used on GM cars, some of those had .045 to .060 gaps on them. There was a rule of thumb we loosely followed, 50k volt HEIs used .050 gap, 60k volt was .060 gap.

    I initially set my NGK gaps to .050, but definitely runs stronger and idles better at .035/.040. I have to change the plugs (need to do a plug check) and run my Holley primary jets down from #69 to #67 or #65.
    stillrunners likes this.
  17. The Shift Wizard
    Joined: Jan 10, 2017
    Posts: 2,098

    The Shift Wizard

    One more for-what-it's-worth tidbit......
    Back about 15 years ago when I was heavy into turbocharging, as we raised the boost pressure the resistance to spark increased so we had to gap down the plugs to compensate. I can see a crossover to high compression situations, though not as extreme as turbo boost, there could be a need to close the gap a little bit in some situations.
  18. It is harder for the plug to fire under higher pressure so this makes sense.
  19. jaracer
    Joined: Oct 4, 2008
    Posts: 496


    That's what I remember from my sprint car days.
    427 sleeper and oldtom69 like this.
  20. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,346


    Guys, I understand a wide gap needs more power to jump across and under heavy acceleration. It’s even harder to jump then , due to more “ insulation “ between the electrodes.
    But my question is how does a narrow gap vs wide gap effect performance/ horsepower?

  21. I honestly don’t know that it would be enough of a difference to matter. A bigger gap will expose more of the mixture to the spark but once it’s burning don’t think it would care. My personal feeling is there wouldn’t be enough of a difference to matter.

    Sent from my iPhone using H.A.M.B.
    31Vicky with a hemi likes this.
  22. saltflats
    Joined: Aug 14, 2007
    Posts: 10,313

    from Missouri

    When I tune my spark plug gap I open the gap by .002 until my E.T. falls off then close the gap to the last setting.
    For me with a points ignition and a few tricks it ends up at .038.
    Boneyard51, Truck64 and Gammz like this.
  23. Since you have real data, does it drop off if you close the gap up?

    Sent from my iPhone using H.A.M.B.
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  24. saltflats
    Joined: Aug 14, 2007
    Posts: 10,313

    from Missouri

    Yes it did.
    Boneyard51 likes this.
  25. Gammz
    Joined: May 10, 2015
    Posts: 793

    from Lincoln Ne

    I can tell you that Saltflats helped us a ton at the Meltdown a few years back. He suggested we close down our gap on the plugs. And the old girl ran the best ever. I think we ended right around .035.... Pay attention to the good ol hot rodders they have a lot of information from experience.

    Sent from my iPhone using H.A.M.B.
    loudbang, Boneyard51 and saltflats like this.
  26. The gap, which is the final part of the chain prior to combustion - when the gap matches everything behind it in the ignition system, air fuel ratio, compression ratio, cylinder head design, bore size, fuel quality- then your HP will be maximized.
    Truck64 and 26 T Ford RPU like this.
  27. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 25,272

    Jalopy Joker

    take a look at Pertronix Flame-Thrower II 45,000 v Epoxy filled $49. 99 : - cheaper than MSD and stay with same brand as distrib conversion - have seen that with this set up a widening of stock gap by .oo5 used
    Boneyard51 likes this.
  28. squirrel
    Joined: Sep 23, 2004
    Posts: 46,864


    If it misfires, because either the gap is too wide and the spark can't jump it, or the spark is too small from a small gap, you will lose power. As long as it's working right, no misfires, I doubt you'll see any performance difference.
    racer-x, sunbeam, Gammz and 4 others like this.
  29. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,346


    Thanks, Jim! I respect your opinions! That’s kinda what I thought, but then I see all those high powered and high priced ignition systems and started wondering!
    With new plugs and a little smaller gap, our engine is running great! And it doesn’t appear to have lost any power, actually seems to have more power!
    We were thinking of up grading the ignition system but the voltage rating of various systems seem to be all over the place! 45,000- 50,000 and the cheapos come in at 65,000 volts!
    Seems odd to spend 3-4 hundred dollars for a 50,000 volt system when you can get a 65,000 volt system for $65!

    VANDENPLAS likes this.
  30. 51504bat
    Joined: May 22, 2010
    Posts: 2,149


    Don't let J.C. Whitney know!
    stillrunners and Driver50x like this.

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