Effect of Spark Plug Gap on FE?

Discussion in 'Fuel Economy' started by visionseeming, Jan 10, 2009.

  1. visionseeming

    visionseeming Well-Known Member

    Hi, I was wondering if the gap on the spark plugs has any effect on FE. I would imagine, if it is too far, the spark will be weak and the gas will not be combusted fully. But I do not know whether decreasing it lower than factory spec will have any impact. Anyone tried this?
  2. SentraSE-R

    SentraSE-R Pishtaco

    The gap definitely makes a difference. I finally changed mine after 75,000 miles when my mileage dropped from 38 mpg to 35 mpg. The gap had gotten so wide, the spark was unable to jump the gap. Changing plugs brought my mileage back up to normal.

    If the gap is set too small, the spark may discharge too early (it's easier to jump the narrow gap), and it may not ignite the mixture. Your mileage would suffer.
  3. abcdpeterson

    abcdpeterson Well-Known Member

    A friend owned a auto parts store. He had a promo video from one of the spark plug manufactures showing how platinum tipped plugs are better and could save you gas.

    In the video they showed the spark from a platinum tipped plug and a standard plug after xx,xxx miles. You could easily see the difference in spark.
    They claimed that consistent larger/stronger spark would ignite the fuel better and in a consistent manner. Giving more power and better gas mileage.
    It was a convincing video. And the same should be true for gap.
    The cost of the plugs was a LOT more. And I could not see recouping the cost in fuel savings.
  4. warthog1984

    warthog1984 Well-Known Member

    Too large a gap and the plug will burn hot and wear out prematurely.

    Too small and the plug will be cold and "quench", leaving unburned fuel fouling the plugs. FE will drop.

    Best to go with a finer plug electrode and a factory gap or a teensy bit larger.

    Platinum has little effect. go with a fine wire electrode and you'll be good.
  5. fanamingo

    fanamingo Well-Known Member

    What advantage does the fine wire electrode have over standard?
  6. abcdpeterson

    abcdpeterson Well-Known Member

    I knew nothing of fine wire plugs.
    looked them up and found this.

    Spark Plug 411
  7. donee

    donee Well-Known Member

    Hi All,

    This is based on my somewhat ancient understanding of spark ignition systems. So, please fill me in on any new technology.

    Spark Plug gap will certainly effect fuel economy.

    Timing and the number of sparks per ignition are both impacted by the gap. The ignitions are designed to give off serveral sparks after the initial, using a capacitor to resonate with the coil inductance. The gap and cylinder pressure set the voltage required for the spark to jump. The collapse of the magnetic field in the coil takes time. And more of the field needs to collapse to get to a higher voltage, effecting how long the delay is from the timing trigger to the first spark. Also, the amount of field that collapses, uses up energy in the field , which ends up in the spark. After the first spark a resonant cycle occures between the ignition coil inductance and capacitor. Spark Gap voltage will again reach break-down potential at the plug. This is all engineered in concert with other emissions controls. The cycle can go on 2 , 3 or maybe even 5 times. Each spark helps ignite more fuel (due to the flow of fresh mixture in vortex fashion back into the area of the spark plug), or initial burn byproducts. Each spark takes so much energy. If the gap is big allot of the energy is used up right away, and the total number of sparks will be less.
  8. fanamingo

    fanamingo Well-Known Member

    Thanks for the response. That's one I should have tried searching myself. :eek:
  9. 99LeCouch

    99LeCouch Well-Known Member

    Yes, there's a definite impact on FE. Changing the plugs on my fiancee's Escort had one plug that was worn to a nubbin, and the rest a bit worn. Replacing the old, worn plugs with a properly gapped set of plugs and fresh wires bumped her highway FE from ~28 to 32-33 (she drives like a maniac) and eliminated a lot of vibration in the car.

    The only reason to run smaller than factory spec is when running a supercharger or turbo. There's no reason for running larger than factory.

    Pretty much Bosch platinum plugs seem to be the devil, and NGK's, Delco's and Autolites work pretty well in everything except German cars.
  10. warthog1984

    warthog1984 Well-Known Member

    Not quite. Running smaller than factory will help if you have a "hot" ignition coil or a knock problem (not a long term solution, but in a pinch).

    Running slightly larger gaps will give better combustion and power, and probably FE, but at the cost of increased plug wear and possible knock.
  11. Nevyn

    Nevyn Well-Known Member

    I had a '97 Escort 5MT base that I put Bosch Platinum+2's in....The others may have been older leading to a larger margin, but the difference was noticeable INSTANTLY. More power on the get-go, it took less pedal to accelerate or maintain speed, and FE went from 33~34 to 36~37.

    I wouldn't put them in anything bigger than a small 4cyl, though - my buddy ran a set through his mustangs V6 and V8, and both cars fouled them real fast.
  12. akashic

    akashic Member

    Is a helpful gap correction something that should show up immediately on the SG in terms of GPH? (or another variable?) Or is it something that the ECU would take time to adjust to?
  13. warthog1984

    warthog1984 Well-Known Member

    Depends. Small corrections will not have much effect. OTOH, if you have a very improperly gapped plug set and severe quenching and carbon buildup or misfiring/knock, a switch to new plugs or cleaning up the old ones will boost FE noticeably as quick as the ECU will adjust, and sometimes immediately.
  14. akashic

    akashic Member

    I'm fishing for a way to know right away, and objectively, so that I can compare the effects of fixing the gap or replacing them. Would my GPH at idle change? Or would it tend to affect GPH under load more?
  15. warthog1984

    warthog1984 Well-Known Member

    Not quite sure. Here's the easy way for the mechanically uninclined to tell if your plugs need cleaning or replacing:

    1) Gather your owners manual, gap gauge, and ratchet with plug socket and extension tool. Also have a spare plug OR a ride to the auto store ready. This is very important.

    2) Open the hood and any covers required to access your plugs.

    3) Carefully remove a spark plug Without cracking the porcelein. This is common and will ruin the plug.

    4) Hold the plug up to the light. Check the center electrode. A worn or badly scored center electrode requires replacing. If its dirty and wet, its quenching. Dirty and dry means carbon buildup. If it looks burned, it may be misfiring.

    5) If plug can be cleaned, do so and gently reseat it. Again take care not to crack it.

    6) Repeat with other plugs as needed or desired.

    *This is meant as a quick how-to and does not cover all conditions and will not solve all problems. I am also not responsible for dead cars caused by removing and cracking a plug without a working spare on hand.* YOU HAVE BEEN WARNED.
  16. SKJ43

    SKJ43 New Member

    Increasing the spark plug gap is an old Hot Rodders trick to increase horsepower. Usually a slight increase from .040 to no more than .050 will give a small increase in horsepower. Since you make more horsepower from the same amount of fuel there is slight increase in FE.
  17. jim isbell

    jim isbell Well-Known Member

    Something that will do more is "Indexing". Race cars do this and the Honda Insight does it. On the Honda 3 cyl there are four plugs available, A,B,C and D. each is made so the electrode id 90 degrees further around when seated in the head. On the head of the engine a letter is stamped next to each cylinder and you are supposed to put the appropriate plug in the appropriately marked hole. If you dont, the car runs, but mileage will be adversly affected.

    To do this on a car that is not marked and designed for it you have to experiment. Mark the plug so that you can look at the porcelain and know the orientation of the electrode. Then screw it in and tighten to spec. Now look and see where the electrode is. You want the electrode to be toward the side of the cylinder so that it does not shade the center of the cyl from the flame front. Now just keep trying different plugs till you find one that seats with the electrode appropriately positioned. Do this again for each cylinder.

    Be sure to mark each plug so that you know which cylinder it belongs in and dont get them mixed up or you will have to do it all over again.:D
  18. The Doctor

    The Doctor Old Dude, New Car

    In 16 years, and four cars, I've only bought one set of replacement (iridium) plugs. They came with some very basic
    instructions, which I actually read. And the manufacturer said, in very plain English, "DO NOT ATTEMPT to REGAP THESE PlUGS!"
    Why? Well maybe because the tips of those plugs are very finely manufactured to close tolerances, and besides that,
    they are brittle, and messing with them can damage them. I ran 150k miles on the original plugs that came in my Suzuki
    Grand Vitara, and when I took them out to change them, I realized that I could have just dusted them off and put them right
    back in the engine, maybe for another 150k miles. They still were like new. But since I had already invested in new plugs,
    I went ahead and installed them. Now, in my last three cars, I don't even mess with the plugs. I'd rather focus my attention on
    things that really impact my MPG. Like oil, gas, air cleaner, tire pressure, and how hard I mash the GO Pedal. :)

    And, I do use my Scan Gauge II, to help me get better MPG, and make myself a better driver.
    BillLin likes this.

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