Spark plug gap
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Cruiser
Joined: Jun 2008
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From: South Dakota
Spark plug gap
Hello out there. Replacing my plugs in my 2005 FLHTCUI tonight. I have always gapped them at .040 in, but tonight I couldn't remember so I looked it up and found that I can gap anywhere from .038 to .043 inches. Now a questioned came to my mind and it might be a stupid question, but here it goes. What gap in the range given would give me the best bang for the buck? I mean should I be setting the gap at .038 or to the other side at .043? Or is right in the middle the best? My plugs look good at .040 in, but my mileage is on low end at 36mpg. I have stage 2 big bore with SE203 cams. Using the standard HD6R12 plugs. Thanks for the time in advance.
Electra 1
Electra 1
Seasoned HDF Member
Joined: Jun 2006
Posts: 28,473
Likes: 3,950
From: Vermont
From nightrider site:
Performance and Technical information on setting spark plug gaps.
Are plugs pre-gapped from the factory?
Yes and no. A spark plug part number might fit hundreds of engines, and although the factory will typically set gap to a pre-selected setting, this gap may not be the right one for your particular engine, and may not take into account modifications that you may have performed to the engine.
Insufficient spark plug gap can cause pre-ignition, detonation, even engine damage. Too much gap can result in a higher rate of misfires, noticeable loss of power, plug fouling, and poor economy.
When you raise compression or add forced induction (a turbo system, nitrous, or supercharger kit), you must lower the gap (reduce gap about .004" for every 50 hp you add on an V-8 engine, for a Harley-Davidson V-2 engine every 12.5 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology,) you can open the gap back up about .002"-.005".
As an example, let's use a hypothetical `96 350 Chevrolet LT1 engine build-up. The standard gap is .050" for an un-modified LT1. We'll add 150hp Nitrous, so we must lower the gap about .012" to .038". We then decide to add that killer MSD 6A/Crane Hi-6 box and, using our guidelines as outlined above, we can now open the gap up .002"-.005" to about .040"- .043". By following this basic guideline should get you very close.
Further experimentation may be necessary, but by always starting with a larger gap than it thought necessary to reduce the risk of detonation, you should be safe.
Performance and Technical information on setting spark plug gaps.
Are plugs pre-gapped from the factory?
Yes and no. A spark plug part number might fit hundreds of engines, and although the factory will typically set gap to a pre-selected setting, this gap may not be the right one for your particular engine, and may not take into account modifications that you may have performed to the engine.
Insufficient spark plug gap can cause pre-ignition, detonation, even engine damage. Too much gap can result in a higher rate of misfires, noticeable loss of power, plug fouling, and poor economy.
When you raise compression or add forced induction (a turbo system, nitrous, or supercharger kit), you must lower the gap (reduce gap about .004" for every 50 hp you add on an V-8 engine, for a Harley-Davidson V-2 engine every 12.5 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology,) you can open the gap back up about .002"-.005".
As an example, let's use a hypothetical `96 350 Chevrolet LT1 engine build-up. The standard gap is .050" for an un-modified LT1. We'll add 150hp Nitrous, so we must lower the gap about .012" to .038". We then decide to add that killer MSD 6A/Crane Hi-6 box and, using our guidelines as outlined above, we can now open the gap up .002"-.005" to about .040"- .043". By following this basic guideline should get you very close.
Further experimentation may be necessary, but by always starting with a larger gap than it thought necessary to reduce the risk of detonation, you should be safe.
Road Warrior
Joined: Jul 2006
Posts: 1,532
Likes: 7
From: Rochester N.Y.
If you look at the gap while it's at .040" is the negative (side) electrode at a perfect 90 degree angle to the center electrode?
You would get the best , highest quality , longest lasting life from the plugs when the electrodes are a perfect 90 degrees.
The result is that the spark will travel around the center electrode and wear it evenly all the way around.
If your old plugs are worn perfectly around the center electrode , that would show that they were set that way
to begin with.
That being said , if the gap was better wider , low RPM spark quality would be better than high RPM.
If the gap is better smaller , the opposite would be true.
Generally a larger gap would produce better fuel mileage efficiency than a smaller gap.
This is how cars have been leaning towards for years.
A larger spark is friendlier to a lean fuel mixture most of the time.
If you want to try it both ways , just "carefully" bend the side electrode so the gap is a perfect 90 degrees for each gap that you try.
I have done this at my shop for years with great success.
Mick
You would get the best , highest quality , longest lasting life from the plugs when the electrodes are a perfect 90 degrees.
The result is that the spark will travel around the center electrode and wear it evenly all the way around.
If your old plugs are worn perfectly around the center electrode , that would show that they were set that way
to begin with.
That being said , if the gap was better wider , low RPM spark quality would be better than high RPM.
If the gap is better smaller , the opposite would be true.
Generally a larger gap would produce better fuel mileage efficiency than a smaller gap.
This is how cars have been leaning towards for years.
A larger spark is friendlier to a lean fuel mixture most of the time.
If you want to try it both ways , just "carefully" bend the side electrode so the gap is a perfect 90 degrees for each gap that you try.
I have done this at my shop for years with great success.
Mick
You ever try drilling a hole in the electrode? I've been wondering if that works. i would expect that it does nothing, since electricity follows the closest (easiest) path.
One thing I want to do is index the plugs, but keep forgetting to; I also suffer from less FE than I want to have. By indexing, I mean marking the plug to show the direction of the electrode, and adding washers to make it point towards the center of the cylinder after tightening it. Or towards the intake valve.
One thing I want to do is index the plugs, but keep forgetting to; I also suffer from less FE than I want to have. By indexing, I mean marking the plug to show the direction of the electrode, and adding washers to make it point towards the center of the cylinder after tightening it. Or towards the intake valve.
Stellar HDF Member
Joined: Apr 2005
Posts: 3,108
Likes: 9
From: Lincolnton, NC
With what most of us have in our tool box, you can't get an accurate enough measurement to know if you're really at .038" or .040". Get it close to the middle of the range, you'll never know the difference between the low side and the high side if you tried it.
Thread Starter
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Cruiser
Joined: Jun 2008
Posts: 129
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From: South Dakota
Thanks all for the responses. My feeler guage gets me close to .040, but like was said how do I know I might be off plus or minus a few inches. Looks like staying where I have them set will be fine. Thanks again.
Electra 1
Electra 1
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