Narrow plug gap = less timing or more?

[John B]

I get conflicting info googling this, one says that a small gap gives a weak start to combustion effectively reducing ignition timing, another says that the spark jumps the small gap earlier effectively advancing ignition timing. Which is correct?

[Vigo]

Offhand i'd say both are wrong in the sense that i dont think the difference is large enough to be measurable. For one thing, once the computer triggers the coil, the current in the secondary circuit moves at the speed of electricity which is ludicrously fast even compared to piston speed, at ~186,000 mph. Adding a few thousandths of an inch to the path isnt going to make any difference in where the piston is when combustion starts and finishes. I think there is way more variance introduced by all the slack between the crank gear and the distributor rotor but that affects all cylinders equally anyway and i think you are looking more towards differences between 1 cylinder and the rest.

I dont think a bigger spark makes combustion travel faster if the bigger spark is in the same place as the smaller spark. But i'd be open to learning the truth if i turn out to be wrong..

[minigts]

I haven't read multiple sources, but I found an article on the MSD Ignition site that mentioned a smaller gap will help with too much spark, basically it reduces the potential for the current. I narrowed the gap on mine and I still have some detonation, well enough that reducing the gap didn't eliminate it, which is what is preferred.

If you can adjust the timing via a cal, I would drop it or retard the distributor and see if the detonation goes away, assuming that is the issue.

[zin]

A smaller gap will do a couple of things, it will give a smaller initial flame "kernel",the bigger the initial kernel, the quicker the flame front will develop, so it "kind of "acts like advanced timing in that you wouldn't need as much time to burn the mixture, but not in a big way.

Second, the tighter gap will make the ignition's job easier, in that the electricity can jump the gap easier, especially when high cylinder pressures are present, such as high boost or a healthy dose of nitrous.

As far as detonation goes, a tighter gap's smaller kernel would tend to give a slower pressure rise, but if things are that touchy, you'll want to make other, more significant changes to fix the problem (IMHO) .

Mike

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A smaller gap will do a couple of things, it will give a smaller initial flame "kernel",the bigger the initial kernel, the quicker the flame front will develop, so it "kind of "acts like advanced timing in that you wouldn't need as much time to burn the mixture, but not in a big way.

Second, the tighter gap will make the ignition's job easier, in that the electricity can jump the gap easier, especially when high cylinder pressures are present, such as high boost or a healthy dose of nitrous.

As far as detonation goes, a tighter gap's smaller kernel would tend to give a slower pressure rise, but if things are that touchy, you'll want to make other, more significant changes to fix the problem (IMHO) .

Mike

[John B]

A smaller gap will do a couple of things, it will give a smaller initial flame "kernel",the bigger the initial kernel, the quicker the flame front will develop, so it "kind of "acts like advanced timing in that you wouldn't need as much time to burn the mixture, but not in a big way.


(IMHO) .

Mike

So, your opinion is that a tighter gap will have an effect something like less ignition timing, but only slightly?

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If you can adjust the timing via a cal, I would drop it or retard the distributor and see if the detonation goes away, assuming that is the issue.

The issue is a hot or lean cylinder. I'm using a plug three steps colder in this particular cylinder, and I'm trying to figure out the problem.

[zin]

Yes,that's correct. Have you had the injectors checked for flow?, or tried moving them to different locations to see if the problem follows the injector? Also, is the injection harness in good shape, or brittle?

Mike

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Yes,that's correct. Have you had the injectors checked for flow?, or tried moving them to different locations to see if the problem follows the injector? Also, is the injection harness in good shape, or brittle?

Mike

[Vigo]

This is the downside of having multiple threads about the same issue. He has already moved the injectors around.

I think it's possible for plug gap to make a difference in when combustion finishes, BUT the difference would be so small you'd have a really hard time measuring it outside a lab and so would be a total non-issue as far as tuning your ignition timing. It definitely would NOT be able to create the condition you're experiencing.

[johnl]

I'm with Zin.

My opinion is based on my experience with a carbureted single cylinder motorcycle engine which had a hemi head and a low angle of entry (i.e., poor) intake port. I got the same seat of the pants change by opening the gap to 40 thou (and installing a coil to support that gap) as I did by advancing the timing a degree or two.

You can really tell the difference when you are working with 20 or 30 horsepower.

[5DIGITS]

A smaller gap will do a couple of things, it will give a smaller initial flame "kernel",the bigger the initial kernel, the quicker the flame front will develop, so it "kind of "acts like advanced timing in that you wouldn't need as much time to burn the mixture, but not in a big way. Second, the tighter gap will make the ignition's job easier, in that the electricity can jump the gap easier, especially when high cylinder pressures are present, such as high boost or a healthy dose of nitrous.
As far as detonation goes, a tighter gap's smaller kernel would tend to give a slower pressure rise, but if things are that touchy, you'll want to make other, more significant changes to fix the problem (IMHO) .

Well said.
Additionally, be conscientious of whether the colder plug also changes the plug to a recessed electrode, which affects the initial flame front position.

[Vigo]

Right. That's what my comment about a bigger spark being 'in the same place' as a smaller spark was getting at. My suspicion is that JohnL's experience with the bike motor would have been even more pronounced if he'd used a longer/deeper plug even at the stock gap because you're actually moving the spark to a more central location.

[johnl]

Right. That's what my comment about a bigger spark being 'in the same place' as a smaller spark was getting at. My suspicion is that JohnL's experience with the bike motor would have been even more pronounced if he'd used a longer/deeper plug even at the stock gap because you're actually moving the spark to a more central location.

Yes, exactly. I neglected to mention that in that engine I also got the same result by switching from a standard to a projected nose plug.

I wound up running wide gapped projected nose plugs. My opinion was that I got better throttle response at the bottom end of the power curve by using projected nose plugs, that with the high compression domed pistons/domed chamber of that engine, the charge was a "melon rind" shape, so shrouded and cooled by all that surface area, that getting that kernel out away from the cold head and into the turbulence was effective.