spark plug gap vs lean mixtures

[Belgian1979]

What is the concensus here on spark plug gap to best run with lean mixtures (range 15-16 AFR) ?
BTW : I run LS1 CNP coils which are pretty powerfull and have a healthy spark.

[Circlotron]

For me with 9.3:1 and lean propane, 60 thou made a nice improvement, 80 thou no obvious further improvement, and 100 thou okay when driving along but slightly uneven idle after idling for more than 20-30 seconds presumably because the plug cooled off a little and so it was easier to arc down the surface of the inside insulator instead of jump the gap. This was with a dead standard inline six with an inductive ignition running 8 amps coil current. Stock motor otherwise, not a hottie.

[Alaskaracer]

You'll have to try different gaps to see what your combo likes best. There is no set gap for any one particular setup. Basically, you want the largest gap you can run without misfiring while still providing proper ignition of the a/f mixture under all the conditions you'll see. There can be some power in larger gaps, but they won't always fire reliably. Best to error to the tighter side.

[David Redszus]

You'll have to try different gaps to see what your combo likes best. There is no set gap for any one particular setup. Basically, you want the largest gap you can run without misfiring while still providing proper ignition of the a/f mixture under all the conditions you'll see. There can be some power in larger gaps, but they won't always fire reliably. Best to error to the tighter side.

Agree completely. A larger gap is necessary to run lean mixtures. But a large gap requires more firing voltage and will
reduce the spark duration. Increased cylinder pressure requires a higher firing voltage while a higher temperature
requires a lower firing voltage. This is a case where actual testing becomes easier than constructing a theory or model.

[Truckedup]

Would Iridium fine wire plugs help in this situation?

[Belgian1979]

My cam has something to do with it, but I see it misfiring if I try to go to 14.7/1 which I'm required now to pass tech inspection/emissions. (recently changed rules here ) Due to the CNP I think I have enough to go wider, but not sure if it would help with that. The car is not exactly one that allows for easy plug replacement without consulting a chiropractician afterwards...(especially left bank).

I think that right now they are gapped out of the factory like .035 or so.

[Lizardracing]

14.7 might be stoic but not always what the engine wants. Much depends on the induction tract design and chamber shape.

[Geoff2]

Iridium/Platinum plugs require less firing firing voltage than a conventional plug, all else being equal. From a performance standpoint, that means you can run a slightly larger gap with fine wire plugs. This what I do & run 0.080" plug gaps with 10:1 CR. Higher CRs & higher rpms will probably require closing down the gap slightly. Common sense tells me that the bigger the gap, more of the mixture is exposed to the heat of the spark & the more likely ignition will occur. Yes, spark duration is reduced slightly with a larger gap, but with inductive ign, spark duration is already quite long, about 10 times longer than CD ign.

[David Redszus]

Iridium is not a great electrical or thermal conductor; it is used for its high melting point, corrosion resistance, and wear properties.
While its electrical properties are not impressive, the electrode can be made using a fine wire which will reduce the required firing
voltage. Since it has a lower thermal conductivity, iridium tends to run at a higher temperature which might lead to pre-ignition in some engines and operating conditions.

Electrical conductivity in descending order for various electrode metals:
Silver
Copper
Gold
Iridium
Platinum

Thermal conductivity usually parallels the electrical conductivity.

[Truckedup]

I have read claims from what appears to be reputable soruces that Iidium has an initial stronger spark kernal.....I have also been told the opposite..Some of my vintage bike buddies use Iridium plugs and claim an improvemt..Most likley from somewhat maginal ignition on older bikes.I used both copper and gold fine wire on my race bikes and can't say it made any difference...

[nitro2]

I have read claims from what appears to be reputable soruces that Iidium has an initial stronger spark kernal.....I have also been told the opposite..Some of my vintage bike buddies use Iridium plugs and claim an improvemt..Most likley from somewhat maginal ignition on older bikes.I used both copper and gold fine wire on my race bikes and can't say it made any difference...

It's easy to not see a difference between one thing and another, under conditions where the difference makes no difference for example a 0.030 gap on a low powered engine with a good ignition system, most any realistic plug choice will make the engine perform the same. Fine wire resistor plugs provide the most stable, repeatable spark from one firing cycle to the next under challenging conditions (i.e. high pressure and/or large gaps), according to our high pressure spark plug tester. Under lesser conditions, there is less difference. Non-resistor plugs at smaller gaps have their place in racing as we all know, unfortunately the spark is not particularly stable/repeatable under challenging conditions. For truly lean operation, lean of stoichiometric, a big gap with a very stable spark would be the best bet.

[BOOT]

My 2 cents Air is an insulator, higher compression means more condensed air so the same gap in a lower comp app appears smaller or with higher comp you need less gap to have the same amount of air/resistance to jump. Lean = more air

What I thought is leaner mixtures were harder to fire(more air) and some in here say larger gap?

Also the benefit of the iridium is as mentioned high temp, so it can hold a sharp edge longer.

Still I'm no spark plug expert but I like to understand how n why many things work.

[nitro2]

My 2 cents Air is an insulator, higher compression means more condensed air so the same gap in a lower comp app appears smaller or with higher comp you need less gap to have the same amount of air/resistance to jump. Lean = more air

What I thought is leaner mixtures were harder to fire(more air) and some in here say larger gap?

Also the benefit of the iridium is as mentioned high temp, so it can hold a sharp edge longer.

Still I'm no spark plug expert but I like to understand how n why many things work.

Lean mixtures are just harder to get to burn from an energy/chemical reaction perspective, irregardless of what you use to light off the mixture. The difficulty with a lean mixture is getting it to burn, the plug will fire just fine through it, but that doesn't mean the lean mixture will light, that's why there are little tricks to improve the light off, such as a large gap.

[David Redszus]

A larger spark plug gap will initiate a larger flame kernel, giving combustion a head start; it's like advancing the timing.
But the larger gap requires more firing voltage and some combination of pressure, gap size, and rpm will produce a spark misfire.

If the mixture is non-uniform, then the mixture in the gap is unstable and will vary from cycle to cycle. Now a larger gap could produce a more stable combustion, but only if adequate firing voltage is provided.

Since the mixture is moving through the spark plug gap, it will change with unit time. Now a longer spark duration is necessary to
stabilize combustion. But larger plug gaps require higher firing voltages and reduce spark duration.

As was clearly stated by Nitro, for a mildly tuned engine (low compression, uniform mixture and adequate spark duration) a change in
ignition will be largely unnoticed. It is at the ragged edge of performance engines that improvements in ignitions cam produce a benefit.

Otherwise, tricky ignition systems provide only bragging rights and little more.

[Roundybout]

I was always under the assumption that larger gaps reach a point of diminishing return. Same as you only need as much octane to keep detonation at bay. Anymore is just wasted. Larger gaps will show you where the weak point is in the ignition. Puts a lot more stress on the components and the weak point will be exposed, and once it is (wires, rotor gap, ect.) the next link in the chain will fail. It appears it can definitely help with an uneven mixture but cycle to cycle variation is still there. The fix is to always strive for a homogenous mixture so a gap can be just large enough to light the mixture without putting undo stress on the sum of components in the ignition system. That seems like a fair statement, no? If you could use a huge gap and reliably fire the plug under every operating condition, would that be a benefit compared to a smaller gap under the same conditions?