Plug gap

[tresi]

Back in the 80's my friend had a Firebird with a very low compression ,7.7 to 1 stock, 400. The factory gap was .060". It didn't really run better or worse at .035 but if the .060 opened up just a little it would start missing. At .035 it would run a long time before you had to mess with it.

...and some Oldsmobeaters had .080 recommended gap. GM revised that figure after a while, went down to .060 or thereabouts.

And it may have .80 as well. I just remember that we set it back to .035

[MadBill]

It's possible that the 'big spark' kicks off a faster burn, rendering the previously-optimized timing excessively advanced.

[gruntguru]

Out of interest from this thread, I experimented yesterday with a wider gap. Bear in mind this was one test, on one engine only, so the result is hardly statistically strong, but nevertheless, here's my finding.

The engine is a single cylinder air cooled vintage bike motor, 38ci/620cc. Modified factory aluminium head, aftermarket crank rod and piston, 10.5:1 SCR, good intake and exhaust, MSD Streetfire ign and tailored advance curve, 4000-7000 power band, and approx 49hp.

I opened my normal 40th gap on an NGK B7ES to 60th. Under load in higher gears, the bike was noticeable flatter, nosed over early, and on one occasion had a brief moment of just detectable ping/detonation. Acceleration in the lower gears seemed unaffected.

Changed the gap back to 40th, and the performance returned. No changes to fuel or timing over the test.

My experience- your result may vary.
Regards
Paul

I would heed MadBill's post above and see what happens at 0.060" with a little less advance. In the absence of mis-firing, it could be that the 0.060" gap is actually doing a better job of starting the burn.

[blown265]

That's an interesting point. Coming into our summer, the air cooled engine usually wants a few degrees less than it does in cooler weather, so the total advance is definitely borderline (hemi style chamber, domed piston, 30deg total, 10.5:1, and premium pump fuel).

I have a dyno session planned for early January, so I'll make the changes at that point and get some quantified results. In the previous simple test, the performance went backwards, but as you mention, there may have been contradicting factors at play. It's likely the potential gains are small- probably undetectable from the Enfields' seat.

Thanks for your feedback.
Regards
Paul

[barnym17]

GM did the wide gaps to expose more spark due to extremely lean fuel mixtures for mileage and emissions in the bad old days. It was very problematic the hei had the power to hand le the gaps but wire leakage caused a lot of issues, hard on the rotos as a lot of time the easiest path to ground was thru it to the distributor rather than across the plug gap.

[barnym17]

rotor not rotos lol

[F-BIRD'88]

GM did not try large plug gaps because the AFR was extreme lean
or even lean. It was because of the use of EGR. Exhaust gas in the air fuel mix does not burn. Thus if a wider bigger spark plug gap better to catch more ignitable molecules of air/fuel close together to get the fire started. Remeber these low cr low perf engines of that era had relatively low cylinder pressure, and had to sneek thru emissions using a (Qjet) carb and egr. DO NOY equate to the igntion spark plug gap needs of a high performance street or racing engine , even with a CD ignition box.
BigJoe1 the late Joe Sherman tested all this on his dyno and found NO POWER from big gaps... With bigger plug gaps expect shorter cap rotor life. .025" to .040" is best for what you and I do.
Gm's general recommendation of .035" is about right. for 99% of what we do.

[Geoff2]

Yes, maybe no extra power at max rpm with large plug gaps. But what about 'hot' street driven cars & drag cars. The long[er] duration cams create EGR because of the overlap, so larger gaps might reduce misfire at lower, cruising rpms & at idle.

[Circlotron]

Yes, maybe no extra power at max rpm with large plug gaps. But what about 'hot' street driven cars & drag cars. The long[er] duration cams create EGR because of the overlap, so larger gaps might reduce misfire at lower, cruising rpms & at idle.

Agreed.
Also, given that there is an optimum AFR for maximum torque at any given rpm, it would be interesting to see if the torque drops off less rapidly either side of the peak with a bigger plug gap. In other words, would it make best AFR less critical?

[Circlotron]

Likely a big plug gap would help smooth out a 2-stroke, especially if it wasn't prone to fouling the plugs so you could use an inductive ignition instead of a CDI.

[RevTheory]

On inductive spark, don't you get longer spark duration with a narrower plug gap by not making the coil blow its wad jumping a wide gap?

[David Redszus]

For a given temperature and pressure, the required spark plug firing voltage (kV) is determined by the plug gap distance.
As shown below, an increase in gap size of 0.010" requires an increase in firing voltage of approx 5.0kV.

As the required voltage is increased, the spark duration is reduced. As engine speed is increased, available voltage is decreased. When the limit of reserve firing voltage is reached, misfires occur.

As the spark plug gap is increased, a larger flame kernel spherical surface area is created. The increased surface area has the effect of changing the ignition firing angle. While the actual firing point does not change, the peak pressure location angle does change.

A larger plug gap, if adequate voltage is provided, will move peak combustion pressure closer to TDC. This may be good or bad, depending on the original starting point of peak pressure. How much the peak pressure location angle will be actually moved is also dependent on chamber turbulence and temperature.

Gap....Volts....Area
(in)......(kV)....(in^2)
0.010....6.9...0.001
0.020...12.2...0.005
0.030...17.3...0.011
0.040...22.3...0.020
0.050...27.2...0.031
0.060...32.0...0.045
0.070...36.8...0.062
0.080...41.6...0.080

[gruntguru]

In those first microseconds after the spark, there is very little happening in terms of mass fraction burned or pressure rise. So any advance of the pressure peak is probably a shift (advance) of the entire combustion process. Put another way, most of the charge is burned at the same rate, its only the first 1% or less that burns faster.

One definite advantage of a wider gap (assuming the ignition system can jump it reliably) is a reduction in cycle to cycle variation which is often caused by "ignition delay" getting that kernel established.

[Circlotron]

On inductive spark, don't you get longer spark duration with a narrower plug gap by not making the coil blow its wad jumping a wide gap?

Yep.
But whether you gain more by a longer spark duration than you lose by a smaller plug gap probably varies on a case by case basis. You'd just have to try it and see.

[Circlotron]

One definite advantage of a wider gap (assuming the ignition system can jump it reliably) is a reduction in cycle to cycle variation which is often caused by "ignition delay" getting that kernel established.

And that might help you to have *all* cycles run closer to the knock limit rather than having only *some* cycles ping. Getting those lazy cycles moving should uncover a small percentage of power.