spark plug gap vs lean mixtures

[MadBill]

Many years ago I experienced the flip side of large plug gaps. During a hasty midnight valve job on a Morris Mini Minor we left the spark plugs in. When we fired it back up it would only run on three cylinders at idle and low load, but #4 would chime in above maybe 25% throttle.
Pulled the plugs and found the gap on #4 at ~ 0.010". Gapped it back to normal and all was well. We figured the wispy low load mixture just didn't provide enough matching molecules in that tiny gap to start the fire...

[Circlotron]

Many years ago I experienced the flip side of large plug gaps. During a hasty midnight valve job on a Morris Mini Minor we left the spark plugs in.

Back in 1980 I also had the misfortune of owning one of those things, a '63 850cc one. Opened up the plug gaps from the standard 0.025 to 0.050 and put on a reasonable inductive ignition and noticed *zero* improvement. Have tried it on a number of cars over the years and it always seemed to make much more differences on big bore engines. Small bore ones like the Mini with tiny chambers were essentially unaffected.

[BOOT]

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.

oem's for emissions use a large gap & lean mixture, but as mentioned with rpm misfires will occur easier or harder to fire! Maybe i didn't put it into the correct term fire/burn but I think we can assume the OP intends to rpm above idle/cruise. Combine the increased load of rpm and lean = harder to fire! Now a 8 coil per cylinder like the OP has is more ideal but still I don't need to explain how a cylinder that doesn't fire isn't helpful.

[Belgian1979]

It is a large bore engine, which may play a role as stated above.

When referring to high rpm misfire I was always under the impression that this was caused by a dizzy type ignition where the time to load the coil deminishes rapidly with rpm. A CNP system doesn't have this drawback and the spark more or less has the same voltage imo.

Seemingly it is worth trying. What type of gap would be best ? 0.060 ? (ps dwell is set at 3.5 ms, I can still increase to about 4-4.5 ms I think)

[nitro2]

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.

oem's for emissions use a large gap & lean mixture, but as mentioned with rpm misfires will occur easier or harder to fire! Maybe i didn't put it into the correct term fire/burn but I think we can assume the OP intends to rpm above idle/cruise. Combine the increased load of rpm and lean = harder to fire! Now a 8 coil per cylinder like the OP has is more ideal but still I don't need to explain how a cylinder that doesn't fire isn't helpful.

Just to be clear a "misfire" with a lean mixture does not necessarily mean the spark plug didn't fire a spark across the gap, it means the mixture didn't light. Lean mixtures are just difficult to light with a spark. At very lean mixtures you can fire the plug all day long with any ignition system and gap you care to use and the mixture will not light, ever.

[Firedome8]

CVCC...

https://en.m.wikipedia.org/wiki/CVCC

[af2]

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.

And $2.49 vs $6.99 per plug I'll take regular please..........

[nitro2]

CVCC...

https://en.m.wikipedia.org/wiki/CVCC

Yes, a lean mixture won't fire off well (or at all depending on how lean) with just a mere spark, so with CVCC the spark lights a non-lean mixture near the plug and that burning mixture near the plug is then used to light the lean mixture in the rest of the chamber. Old as the hills, sadly I remember when it was new

[ClassAct]

CVCC...

https://en.m.wikipedia.org/wiki/CVCC

Yes, a lean mixture won't fire off well (or at all depending on how lean) with just a mere spark, so with CVCC the spark lights a non-lean mixture near the plug and that burning mixture near the plug is then used to light the lean mixture in the rest of the chamber. Old as the hills, sadly I remember when it was new

Other than minor added complexity, why not continue to develop the CVCC system? Unless direct injection eliminates the need for it. That's my best guess.

[peejay]

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.

Try retarding the timing a bit, that will reduce engine vacuum and allow the engine to tolerate leaner mixtures due to reduced exhaust gas residuals.

[peejay]

CVCC...

https://en.m.wikipedia.org/wiki/CVCC

Yes, a lean mixture won't fire off well (or at all depending on how lean) with just a mere spark, so with CVCC the spark lights a non-lean mixture near the plug and that burning mixture near the plug is then used to light the lean mixture in the rest of the chamber. Old as the hills, sadly I remember when it was new

Other than minor added complexity, why not continue to develop the CVCC system? Unless direct injection eliminates the need for it. That's my best guess.

Direct injection eliminates the need for it. The injectors are placed, and the piston is shaped, so as to do the same thing CVCC does.

[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.

Try retarding the timing a bit, that will reduce engine vacuum and allow the engine to tolerate leaner mixtures due to reduced exhaust gas residuals.

Thanks Peejay, I didn't consider this, but you may be correct.

[MadBill]

On the other hand, lean mixtures burn slower and need more advance for optimum fuel economy.

[David Redszus]

Many years ago I experienced the flip side of large plug gaps. During a hasty midnight valve job on a Morris Mini Minor we left the spark plugs in. When we fired it back up it would only run on three cylinders at idle and low load, but #4 would chime in above maybe 25% throttle.
Pulled the plugs and found the gap on #4 at ~ 0.010". Gapped it back to normal and all was well. We figured the wispy low load mixture just didn't provide enough matching molecules in that tiny gap to start the fire...

Bill,
you raise a very interesting point. The issue that you described relates to "quench distance".

If two disc conductors are placed at some distance apart with a combustible gap between, the
voltage required to ignite the mixture will increase as gap size increases. At some point firing
voltage is inadequate to ignite the mixture.

As we close the mixture gap the required firing voltage diminishes...up to a point.
When the gap becomes less than the quenching distance, the spark will still jump the
gap and ignite the flame kernel. But the kernel size will be too small to sustain the
heat loss to the disc electrodes and the flame kernel will extinguish itself.

No amount of additional electrical energy will produce a sustainable flame.

But, if the gap temperature is increased, the quench gap distance can be reduced.

Combustion misfires are determined by local temperature, local pressure, gap distance,
mixture autoignition temperature, electrode temperature and air/fuel ratio.

[MadBill]

Ah, of course: quench! Had fine wire plugs not been forty years in the future at the time, we might never have noticed the miniscule gap.