Plug gapping and cylinder temps?

[TripleTransAm]

Any relation between spark plug gaps and cylinder temps? Combustion flame front speed?

[TripleTransAm]

I came across this quote on a search:

"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."

Sound right?

[BillyGman]

I cannot say for sure, but while I don't doubt that a gap that's too wide will cause misfires(that to me is believeable), I question the claim about a gap that's too narrow causing pre-ignition. How can that be? The system delivers the spark to the plug when the timing calls for it. No matter how close or small the plug gap is, the plug cannot create a spark on it's own.

One of the possible causes of detonation and pre-ignition is carbon deposites in the combustion chamber getting so hot that they can actually ignite fuel vapors, but I don't see how a plug can do that before it sparks. I would think that the plug in question would have to get so hot that it would act as a glow plug in a desiel engine. Not very likely as far as I know.

An interesting side note here:
Supercharged engines call for a smaller plug gap. This is neccessary so that the increased cyclinder pressures created by a blower engine will not blow the spark out. The plug gap for the Trilogy blower set-up is .032" which is ofcourse .020" tighter than the stock Marauder gap spec. And my car runs fine w/out any pre-ignition nor detonation that I've ever noticed.

[sailsmen]

Could be retained heat is increased if the gap it too small?

[BillyGman]

Could be retained heat is increased if the gap it too small?


But retained heat where? And why? I've edited my last post, so check it out.

[TripleTransAm]

I suppose it all hinges on the fact that a spark across two points is probably not instantaneous. So a spark has to travel a longer time across a wider gap than a smaller one, hence the possible change in behaviour there. But why the link with pre-ignition?

I'm running an experiment now, where I've gapped my MM at 0.050" instead of the stock value. I know, not a large change, but I have my reasons and I'm looking for one particular side effect. Anyways, one thing I noticed yesterday was an occasional audible pinging in 3rd w/ TCC lockup (stock pattern) under very light load. (why very light load? Do you folks have a clue how much gas is costing up here right now?). So that got me thinking...

In any case, it all coincided with a fresh (?) tank of 94 octane, so there are too many variables in there for now. I don't recall any pinging from the previous night's drive, so perhaps it's just a weak batch. Damned Petro Canada...

[BillyGman]

HMMMM, that's interesting Steve, but I guess you would have to rule out the other variables like you said. I mean the spec is .052" to .056" and you merely went from that to .050"??? That's only a matter of several thousands at the very most, so I'd have a tough time believing that minor change in plug gap can be the cause of pinging. If that were the case then I'd think that my car running w/a gap of .032" would need a much greater timing retard then it's recieved, otherwise it would be pinging all over the place.

[TripleTransAm]

In your case, as you mentioned, you need that gap reduction because of your increased cylinder pressures. So we can't compare your gapping requirements to a stock N/A application.

I suppose when I resume my daily commute from hell tomorrow, I'll have a better idea of how it's behaving overall.

BTW, don't knock small gap changes... I was really surprised at the results last time I fooled around with this little detail! :up: Wondering what effect it will have on this motor in the end, though... never fooled around with a Ford before.

[sailsmen]

The closer an objec t is to a heat source the hotter it gets. The difference in electrode gaps my not be enough to have an effect on retained heat.

[MENINBLK]

I suppose it all hinges on the fact that a spark across two points is probably not instantaneous. So a spark has to travel a longer time across a wider gap than a smaller one, hence the possible change in behaviour there. But why the link with pre-ignition?

I'm running an experiment now, where I've gapped my MM at 0.050" instead of the stock value. I know, not a large change, but I have my reasons and I'm looking for one particular side effect. Anyways, one thing I noticed yesterday was an occasional audible pinging in 3rd w/ TCC lockup (stock pattern) under very light load. (why very light load? Do you folks have a clue how much gas is costing up here right now?). So that got me thinking...

In any case, it all coincided with a fresh (?) tank of 94 octane, so there are too many variables in there for now. I don't recall any pinging from the previous night's drive, so perhaps it's just a weak batch. Damned Petro Canada...

Before you blame the Marauder I would go try some fuel from a different source.
Your gas station could be putting a lower octane in the high octane tank.
It increases his profit margin by a factor of $$$ and not most people would notice the difference,
unless they drive a car with a high compression engine like ours...

[MitchB]

The timing/gap relationship is based on the time it takes the coil to build sufficient voltage to jump a given gap. This used to be significant years ago, but with the high powered ignition systems in cars today, this is no longer true. Still, if you have gaps that vary widely, your spark timing will vary to some degree. Remember, at 6000 RPM, it takes 0.005 seconds for a piston to travel the full length of the stroke from TDC to BDC. The slightly greater time it takes for a coil to saturate with larger gaps becomes increasingly more significant as engine speeds increase.

Mitch

[TripleTransAm]

Before you blame the Marauder I would go try some fuel from a different source.
Your gas station could be putting a lower octane in the high octane tank.
It increases his profit margin by a factor of $$$ and not most people would notice the difference,
unless they drive a car with a high compression engine like ours...

I totally agree, except for the fact that this is the gas station that I use for about 95% of my fillups, especially since my previous favorite dropped their best offering from 92 octane to 91 octane (and I did begin to hearing pinging on that gas, hence my switch in fuel preferences). The 94 octane from that pump has never given me cause for concern, but as an engineer I totally accept the fact I'll need to spread my focus over a greater number of fillups.

Could have been a bad batch, could have been simply old gas due to the fact that 94 octane has become so bloody expensive up here during the last 7-10 days and no one's been buying it... but nonetheless that shouldn't stop me from examining all possibilities as the experiment progresses. We'll see how things go now that the prices of fuel have dropped a few micro-cents... ;)

[SergntMac]

Before you blame the Marauder I would go try some fuel from a different source. Your gas station could be putting a lower octane in the high octane tank. It increases his profit margin by a factor of $$$ and not most people would notice the difference, unless they drive a car with a high compression engine like ours...
At the next main intersection south of my home, I have "dueling" Citgo dealers. Dealer #1 has 20 pumps, diesel, free air, lottery, milk, pop, candy, chips and coffee, and one keyed unheated john. Dealer #2 has 4 pumps, but he is also a full conveinence mart offering hot food, delli sandwiches, designer coffees, liquor, wine and beer, rest rooms, and a whole lot more stuff than Dealer #1.

Dealer #2 is always 1 penny under dealer #1, so, most of us from the neighborhood tend to fill up where our time is better spent. One penny per gallon isn't significant, but if you have some shopping to do too, well...And I'm guilty of this myself. I often pick which dealer by what other needs I can attend to while stopped. Anything I need under a buck, is almost free by what I save in pump prices. It's not something the average busy consumer thinks about.

I mention this because dealer #1 has 20,000 gallon tanks underground, it's a big filling station that fills a few needs on the side. Dealer #2 has 5,000 gallon tanks, it's a "general store" that sells gasoline on the side. BTW, the price difference between 93 and 91 octane at both stations, is 9.99 cents per gallon. Identical.

All either dealer has to do, is flip the color coded tank caps (small manhole covers in red, green and blue) and slip the delivery man 1K in cash to be in a real hurry today, and try to be not so familiar with where the tanks are located today. The rest is undetected profit, unless you drive a car like mine.

My math is crude, but Dealer #1 could be 19K ahead, and Dealer #2 4K ahead, should such an (cough) accidental delivery occur.

C'mon now...If YOU were one of THEM, would this be tempting?

/Steve, I think the only way you could conduct a fair test here, is to include a second set of spark plugs with opposing gaps, and test both per tank of gas. Averaging the results over 3, maybe 4 fill-ups from the same pump should tell you what you want to know. A PITA, yes, but the only way to side step, or, rule out gasoline quality issues. Keep us posted?

[TripleTransAm]

/Steve, I think the only way you could conduct a fair test here, is to include a second set of spark plugs with opposing gaps, and test both per tank of gas. Averaging the results over 3, maybe 4 fill-ups from the same pump should tell you what you want to know. A PITA, yes, but the only way to side step, or, rule out gasoline quality issues. Keep us posted?

Agreed. Unfortunately, time is not on my side so the only approach I can take is keep a close eye on the pinging situation, in that one particular scenario (seems to be after a long highway haul, followed by stop and go, and only once the TCC locks up 100% (ie. shift to 3rd, possibly 3-4 upshift as well) and only at the very low RPMs awarded by a factory calibration in those instances.

If the pattern continues (including filling across several known quality fuel sources), I might consider a regap to the stock range and then see if the pinging disappears. In the meantime, I was wondering if I was overlooking some everybody-should-know-this kind of relation between plug gaps and pinging, to reduce me worrying that I was just more sensitive to the engine's operation because of my change, and that the pinging was always there.

This 'experiment' wasn't the sort of thing I was prepared to do, but I found myself with a 1/2 hour of spare time on Saturday late afternoon before having to get ready for a dinner rendezvous with some visiting friends from out of town, hardly enough time to put a dent on my to-do list. So before I could stop myself, I was digging into the plugs for a quick look at the driver's side (mostly to check for cylinder overheating) and decided to go the extra distance with a regap.

But if anyone's interested, I'll feedback whatever I experience over the next few weeks, including if my original idea worked out.

[1 BAD 03 MM]

After reading through Sean Hyland's Book (Multiple times). I went out got some copper core plugs, gapped em at 0.038", haven't had any problems in 3000 miles. My car is not supercharged, and the plugs cost 1.69 ea. (Autolite 103's - Full thread design)

[CRUZTAKER]

I'm not gonna get involved because I have no clue about any of these tests.

But...as long as we are on the subject of gap...what was the recommended gap for those of us running cooler plugs?

I will be picking up some Motorcraft AWSFA12C's tomorrow.

[BillyGman]

Hmmm....I didn't think that the gap is dictated by what heat range the plug in question is. What I mean is that I thought that the need for gap is independent of that of the heat range. Anyone please feel free to correct me on this IF you happen to be positive of what you're talking about. :)

[drobin]

I'm not gonna get involved because I have no clue about any of these tests.

But...as long as we are on the subject of gap...what was the recommended gap for those of us running cooler plugs?

I will be picking up some Motorcraft AWSFA12C's tomorrow.

Barry, proper gap is .054

drobin
"Donald"

[BillyGman]

Barry, proper gap is .054

drobin
"Donald"
Yep, I believe you're correct. Like I said, I don't see any reason why it should be any different than that of the factory stock plugs(.052"-.056") just because the heat range has been changed.