"Special" spark plugs--what do they do??

The part that you bend to set the gap is called a side or ground electrode. The spaqrk moves from the center electrode to the ground electrode.

I am not a fan of spark plugs with a ring of ground electrodes because the extra metal surrounding the center electrode may actually impede the spread of the flame when the air-fuel mixture is ignited and I would imagine that if the spark moves along several paths to the ground electrode, the spark might not be as hot as if it were going only to one electrode.

What kind of car was it? I can't see spark plugs increasing idle; spark is spark. Even burning fuel more efficiently shouldn't change the idle.

I had an old Yamaha dirt bike that had oil induction, so you didn't have to mix oil with the gas. It wasn't working properly, so I was adding oil to the gas. But then I found the oiling system was stuck *open* and really fouling the plug.

So I switched to Nippon Denso (or NGK?) U-Groove. The plug lasted a LOT longer, since the ground electrode had a square notch cut into it. It is easier for a spark to jump a sharp edge than a round edge.

This is the same basic idea behind Split-Fire, thos Bosch plugs, etc. You are providing a larger area of sharp edges for the spark to jump.

Since you're getting a 'fatter' spark, the idea is the fuel will burn more completely, thereby getting beeter economy.

Of course, if you START with a cheap-@$$ plug, you still have a cheap-@$$ plug! I get the U-Grooves whenever I can, and used the Bosch dual platinum in a Tercel that called for dual-electrode platinum plugs.

Otherwise, I stay away...

BTW, your sister's car probably needed to have the timing checked after the new plugs were installed. I remember those things. Since I had never heard of the MFGR, I stayed away from them.

Thanks!

I agree that that would be the case IF you had multiple sparks, but you'd never have that. There would only be one path that the spark would take per firing. In other words, you would never have more than one spark per firing. This is because there can only be one path of least resistance at any given time.

If you think about it, why doesn't a regular spark plug have multiple sparks? It's because one point on the ground electrode is closer to the center electrode than any other point, OR if that closest point is covered with insulating crud there is another nearby point which has no or less insulating crud on it.

It's the same way with a ring of ground electrodes. One point on the ring is closer to the center than any other point (due to manufacturing tolerances) and that is where it will fire, until that point gets burned away, worn down, covered with crap or whatever. It's just that there are many points to wear away before the shortest distance between the electrodes is appreciably affected.

And her engine did run faster, but damned if I can figure out why. Only thing I can think is that all engines have a peak efficiency RPM and a car engine is likely to be tuned so that RPM is somewhere in the driving range rather than at idle... so if engines are less efficient than normal at idle, there might be a way to make them more efficient at that particular speed but it might take away some efficiency at higher speeds and reduce overall economy... Just grasping at straws here.

She did get the car used. Maybe someone incorrectly gapped the plugs before she got the car? Would that cause a car to idle more slowly than normal?

Buick Skyhawk, six-cylinder, stick shift. I'm not sure the exact year but probably somewhere around the mid-1970s.

Exactly! That's why it's still a mystery to me after 25 years.

Are you sure? If the idle speed adjustment is set to hold the throttle plate in a certain position, increasing the amount of power on each stroke would pretty much have to boost the speed, wouldn't it? Remember, we're talking way back before most of the electronic stuff that could sense the engine RPM and adjust the speed on its own. But even so, how on earth could a spark plug make it burn fuel more efficiently?

If that was actually what it was doing, I think it has to have something to do with the fact that car engines aren't designed to have peak efficienct anywhere near the idle in the RPM range. Idle burns little fuel and not much time is usually spent idling, so you might expect a little slop in that range. But that doesn't answer how the spark would make a difference.

This is something I didn't know. Interesting.

That would describe the plugs she had very well.

How? I don't doubt that what you're saying is true, but I want to know how. It seems that because the flame front is self-perpetuating once ignition begins, it shouldn't matter how big the spark is so long as it's just hot enough to cause ignition... Maybe a stronger spark ignites a slightly larger area of mix and thereby changes the shape of the flame front?

Good point! I would guess she did as she was always a good gearhead, but I'll have to ask her to be sure.

Yep, that's them. Brand X.

A regular sprak plug does not have multiple sparks because the ground electrode provides the path of least resistance.

If there are multiple ground electrodes and they are all continuous with the threaded body of the plug, then in theory, they they share the path of least resistance and so the spark could spread in multiple directions. If manufacturing tolerances made enough of a difference so that the spark moves to only one ground electrode, then there is no purpose to having all those other electrodes.

If those multi-prong spark plugs were such a great device, all spark plug makers would be marketinng them and all automakers would be using them.

I can only speculate on why the engine idle RPM rose with the miracle spark plugs, but my guess is that the spark plugs they replaced were not doing the job.

Could it also be that the engine had to idle faster to compensate for a poorly running engine or a problem with the engine because of a poor spark?

Jeff

I doubt it. That vintage car was probably carbureted and did not have a feedback system that would compensate for engine problems.

If the spark plugs that were being replace were mis-firing, then idle speed would be lower, and the replacement spark plugs merely brought idle speed up to where it should be.

Exactly! There is no point in having multiple electrodes - except - as the one most likely to fire wears, it becomes less likely to fire and another electrode now becomes more likely to fire. You share the wear over multiple electrodes. No matter what the commercials show, in an engine you don't get multiple sparks per combustion cycle. You get your one spark and that is it. The advantage of multiple electrodes is that you have redundancy - you are limiting the maximum voltage needed to bridge the gap to the least resistant path. Over time, the least resistant path changes. I've see engines that used dual electrode plugs as a means of increasing the life. However, with platinum plugs and unleaded fuel, I hardly see the need for multiple gap plugs. I've run platinum plugs to over 100k miles and they hardly seemed worn when I replaced them.

As for why the idle speed increases with the surface gap plugs, I have three theories:

1) Different heat range - plugs ran hotter which effectively advanced the ignition timing at idle. This might be good for idle speed, but is probably bad for high speed operation. 2) The plugs described by the OP are surface gap plugs. Perhaps the open design allowed for a faster flame front advance, which again effectively advances the ignition timing (a faster advancing flame front is about the same as starting a slower advancing flame front sooner). Same comment as above, good for idle, bad for high speed operation. 3) Bad data.

Surface gap plugs are used from time to time, but are prone to fouling. The location of the spark clearly has an effect on performance - race car builder often go to great lengths to orient the gaps to prevent the ground electrode from shading the flame path (indexing the plugs). This is probably trivial for passenger car engines, but there is an effect there.

Personally I only use OE spark pugs.

Ed

That's a good idea, but I'm pretty sure that car had a carburetor with a throttle-plate adjustment rather than a sensor for setting idle speed.

Are you sure? Since the ground electrode isn't a single geometric point *physically*, there are an infinite number of points on a single ground electrode in theory and in practicality there are many points at which the spark could jump from and to. For example, one spark could start toward the open end of the electrode and jump to the closest point on the center electrode, and another could jump from farther back on the electrode to its closest point to the center electrode at the same time as the other spark. but I don't see how that would happen, because there is only one path of least resistance, and it's not necessarily the closest points of the two contacts though it's almost always very close.

I don't get what you're saying at all. First, there's only one ground electrode though it may be split into the ring of five or six segments. Second, the path of least resistance (POLR) is the path of least resistance, not one of the paths that's close to the least resistance. There are infinite paths but only one of them is the least resistant. Just like there are many points on the so-called "single ground electrode" plug that the spark can take but it only takes one (and it isn't necessarily the point closest to the center electrode, because chemical impurities and deposits on both electrodes combined with the resistive properties of the fuel/air mixture can alter the POLR from the two closest points to two other points near them.

There can't be multiple ground electrodes because electrically speaking, there is only one ground. Ground is in theory one point electrically even though it can be pretty much any size or shape electrically. There is one path from center electrode to ground that is the path of least resistance. There can't be two paths of least resistance.

As I mentioned before, the ring ground electrode is always going to have one spot that's closer to the center electrode than any other, because unless the electrodes are assembled molecule by molecule, the manufacturer won't be able to make a plug that doesn't have one point on each electrode that is closest to the other electrode.

If it is possible for the spark to split, then it could also split with a standard plug. Why would it be any different with a standard plug?

There is in theory but not for practical purposes.

True, unless the patent holder is a moron. But I agree--there's no real advantage to multi-prong electrodes.

ROFL more like trick plugs. CRAP, I wish I'd have thought about this before, I could have experimented on the old Geo Metro. I don't want to mess around with my Corolla. Come to think of it, I think I must know some people who have old junk cars I can experiment on...

That's probably true. How long did average-grade plugs tend to last before electronic ignition and unleaded gas? I don't know what kind of plugs were in the Skyhawk beforehand, but guess they could have been bad enough for that to happen. They didn't look bad IRCC, but the gaps could have been way off or they could have been cheap-junk plugs to start with.

The only other alternative I can see is that there's some trick to the plugs that makes the engine idle faster. Unlikely I admit.

I have a Yamaha Vino 125 Scooter and an Engineering Professional in the Yahoo Vino125 newsgroup has pretty well documented increases in idle speed and mileage when switching from the stock plug to an Iridium plug. I changed to an Iridium Plug and noticed a 200 rpm difference in idle (which I had to adjust down from). My scooter is still within the break-in period of 600 miles so I can't say much about mileage increases since the mileage improves significantly during break-in. The gentleman in the newsgroup who closely documents each change, plugs, windshields, and other items, rides daily and on fairly flat Phoenix AZ roads. He notes a slight increase in fuel economy

1-3 mpg better than several miles before changing. Just some fodder to add to the conversation.

I think we're saying the same thing here.

I think the term "ground electrode" is causing confusion. The "electrode" is the piece of metal sticking out from the threaded ring, and "ground" is the type of electrode, as opposed to center electrode. "Multiple ground electrodes" means that there are multiple electrodes that are grounded, not multiple grounds. Substitute the terms "side eletrodes" for "ground electrodes" to reduce the confusion. The spark plug we're discussing has multiple side electrodes that share a common ground.

As far as there being only one POLR, that is my point - only 1 POLR means only 1 spark so the extra side electrodes are doing nothing.

It is theoretically possible for the spark to split when there are multiple side electrodes if 1 or more of the electrodes provides the same amount of resistance. In reality, as you point out, this is not going to be the case and there will be only one spark from the center electrode to one of the side electrides.

The theory is different with a standard plug because there is only 1 general direction for the spark to travel, and that is towards the single side electrode. Although the spark can theoretically jump to any point on the side electrode, in reality, it tends to jump to the tip of the side electrode and migrate down towards the base of the elctrode. I don't know the reason why the spark migrates, but my guess is that the point on the electrode where the spark touches gets very hot on the surface, increasing its resistance (resistance in metals tends to go up with temperature) and so the spark moves down a little from the tip, heating that spot up, and moves a little more, and so forth for the duration of the spark.

Spark plug technology has advanced quite a bit since the 1970's. Spark plugs used to have a life of 12,000 to 15,000 miles in the days before electronic ignition and unleaded gas, then 30K miles, then 60K and up.

Ed White pointed out another possibility, and that is that the hotter plug had the effect of advancing ignition timing. Experience tells me that those multi-side electrode spark plugs were a gimmick, so the RPM increase was more likely due to the previous plugs being so worn.

Why did my '95 Tercel specify dual-tip Platinum plugs?

I never understood why Toyota started using the dual-tip plugs, except to extend electrode life. As a tip wore or picked up deposits, the other tip would offer less resistance, and as the second one wore, the first one would offer less resistance. I guess having 2 electrodes makes some sense, but having 6 or 8 makes no sense.

Why wouldn't having 6 electrodes make a much sense as having two, for the reason you just stated?

Because no matter how many electrodes, they're eventually going to get carboned up.

I see. And if they aren't worn out but just carboned up, you could just clean them and reinstall them... but as long as you're going to the trouble of removing them, you might as well put in brand new ones, because they're fairly cheap. So it doesn't make sense to have a plug designed to last that long as it's a wear part anyway.

As hachi mentioned, the additional electrodes provide additional surface area for carbon and other deposits to form, perhaps more than would be burned off. If 6 electrodes were such a great idea, they would be in widespread use today, and for whatever reason, they are not.

I believe that the six-prong plugs would last quite a bit longer, but it's a moot point because there's no reason to leave plugs in that long anyway.