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Iridium Spark Plugs
Old 08-03-2008, 09:38 AM   #1
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Iridium Spark Plugs

Anybody familiar with Iridium spark plugs? How important is it to use them if the owners manual specifies them? What are the benefits?
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Old 08-03-2008, 09:54 AM   #2
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They cost a lot more, they may last longer, and they require less voltage to produce a spark.

But every spark plug just does one thing...make a spark. Some are more resistant to high operating temperatures.

The rub may be that in some engine designs that call for iridium plugs, the head design may be unique to the point of only accepting a specially designed plug and those may only be made in iridium. I'm aware of several cars that actually have four unique plugs labeled a-b-c-d and those have to go in the matching cylinders.

But in general...gimmick.

If your warranty calls for them, you probably oughta get them and pay the $20-something a pop. Cheap insurance against being denied a warranty claim.
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Old 08-03-2008, 11:36 AM   #3
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Originally Posted by cute fuzzy bunny View Post
and they require less voltage to produce a spark.
Got me on that one.
I thought the voltage required to generate spark was dependent on the spark gap, fuel mixture, cylinder pressure, somewhat on the shape of the electrodes. The material I think had to do with the rate of erosion. IIRC the voltage for automotive ignitions is in the range 10 to 18 Kilovolts. A few hundred volt variation from plug to plug is common. Mostly due to uneven mixtures.

What would be the magic about iridium besides the price and longevity?
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Old 08-03-2008, 11:44 AM   #4
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... they require less voltage to produce a spark.
What law of physics comes into play to achieve that feat?

The spark jumps an air gap - the air gap is what determines the voltage requirement.

Sure, lower resistance in the materials would reduce the voltage lost before reaching that gap, but compared to the run of wire, and the built in resistor that is in most plugs, the resistance of the tip material is going to be miniscule, probably micro or nano-scule (if that's a word).

At any rate, copper conduct more than 2X better than Iridium, so not even that would explain it. Iridium may last longer, and if the manual specs it, I'd go with it. But I wouldn't expect it to affect the voltage required to produce a spark.

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Old 08-03-2008, 12:29 PM   #5
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anecdotal and way second hand, but: a year or so ago on another site, probably BMW related, a mechanic who was well respected was singing the praises of iridium plugs and claiming fuel economy and ease of starting advantages on customer's cars he installed them on - superior to other plugs. had me tempted, but i'm really cheap, and as long as the plugs are making fire i tend to just keep driving.
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Old 08-03-2008, 01:38 PM   #6
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What law of physics comes into play to achieve that feat?

The spark jumps an air gap - the air gap is what determines the voltage requirement.

Sure, lower resistance in the materials would reduce the voltage lost before reaching that gap, but compared to the run of wire, and the built in resistor that is in most plugs, the resistance of the tip material is going to be miniscule, probably micro or nano-scule (if that's a word).

At any rate, copper conduct more than 2X better than Iridium, so not even that would explain it. Iridium may last longer, and if the manual specs it, I'd go with it. But I wouldn't expect it to affect the voltage required to produce a spark.

-ERD50
This article claims that the smaller central tip of an iridium plug does in fact mean that less voltage is required to produce a spark. I don't know enough about it to tell if it's true.

Spark plug comparison - iridium vs platinum
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Old 08-03-2008, 02:25 PM   #7
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From the article:
"Reasoning that it was an engine-management or ignition-system problem, he replaced both."

"Kenny had correctly diagnosed the problem, but as far as he knew, there was no solution. He was already using the most powerful engine management and ignition systems on the planet, "
Edit Add: In the trade this called "shotgunning" the problem, hoping, praying, maybe....
Ah, what brilliant analysis, splendid troubleshooting skills. This guy is pro!!!!. The worst tech I ever had could do better in his sleep.

Nowhere do they show dyno curves. No voltage graph. It is nothing but a very thinly veiled hype for iridium plugs.

The article is not worth the paper it was written on.
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Old 08-03-2008, 03:40 PM   #8
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From the article:
"Reasoning that it was an engine-management or ignition-system problem, he replaced both."

"Kenny had correctly diagnosed the problem, but as far as he knew, there was no solution. He was already using the most powerful engine management and ignition systems on the planet, "
Edit Add: In the trade this called "shotgunning" the problem, hoping, praying, maybe....
Ah, what brilliant analysis, splendid troubleshooting skills. This guy is pro!!!!. The worst tech I ever had could do better in his sleep.

Nowhere do they show dyno curves. No voltage graph. It is nothing but a very thinly veiled hype for iridium plugs.

The article is not worth the paper it was written on.
"Easter egging" (as we used to call it in my Navy days) is indeed a poor means of troubleshooting. But that doesn't necessarily mean that they are wrong about everything in the article. Here is a 1999 SAE paper produced by two Denso engineers, stating that a smaller central electrode requires less voltage to generate a spark.

http://www.globaldenso.com/PLUG/topi...Manuscript.pdf

Due to their employer, they may have a vested interest and therefore you can take their paper with a grain of salt, although, on the other hand, I suppose the engineers who know the most about spark plugs are the ones who work for the plug manufacturers. It would be interesting to see an independent test on actual plugs, but I haven't yet found one.

P.S. -- I have no dog in this fight. I don't use iridium plugs and probably never will. It is just an interesting question.
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Old 08-03-2008, 06:07 PM   #9
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Neither do I have a dog in this. The reduced voltage requirement is not the result of iridium. It is the result of smaller diameter center electrode. There are some platinum plugs with center electrode that is hair thin.

In any case I suspect the electrodes are coated with, not made out of iridium. I believe there was some method called "sputtering" used to apply micron thickness of precious metals over other metals.

You could cut 1/8" off the ground electrode's tip, thereby exposing more of the spark to the mixture, same result, far less $$.
An ancient racer's trick it is.

I use neither platinum, nor any of the trick plugs. I rarely go to full power on any of my wheels. On a suburban with a 7.4 liter engine I do have the computer re-mapped and a few other tricks, but still use OEM GM plugs. Can burn rubber, but that is a function of low end torque, not high rpm HP.

I just get maximally annoyed at "product placement" articles, masquerading as technical info. With a smidgen of truth.
In reality the mechanic/builder mentioned is likely very good at what he does, but lent his name for a few drachmas.
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