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Spark Plug Indexing


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I would love to see a scientific test on something like this but I think the gains would be so small that they would fall within the margin of error of the dynos. It seems like it might be a waste of time with stock plugs since there are 3 other ground electrodes in the way of the flame anyways. Who knows though. I personally think the gain wouldnt be noticable.

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In years past this was standard procedure in any race type engine, especially high compression motors, re:GT3 (11.8:1). Spark quality and it's relative location was and probably still is important to efficient ignition. If you're just driving to pick up the groceries, the difference is certainly not going to be an issue. But in a racing environment, you take all those little things that don't make much of a difference and add them up, it can make a win or loss difference. It would be interesting to know, of all the folks that finished in GT at Daytona, what type of plugs they were using. It's kind of ironic that folks will spend $6000 for a 10 hp gain but are very quick to discount a very inexpensive possible couple of hp. Maybe I need to go back to school but I have a difficult time understanding the benefit of the multiple electrode over the single. In most cases, if not all, you would only have one spark from the center electrode to the matching ground electrode having the least resistance path, regardless of the number of electrodes. The other ones are only there to get in the way. Color me confused, not the first time and not the last, I'm hoping someone can explain what I might be missing. Please don't think I'm being argumentative, I just like a good, informative discussion. This is how I learn. I'd love to hear everyones comments.

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Next time I am at RennWerks I'll ask what plugs they use.

With multiple electrodes you are still going to get just one spark, to the electrode with the least electrical resistance. But once in a while someone claims you get multiple sparks. But I was a history major....

The benefit of multiple electrodes is that the plug does not have to be changed as often as a single electrode plug. As the electrode with the least resistance wears then the spark jumps to the next electrode with the least resistance.

The 2.5 Boxster had 2 side electrodes, and were to be replaced at 30,000 miles. When the 2.7 and 3.2 came out Porsche switched to 4 electrode plugs, and increased the change interval to 60,000 miles.

Then you get the platinum debate. Platinum last longer than copper, but does not conduct electricity as well as copper. It is said that the platinum plug with or without multiple electrodes is used to get that 100,000 mile plug.



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  • 3 months later...
  It seems like it might be a waste of time with stock plugs since there are 3 other ground electrodes in the way of the flame anyways. Who knows though.

Read this from PCA.org tech article. The next to last paragraph (though confusingly written) provides some insight.


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I like the questions in the article. I have suggested that someone with a 2.5 that has 2 side electrodes might want to try the 4 side electrode type plug used on the 2.7/3.2. As pointed out in the question the problem is the 2 electrode plug sold by Porsche has a heat range 7, while the 4 electrode plug sold by Porsche is a 6. Some time ago I searched the net for a Bosch Super plug with 4 electrodes with a 7 heat range for use on a 2.5 and came up with nothing.

The question also points out a question I have had with the Bosch Platinum +4 plug with a part number 4417. What is the heat range. Companies that sell this plug list the same plug for the 2.5/2.7/3.2, but we know the heat range for a 2.5 is different than a 2.7/3.2. I have not been able to verify the heat range of this plug, other than to assume, like in the question, that the 7 at the end of 4417 is the heat range.

As pointed out by Scott, more electrodes does not necessarily mean it is a better plug, rather it is the design of the plug. I do not follow Scott when he mentions indexing, other than as a generic response. If you have a 4 electrode plug in the middle of the combustion chamber and 4 valves per cylinder, then the most electrodes that can face the intake valves is two.

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That is a handy Bosch chart. I can decode the Boxster oem 2.5 Super plug with the Bosch part number FR 7 LDC4 as an example.

F is the thread size or 14 mm x 1.25 mm.

R is for resistor.

7 is the heat range.

L is the length of the threads.

D is 2 side electrodes.

C is copper.

4 is an extended insulator.

Can't do that with the Platinum +4 plug as the number is simply 4417.

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