You know... this has been on my mind for a while here. Back in the day when I was in the F&F tuner crowd and dealing with basic 4-cylinder econoboxes, the general consensus was that the multi-pronged plugs were just a gimmick and that we were better off with the traditional single prong plugs.
So I was surprised to see these as OEM for these extremely advanced motors.
Then you've got pics showing that, as sort of expected, one of the prongs will be obviously more "used" than the others indicating that the spark is going to jump the easiest gap. Again... as expected.
So what's the deal here? Why do they use these? I mean... I know we use cross-drilled rotors because they're pretty even if they're not the best choice... but I can't bring myself to think that Porsche is using fancier looking spark plugs just because they're fancier looking.

 

Sparks leaping from one surface to the other actually has the effect of rounding the edge (due to localized heating). So multi-prong plugs allow for eventual transition of the spark occurrence from one ‘edge and prong’ pair, to the next better one.
In general, GT3’s just don’t cover enough miles within the two year change interval, for this to happen, but it ‘could’, and so they specify such a plug.

 

If I'm remembering correctly, Trakcar posted photos where all 4 of the prongs were worn down to the nubs. So, maybe once the primary prong is worn the next closest prong begins to become the spark path, etc. So you have 4 times the prong length, in effect.

 

Maybe insurance. You've got 4 ground electrodes on each plug. Maybe one gets used more than the others, but sometimes the others get used instead because for whatever reason, the spark "likes" them more for that ignition cycle. Less chance of a misfire maybe?

EDIT: Here's a paper discussing this actually: https://www.sciencedirect.com/scienc...90447912000883

"This type of spark plugs was introduced by different manufactures primarily to extend spark plug durability. It is clear that there is a higher rate of combustion variability when this spark plug was used. This is explained firstly by increased contact area and heat transfer to the ground electrodes in the initial period of flame kernel growth. Secondly, it is well known that as a spark channel forms between the two spark electrodes, the local resistance increases as the localized temperature grows due to the energy flow during the spark event. After a few sparks, the local resistance becomes high enough to cause the spark to seek paths with less resistance and this moves the spark channel around over a series of sparks to prevent the “arc welding”. The rotating spark in this case changes the spark plug orientation which affects smoothness of engine operation as mentioned earlier. In circumstances where the charge motion carried the kernel into the ground strap, the flame growth would be slowed by the presence of a cold wall. With a different orientation, the same plug could have much different performance."

[So basically, as the contact area heats up, resistance increases, hindering spark formation. So the spark will seek a lower resistance path to ground and it "chooses" a different prong for that ignition cycle. This helps durability and longevity of the plug since higher resistance hinders spark formation and can lead to electrode erosion.]

"In spite of continues investigations to explore the impact of spark plug designs on combustion and engine performance, a complete clear picture is still far from our understanding. In the present study, the impact of spark plug design, represented by number of ground electrodes, on the engine stability was investigated using four commercially available spark plugs. The spark plugs were carefully chosen with the same point central electrode to eliminate any effect from the central electrode."

"Multiple ground electrode designed spark plugs significantly slow down the early flame development, due to the increase in heat loses and the reduction in flame growth due the restricted flow directions."

 

A huge thanks to @SmokinGTS for this thread! Just finished my first plug change on my '15 GT3. It has the replacement G6 motor so it got the plugs ending with 91 that the previous owner was kind enough to include. Like others have said, pretty easy if you have the right tools and go slowly. Loosening the 10mm bolt holding the Bilstein sensor made access to cylinder 3 much easier. Getting the centerlock wheels off and back on was the most tiring part.

All the debris that fell out when removing the shields. That is a big piece of gravel.


Plug boxes marked using the clever positional notation from other posters.


They all seemed to have about the same color and amount of wear.


Started up after the replacement so all set for WSIR this weekend.

 

Hey... I thought we didn't have to remove any of the underbody covers to do this?

 

The undertray stays in place.
The ‘tin’ cover (one each side) that protects the coils, gets removed and (in my case) almost a pound of gravel was lodged behind each one!

 

i know this is a fairly old thread, however just for everyone's FYI, i just contacted BOSCH and they deny that they make a plug for the 2018 991.2 GT3 4.0. they have no listing for such a plug. This coming from their Technical Service Line.

 

Yeah, I think Porsche worked out an exclusive - gotta buy through a dealer network or genuine Porsche parts seller.

 

Just changed spark plugs on my 991.1 GT3 with G6 engine and torqued to 24lbs would that be okay?

 

Sounds a little tight for a 991.1 (17-21 ft-lbs recommended in first post), but you're probably fine. Maybe require a little more effort to remove them.

 

Just checked with the service adviser that I know using my VIN, 24lbs/32nm seems to be correct torque settings even for .1

 

Heat expands, so I'd opt for your recommendation, and stop @ 19.

 

You’ll making me worried haha, so why do we even bother referring to factory specs 😅

 

Has anyone recently replaced their plugs on a 991.1 GT3? I was prepping the parts for my next service and realized the PN has changed to 99917015390. This is also listed for 981 as well. Doesn't make sense the thread on this is different from GT3.

https://www.suncoastparts.com/produc...917015390.html