911 SC twin plug 12mm or 14mm plug

[florent78]

hi guys,


before twin plugging the head and after reading some articles / informations I would like to know if I need to ask for 12mm plug or 14mm plug for the second plug


it seems racing porsche twin plug have both 12mm hole.

I have a 911 SC 3.0 head so the upper plug is already 14mm

what are the advantage / disadvantage if I choose a 12mm or a 14mm plug for the second plug ?

from my point of view, no difference but I can imagine if we discuss about 12mm
there is a good reason


I'm using BR8EIX NGK plug for the moment
if I want to go for similar spec plug in 12mm I cannot get the same technical spec:
14mm plug inflammation point is at 1.5mm
12mm plug inflammation point is at 2.2mm

seems it's better to get a second 14mm plug ?

thanks for your help

[chris_seven]

This debate, like most things to do with 911 engines goes around every year - a bit like the Christmas Fairy

I have never been keen on the idea of using mismatched spark plugs but I do realise many people will disagree.

We machine all of our Twin Plug heads to use 14mm plugs and match them top and bottom. We generally use a 964 plug with a reduced hexagon.

If we had new castings I would possibly use 12mm but even then I am not entirely sure.

In the Seventies I used to run a Lancia Fulvia 1600HF as my daily driver and this had a single 12mm diameter plug.

The reason for the change was the valve sizes were so large that there was no room for the 14mm plug used in the 1300cc version.

The car was very, very good but it was always a nightmare to cold start.

It flooded as soon as you looked at it and the plugs always bridged across.

The only cure was to change plugs as even towing the damned thing didn't make it clear.

I guess this experience has coloured my opinion on small plugs.

The 1300cc version never suffered from this issue.

[prschmn]

K.I.S.S. just do the 14s. I can't think of any reason you'd be better off with 12s

[panzerfaust]

Yes there is an advantage running 12s over 14s. Less crevice area for in burnt HC to hide in for a more complete burn. As mentioned earlier more valve clearance for ported heads big valves without the need for indexing for extreme cases

[florent78]

thanks guys

we will go for 14mm
even if it's a big "hole" 32mm


calling a NGK tech this morning I learned that depending of the plug choice and without correlation with heat code the flash point could be different

for exemple for the BR8EIX plug it's at 1.5mm
for the DCR8EIX plug (12mm plug) it's at 2.2mm

so I agree with chris I prefer to get the same plug specification

thanks guys

[cgarr]

I never had good luck with Iridium plugs, have them check the BPR8ES and DCPR8E Whats your motor running that cold of plug?

[Jeff Alton]

We use 14mm all around as well. Never had an issue. We tend to run BPR6ES or BPR7ES NGK plugs depending on the build.

Cheers

[Mark Henry]

14mm here as well.
You do have to run a different nut than the factory barrel nut, no problem with Supertec or ARP stud sets clearance wise. NCIS
I agree with Craig, VW or Porsche aircooled never have had luck with platinum, Iridium, etc.
Good old school plugs work the best.

[Walt Fricke]

Well, mine are 12mm. They were machined that way to avoid problems with the barrel nut. With those, even with the 12s those particular barrels had to have their tops tapered. Of course, with the use of shouldered nuts (I think everyone should use only these no matter what flavor of 911, or how many plugs) this is less of an issue. Can't say I have ever had a problem matching NGKs, though the 12s only come without the round screw tip (I think it is this way), so you end up scavenging those off of plugs you are replacing.

For a race motor, I like the idea that you have not increased the head volume by as much. It isn't much, but I guess I don't see the downside of having two plug sizes. I always have a bunch of pretty good used ones of all sizes anyway.

Porsche now uses one 12mm plug, doesn't it?

[Steve@Rennsport]

One of the reasons the factory used 12mm plugs was to help preserve mass in the head casting. This practice was used in the 917 engines as well.

Cylinder head cracking was not uncommon under racing conditions so when the 2.8, 3.0 RSR engines were done, the small plugs helped contain that problem. 935's used the same 12mm plug sizes, too.

Its not too practical now for street use due to limited choices of heat range since most 12mm plugs are fairly cold.

[Walt Fricke]

Oh - how common are reduced head 14s?

[Steve@Rennsport]

Originally Posted by Walt Fricke

Oh - how common are reduced head 14s?

Hi Walt,

Can you clarify the term, "reduced head 14's"?

[Walt Fricke]

Whatever Chris Seven meant by this:

"We generally use a 964 plug with a reduced hexagon."

Your plug wrench would be narrower, so more clearance. I have a regular 14mm plug wrench with the OD of the socket's end ground on my grinder to get more room on the one 14mm lower twin plugged motor I have. When I rebuilt it of course I went to flanged nuts.

And not every machine shop puts this second plug in exactly the same place as every other shop. What did Porsche do with the 2.8 and 3.0 RSRs? And when the street 964 and 993s went to twin plug, all 14mm?

[toddu]

If you look at the NGK numbering system, you can get 14mm plugs with 5/16 hex. They work fine (at least on my car) on the bottom.

Todd

[Stevenlinh1]

Originally Posted by florent78

hi guys, before twin plugging the head and after reading some articles / informations I would like to know if I need to ask for 12mm plug or 14mm plug for the second plug it seems racing porsche twin plug have both 12mm hole. I have a 911 SC 3.0 head so the upper plug is already 14mm what are the advantage / disadvantage if I choose a 12mm or a 14mm plug for the second plug ? from my point of view, no difference but I can imagine if we discuss about 12mm there is a good reason I'm using BR8EIX NGK plug for the moment if I want to go for similar spec plug in 12mm I cannot get the same technical spec: 14mm plug inflammation point is at 1.5mm 12mm plug inflammation point is at 2.2mm seems it's better to get a second 14mm plug ? thanks for your help

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[Steve@Rennsport]

Thanks Todd!

3.6 engines use that 14mm plug with the 5/16" hex drive.

The factory positioned the plug(s) in the same place and the same angle in all of the 911-based engines.

[prschmn]

I think that's a 5/8 hex-Bosch f series. Using the late style barrel nuts is OK but you MUST
torque by angle. My understanding is that the new integrated washer was introduced after Porsche
had "efficiency" people in. It would reduce costs by virtue of quicker assembly and lower hardware
cost. correct torque is highly dependent on frictional losses between the washer face and the mating face and twisting a hard steel surface against aluminum can be a recipe for problems. You need the added surface area of a washer due to cast Als low compressive strength. Try reading
ARPs tech pages--best practice is to tighten by stretch but not terribly practical with head studs.