Twin PLug Heads and Air Injection

Tom Oxner · 12-07-2007, 08:07 AM

I am thinking about twin plugging my 85 930. A local Porsche tech mentioned that he had noticed cracking in twin plug heads that had the air injection ports in the exhaust side-even when the holes were plugged. Has anyone else noticed this problem?

While examining my engine during disassembly, I noticed that there is certainly not much room on the exhaust side to insert the second plug-It looks really close.

Thanks for your help.

Tom

sand_man · 09:05 AM

I have never heard this, before. FWIW, my '87 is twin plug and was equipped with the air injection (now removed and plugged). When I was shopping around for a machine shop to handle my parts, not one of them mentioned this issue, either - I dealt with some pretty well respected shops, too.

As for the second set of spark plugs, your lower valve covers are machined (holes drilled) for the plugs to be inserted into their respective places. The only plug that I have accessibility problems with is my lower number three, as the valve cover hole isn't very large (looks like a half moon) and the turbo scavenge oil pump butts up against the cover. No biggie, I just remove the valve cover if I never need to get that one.

I can post some pictures of the twin plug process if need be...

sand_man · 12-07-2007, 08:52 AM

Pictures of my heads being drilled and such:

http://forums.rennlist.com/rennforums/showthread.php?t=255567

sand_man · 12-07-2007, 09:00 AM

Look at this picture. If you look at where the air injection hole is, I don't see how either spark plug would have an impact....of course I'm not paid to think!

les_garten · 12-07-2007, 09:05 AM

Quote:

Originally Posted by sand_man

Look at this picture. If you look at where the air injection hole is, I don't see how either spark plug would have an impact....of course I'm not paid to think!

Sand_Man,
What Plugs do you run?

Les

sand_man · 12-07-2007, 09:20 AM

Running Bosch W4CS, which I think is a "silver" plug. It's a tad hotter than the factory W3DPO (and cheaper). I ordered them from Pelican.

911boost · 911boost's Garage

Sandman, it looked like a total hack or monkey, had hole sawed my valve covers for the second plug, so I got a set of the IA Billet ones. I am having a little leaking issue with them but not bad. I might look for a set of stock lower valve covers and then drill them myself....

I also run the same plug.

Bill

Tom Oxner · 12-07-2007, 11:30 AM

You guys are great! Thanks for the great reply Sand_man. The machine work on your engine is very impressive. I agree with your assessment that the location of the air injection holes would not cause any problems. Is the second plug a "small" plug or is it the same size as the other plug?

Once again, thanks for the help and information.

Tom

sand_man · 12-07-2007, 11:51 AM

You are most welcome. The second plug on mine is the same thread diameter as the ones on top. 14mm comes to mind, but I'm not certain - whatever the stock plug thread is. I do think some guys have run 12mm, but again, I'm not 100% certain I've got my diameters correct.

les_garten · 12-07-2007, 12:35 PM

Quote:

Originally Posted by sand_man

You are most welcome. The second plug on mine is the same thread diameter as the ones on top. 14mm comes to mind, but I'm not certain - whatever the stock plug thread is. I do think some guys have run 12mm, but again, I'm not 100% certain I've got my diameters correct.

Both Plugs in mine are 14 mm also. Sand_Man are you running the same plug top and bottom?

Les

sand_man · 12-07-2007, 01:22 PM

Quote:

Originally Posted by les_garten

Both Plugs in mine are 14 mm also. Sand_Man are you running the same plug top and bottom?

Les

Yes, same in top and bottom on mine. I have heard of differing them, though. I think user 125Shifter was running one model plug on the top and a different one on the bottom...

911boost · 911boost's Garage

Both same plugs here as well.

spuggy · 12-07-2007, 04:03 PM

Quote:

Originally Posted by BSiple

SI might look for a set of stock lower valve covers and then drill them myself....

I'm told 964 lower covers are a bolt-on, except that the actual stud holes are 2mm(? - think they're M6 instead of M8) narrower. Of course, they already have the lower plug holes.

Now that's gotta be easier with home tools....

I've heard some people say they prefer the smaller plugs in the bottom - some say it leaves more metal in the head, for better heat dissipation. I've also heard people make dismissive noises at this.

Pick your expert and follow their advice? Both ways seem to work though...

David · David's Garage

I went with the 12mm lower plug. The smaller plug requires less material removed to unshroud the plug tip so compression is a little higher. I hate to admit, but probably the big underlying reason was that I did the machining myself so if any mistakes were made, I could go up to the 14mm plug. I haven't had any problems with the smaller plug and they seem to be burning the same color as the upper plug.

Porschephd · 12-08-2007, 08:53 AM

Quote:

Originally Posted by BSiple

Sandman, it looked like a total hack or monkey, had hole sawed my valve covers for the second plug, so I got a set of the IA Billet ones. I am having a little leaking issue with them but not bad. I might look for a set of stock lower valve covers and then drill them myself....

I also run the same plug.

Bill

If you are having leaking issues something is not right. They should never leak. Drop me a call or email and let me see if I can help.

930gt-40r · 930gt-40r's Garage

I agree about the smaller lower plug (12mm) I just witnessed my friend's heart break (he knows who he is) as 3 of his twin plugged heads were cracked between the thread tap to the exhaust valve seat (this really sucks) However he had 3.2 heads which have a more porous alloy than the actual 930 heads do- so I dont know- I would go 12mm for safety.

Tom Oxner · 12-08-2007, 11:10 PM

I checked out the difference between the ports on the 3.2 intake manifold and the 930 heads-pretty big difference. I have a set of 79 911SC heads setting in the shop (from another partly finished project). The ports in the 911SC heads are almost a match for the 3.2 intake. I was thinking of using them instead of porting the 930 heads to match the 3.2 intake. If the 3.2 heads are made of more porous aluminum, then I would assume that the 911SC heads are also made of the same material. Opinions on using SC heads on a 930? Thanks for your help.

Tom

930gt-40r · 930gt-40r's Garage

Im just going by what the men from Verdan tool tell me- but these guys have been into these things for a while (I think 40 years) and they said that the 930 heads were made of a stronger alloy than the 3.2s.

Porschephd · 12-09-2007, 08:16 AM

930gt you are 100% correct. The 930 was made of a alloy that was designed to cope with the heat a little better. That doesn't mean that they will not work, but they do not hold up to long use. Meaning don't expect to get 100K out of those heads on a built turbo motor.

Additionally the 930 heads take some profiling which is not a big deal and you have to open up the 930 gasket on the base of the head/spacer because the 3.2 gasket doesn't match worth a snot and gives only a few mm sealing on the outer edge. You can also tig a little edge on the port and use the 3.2 gasket if you wish

911boost · 911boost's Garage

Thanks Stephen, I will reach out to you. I love the look of the lower valve covers, but the drips are driving me crazy....

Bill