Did I get a special SC motor from the factory?

munsonbw · 10-21-2007, 09:54 AM

I have heard of some deviations in the accepted cut off dates for changes to the SC motor, but mine seems to be a little of everything. I am not sure if I got the best of everything, or basically a 78-79 motor.

Here's the deal: I have a 1981 SC Euro that had a couple of broken head studs. The car had 58K on it when I bought it and the Carfax indicated a consistent mileage history. When tearing into the motor, I would say the mileage looks correct based on wear. Is there anyway to know if this is really the motor that came in the car? The serial # is a 631 2xxx indicating it is a 1981 930/10 motor. However, the cams are pre-79 style with the big nut, heads have 39/35 diameter ports and the piston rings are the pre-79 style 4.0mm wide. So, the first question that comes to mind is do I have 9.8:1 pistons or not? The have 951 70 cast into them and are Mahles with Nikasil cylinders. It would stand to reason that if the pistons have the earlier ring style they are then the lower 8.5:1, or are the heads different changing the compression ratio? The big question is why does my 930/10 motor have all the parts typically found in the earlier motors? Is it possible I got the best of everything... or did I get the left overs?

Nothing seems sketchy about the engine itself, but I am very interested in finding out what is going on. Any ideas?

Thanks,
Ben

YTNUKLR · YTNUKLR's Garage

the serial number indicates '81, like you said. sometimes even '82s had the nut style cam end, so that's not too weird. since your car is Euro/ROW that is why you have the big ports on the heads. As far as 4.0mm oil ring, didn't all SC's have that? I would expect you have the 9.8:1 pistons, but maybe not if the motor was rebuilt at some point..

munsonbw · 10-21-2007, 01:56 PM

According to Wayne's rebuild book and 911 performance handbook, '81 RoW cars were supposed to have 34/35mm ports. The pelican catalog shows 78-79 have 4mm compression rings and 80-83 have 3.5mm. It is certainly possible that it was rebuilt... not sure how I would know this. Even if it were rebuilt, it would be odd that everything but the case was from an early SC motor. But again, I suppose anything is possible.

Oh well, I am not going to swap pistons out for the 9.8:1 even if I did know they weren't. I'll just get it back together and do a compression check and see what kind of psi it is making.

munsonbw · 10-21-2007, 06:03 PM

Well, I went back and double checked my pistons and found I read the number wrong. I have 95L70 Mahles, which are the correct 9.8:1 CR for an '81 Euro SC. It appears that I did get a special engine... I have 39mm runners, 39/35mm heads, and 9.8:1 CR Mahle Nikasil cylinders. The heads are the funny thing... 930/10 heads should have been 34/35 for 1981. Lucky me!

Jim Williams · 10-21-2007, 06:16 PM

I don't know what Wayne's book says but the 39/35 heads were the stock issue for ROW SCs all the way though '83. Bruce Anderson's book also says the smaller port heads were stock for ROW SCs, but that's not what the PET says if you do the research. And the heads match the runner diameters, as they should.

So, it's not really a special engine.......

munsonbw · 10-21-2007, 06:23 PM

I always thought it was odd that they would have large port runners and small port heads, but two different sources had the exact same information. What is the PET? Can't say I have heard of it, so I certainly wouldn't have known to to research it, nor would I have thought to question a couple of pretty respected Porsche authors.

Oh well, it is still a nice surprise... although knowing won't make it any faster

I am also backdating the exhaust, wonder if it will have any more (or less) significant impact on the large port motors than the small port.

Jim Williams · 10-21-2007, 07:06 PM

Ben,

The PET is the electronic version of the Porsche Parts catalog, available on CD from several sources. It used to be sort of contraband, but available widely across the internet. I think lately it is available for the Porsche dealers.

Google on "Porsche PET", you'll find several sources.

The PET has two sections - US and ROW. You would choose the ROW section and start looking up part numbers for the intake runners and for the heads for the ROW cars for the '78 all the way through the '83 911s, and comparing the part numbers.

I spoke with Bruce Anderson about this a couple of years back. I don't know if he did any further research or not, I had hoped he would and include whatever he found in his new book, which I don't know whether is out or not.

This was my finding, and I have told several people about it, and no one has come back yet to say I misinterpreted anything........ But, I could be wrong.

HarryD · 10-21-2007, 08:42 PM

Quote:

Originally Posted by Jim Williams

Ben,

The PET is the electronic version of the Porsche Parts catalog, available on CD from several sources. It used to be sort of contraband, but available widely across the internet. I think lately it is available for the Porsche dealers.

Google on "Porsche PET", you'll find several sources.

The PET has two sections - US and ROW. You would choose the ROW section and start looking up part numbers for the intake runners and for the heads for the ROW cars for the '78 all the way through the '83 911s, and comparing the part numbers.

Jim,

FWIW, Porsche has make PET available on the internet. Just Click this link: PET @ PORSCHE.com .

About the only downside is that when you click on the parts diagram, it does not highlight the part in the list.