9:8:1 mahle cylinders

[1987 Porsche]

I have a 78 911 that I am doing a top end rebuild on due to a broken head stud. I currently have alusil pistons and cylinders and am considering buying a set of mahle p/c. The stock compression ratio on my car is 8:5:1. I am noticing a lot of the mahle sets have a 9:8:1 compression ratio. Is the CR ok to use on my setup or should I stick to the original CR of 8:5:1. I plan on having my cams regrind to 964 specs. Thanks.

Lou

[Flat6pac]

To run the 9.8 to their efficiency you need to find the proper fuel distributor and warm up regulator to match your system
I have the WUR for your 9.8 system.
Bruce

[1987 Porsche]

Bruce,

My goal at this point is to have a good performing street car. Do you think I should stay to the stock compression ratio of 8:5:1 or go ahead and upgrade to the 9:3:1 or 9:8:1 since I am looking to purchase the mahle pistons and cylinders? As of now my stock cylinder heads are getting refurbished and I will be sending John Dougherty my cams for a regrind.

Thanks

[colingreene]

Id add the compression.

[Peter M]

Lou,
Why don't you reuse your existing cylinders by getting them Nikasil'ed and buying a set of new pistons in the compression ratio you want? May save you some $$ and is a well proven approach.

[1987 Porsche]

Pete,

Thanks for the info and I will be looking into it.

[emcon5]

Originally Posted by Flat6pac

To run the 9.8 to their efficiency you need to find the proper fuel distributor and warm up regulator to match your system

Nonsense. My 9.8:1 3.2 Short stroke runs great on the stock CIS, as did rdane's 3.4 CIS basic engine. He got 217 HP to the ground with a stock early SC CIS system, with 9.8:1, 98mm bore and a 3.2 (74.4mm stroke) crank.

An interesting look at CIS

[Flat6pac]

Then why did the factory make specific components for 9.8?
The 77 carrera 3.0 is 200 hp but exactly the same specs as the 78 sc at 180hp.
Where did the 20 horses go?
Bruce

[emcon5]

Originally Posted by Flat6pac

Then why did the factory make specific components for 9.8?

Aren't the ROW Late SCs CIS Basic? The Lamda and non-Lamda cars have different parts.

Originally Posted by Flat6pac

The 77 carrera 3.0 is 200 hp but exactly the same specs as the 78 sc at 180hp.
Where did the 20 horses go?
Bruce

They were not the same specs. For starters, the 930/02 had the lighter rotating assembly of the earlier cars, and the cam timing was different, and the US market cars had a different exhaust (a cat).

What I am not sure of is the Intake port sizes of the ROW SC, Wayne and Bruce Anderson's books show them having the same size intake ports as the US models, but there have been a lot of reports on this forum that the books are incorrect, and the 930/10 has the large port heads found on early SCs. The half point compression in the late SC engines (930/16 @ 9.3:1 vs 930/10 @ 9.8:1) is not enough on it's own to boost power by 24HP (the cam timing is different as well, as well as the lack of smog crap).

The fact remains, there is plenty of headroom in the stock CIS system to handle the extra compression. It has been proven many times by a bunch of different people on this forum, not to mention a bunch of VW guys. There were a lot of cars with CIS, and the water-cooled VW guys are not as averse to making big modifications. My GTI race car put 100HP to the ground with the stock CIS which was made to produce 90HP at the crank. It has been a while, but IIRC the conventional wisdom for the VW guys was like 140 HP before you ran in to fuel problems. That is about 50% more than stock.

[G450X]

Originally Posted by Flat6pac

Then why did the factory make specific components for 9.8?
The 77 carrera 3.0 is 200 hp but exactly the same specs as the 78 sc at 180hp.
Where did the 20 horses go?
Bruce

I don’t think those horses went anywhere. Early U.S. spec SC’s were known to be very quick - even with relatively low compression (8:5:1).

Search for the Car & Driver ‘78 SC road test - that targa really scoots...

[nomore]

Interesting topic as I have a 74 that I will undoubtedly have to address this exact question. Would increasing the CR not require doing more than just the top end, meaning should you at least split the case and verify tolerances on the bottom end first.