Thoughts on 9.3:1 cr on early SC with CIS

[1979911sc]

I have found lots of great info on here regarding 3.0 rebuilds and wanted to see who is specifically using a 78-79 engine (larger intake and heads) with higher compression pistons from 81-83 USA SC (9.3:1)? Mine is a 1979 3.0 USA version w stock 8.5:1 and will be used 90% as street car. The plan I am considering is 9.3:1 or possibly 9.8:1 in favor of the stock 8.5:1 and cis with of course the usual ssi upgrade and either 20/21 or 964 cams (not sure yet). Due to broken head studs I am looking to do a budget minded rebuild and have also recently acquired a nice low mileage set of mahle p/c with 9.3:1 cr for cheap money that I will re-ring or find some serviceable 9.8:1 pistons(depending on price). I do not want to (can't afford to) twin plug or shell out big bucks on carbs or short stroke 3.2 either and the best gas available is currently 92 octane. I am looking to make the best single plug motor cis motor I can within reason. Anybody running this specific combo? If you were me would you try to find some 9.8:1 pistons or use the good used 9.3:1's ? I am hoping 9.3:1 should be a decent setup but any experiences with this combo or thoughts would be appreciated.

[will hung]

I purchased my 1979 SC with zero miles on a rebuild exactly as you describe. Mine is a 1979 ROW Carrera SC, the previous owner was going to rebuild the engine as a result of broken head studs. When he priced brand new piston and cylinder sets, he shifted gears and bought a very low mileage P&C set from a 1982 SC.

He had the rotating assembly balanced, rebuilt the heads, and had the ports polished. It has 964 cams. I broke it in and recently added SSI's with a Dansk 2in/1out muffler.

The car runs incredibly. It pulls hard from any RPM, starts on the first try (cold or hot), runs at a steady 180 degrees, and has no problems with pump gas. At the moment I have no desire for more power. As long as it continues to run as it does now, I have no plans to change anything. If a NLA component of the CIS fails in the future, I'll probably convert to EFI, but as I said, I love this set up. I suggest it wholeheartedly. The increase in cost going to 9.8:1 is not worth the small increment in power you'll get by doing so.

[G450X]

I may go this route as well. Max Moritz 9:8:1 P&C's are silly expensive - I may "backdate" my '82 3.0 a few years (early 3.0 CIS & heads, 20/21 or 964 cams) and use my later 9:3:1 P&C's and call it good for my pending engine build.

I've got a good 7:31 R&P for my 915 that should make up the difference in the small displacement bump. When I've saved the coin, JW in Seattle will hear from me!

Will Hung - have you had this set-up dyno'd?

[Scott R]

Mine's an 80' but this is my configuration. 9:8:2 964 cams SSI's and CIS single plug. My CIS system works like a champ, ignition system is perfect, I love it over stock. The main thing being the cams, they pull a lot longer than the original SC cams did.

[1979911sc]

I will look into the 7:31 r and p and see if it it feasible as I know tranny will be rebuilt to some extent while out. I have seen some dynos on here of late 81-83 sc's with early early intakes but not sure if they had early heads or not. I believe superman's and a couple others but I would be real interested in seeing a dyno of a 78-79 w/large intake and heads and the 9.3:1 compression if anyone has one....

[will hung]

Unfortunately, I've never dyno'd it. I'd love to, just for curiosity's sake. Maybe someday... The only upgrade I'd consider in the near future would be a 7:31 r&p swap, but as I said, the car runs great, so I'm in no hurry to change anything.

[bullethole]

JE Pistons in 9.5:1 are not that expensive, and I know of at least 3 other folks who have put these into the Mahle cylinders with no problems... About 1K for the Pistons..

[Bob Kontak]

When I had my 81C heads reworked by EBS in the 1990's they shaved 0.006" off the face of the head. I did the calculations and it ended up taking the compression from 9.3 to 9.5 to 1.

There is a little compression available without changing pistons.

[1979911sc]

I will probably just use the 9.3:1 mahle CIS's since they are in good shape as opposed to spending $1000+ for 9.5 JE's and for .2 point bump. Now milling down the heads a little sounds more cost effective. Wonder if shaving off any more than .006" is safe for clearance and pump gas?

[snbush67]

Even more cost effective is leaving out the cylinder base gaskets but, I do not think the pay off you are going to get from compression ratio is worth the risk of losing deck height or valve clearance.

['78 SC]

1978SC - SSIs, 964 cams, JE 9.5 and stock CIS



These are 3rd gear pulls.

[Bob Kontak]

Originally Posted by snbush67

Even more cost effective is leaving out the cylinder base gaskets but, I do not think the pay off you are going to get from compression ratio is worth the risk of losing deck height or valve clearance.

Here is an article regarding deck height and a little bit about the clearances.

"Competition Engineering - Easy Deck Height"

[303462]

I would avoid the 9.8-1 ROW 204hp SC pistons. These are documented in Bruce Andersen's Technical notes to have a considerably higher than normal failure rate. I am not an expert but I think 9.8-1 is to high for a cast piston, and these are cast pistons. On a personal note , I have an 83 ROW 911SC that had this piston failure. (You also lose your oil pump from the debris). I bit the bullet and got the Max Moritz 3.2 pistons and cylinders and 964 cams for the rebuild. With a backdated exhaust and sport muffler, the performance is similar to my 2.7 RS spec weber carbed car, but the mileage is much better. (20+mpg). Best of luck with your build.

[1979911sc]

Great info everyone. Steve, Those hp numbers look good and lots of top end up to 7k ish? Wow. I can't imagine much difference in power w/ JE 9.5 opposed to 9.3:1 CIS dome is there? I have some seen some small port #s but wish I could find some torque dyno figures with the 78/79 large port heads as most of my driving is in town. Anyone?

[GaryR]

I believe JE will cut forged pistons to match the profile of stock 9.8:1 euro profile but I may have got that wrong...

[1979911sc]

So is anyone else running the cis w/ 9.3:1 pistons with a 964 or 20/21 cams on early 78-79 3.0 with large intake and heads? Just wondering if there is much of a noticeable improvement in the torque especially at lower rpms over stock..

[LakeCleElum]

Well, close to what you ask, so I'll chime in: Bought a used engine (81 3.0) and had it put in my 73.5. Recent documented rebuild:

Max Moritz 3.2 kit - 9.7 CR I think
364 cams
Updated oil pump
Early exhaust
MSD
CIS

The person I bought it from had it on the dyno with SSI's before his car was crashed and he sold me the engine. 209 RWP. If you run higher compression with CIS, fuel and ignition timing are critical.

JW set the timing at 29 BTDC if I recall. I use 92 octane with no sign of pinging........In a light car with short gears, it's a hoot.....

[xpensivewino]

I built a motor for my SC with a 78 core, large runner injection, and purchased new 9.8:1 P&C's from Mahle, last set that Andial had at the time, as Mahle doesn't make these very often. The price as stated earlier was stupid, but it made a very stout motor. I ran headers, and a Alan Johnson sport muffler. I was very happy with the results. I have used JE pistons in the past with Mahle cylinders, and had good results. JE makes a very nice product for the price in my opinion.

[turborat]

When my '78 Euro 3.0L was rebuilt, I decided on 3.2 cylinders with 9.5 JE pistons. Cams are 964 profile. Running stock CIS with backdated headers. The car pulls pretty hard and is a light track only car. It will pull 3.2L MFI Carrera in a straight line. It runs fine on pump gas.

Prior to rebuild the engine had the 9.3 JE pistons in 3.0L Cylinders. The same 964 profile cams were used with stock CIS. It pulled much better that the stock SCs I ran with on the track. It also ran on pump gas with no issues.

I am now considering carbs/cams for more torque.

[fintstone]

Almost exactly what I run in my '74 911. 7:31 and 3.0..... A few years back I had Henry at Supertec build me a 3.0 with 9.5 JE Pistons/Mahle cyl.,20/21 WebCams, 39mm intake and 38mm exhaust ports, SSIs, and early SS muffler, early CIS w/no smog, oxygen sensor, etc. Basicallly a '78 top end and and 81 bottom end. Very fast and reliable combination in a light car. Certainly quicker than my 3.2 and bulletproof.