78 911SC Upgrade Options (Advice needed)

[cmovic]

Hi all: I’m doing a top-end rebuild of a stock 3.0 1978 911 engine. This engine lives in a PCA D-stock club racer that is used for DE / track days but is no longer raced. I did a full stock rebuild in 2004 (back when I was racing) and have since put about 15k (track) miles on the car. The engine runs strong but I made the mistake of installing ARP head studs, so after 18 years of retorquing my heads, I’m ripping them out and replacing them with Supertec, Raceware or (probably can’t afford) 993 all-thread.

The point of this post is: While I’m in there, I’m considering some upgrades. I want to keep CIS, though if I have to spend another $2k on unobtanium CIS parts again I might switch to carbs. I also want to keep the option of returning the engine to stock if I decide to sell the car. Another requirement is it needs to run on 91 octane pump gas. Finally, given the stock bottom end I mostly keep revs under 6k (and have stock rev limiter at 6.7k). The exhaust is 1 5/8" European racing headers into a very free flowing exhaust (spiral max with a crossover).

Here’s what I’m thinking in three parts:
P&C:

- Option 1: 3.2 SS. I’ve read many of the 3.2SS posts and haven’t found one specific to CIS. Based on these it seems like the Mahle PS98-015 10:1 P&C can be lowered to 9.5:1 with a 0.5mm base gasket.
- Option 2: 3.0 Euro. Same idea as above, set static CR at 9.5:1 w/0.25 base gasket.
- Option 3: 3.0 with 9.5:1 JE pistons & replated existing cyls with stock cams. Simplest and safest option. Will probably end up below 9.5:1 (prior build measured out at 8.4:1 probably due to valve job).

Cams:
From what I’ve read, it seems like 964 cams are the way to go and will reduce the dynamic compression ratio (important to the first two P&C options above). The M1 cam looks interesting but I think I need to learn a lot more about camshaft options to make this decision.

Carbs – This is the wildcard. I planned to stick with CIS but I’m now getting a little concerned about parts availability and cost for a 45 year old induction system. Is CIS dead or can I build a reliable engine with a combination of the P&C and cam options above? I’ve had great luck so far until this year when my Auxiliary Air Valve went out and later a backfire popped the whole pop-off valve out of the airbox. My “budget” for the build can handle the first two items but adding carbs will be a stretch.

Thoughts? Or pointers to other threads where this is covered?

Thank you,
-Vic

['78 SC]

One data point:

I upgraded my stock 1978 911SC with 9.3 CR JE pistons, cams reground to 964 spec and SSI exhaust with a 2-in-1-out Dansk muffler. At the time (2004) this was a popular combination and cheaper than a 3.2SS (I used the original cylinders and stock CIS).

In terms of performance, the car was noticeably quicker. More objectively:


Last year I returned the car to all stock.

[Dpmulvan]

Originally Posted by cmovic

Hi all: I’m doing a top-end rebuild of a stock 3.0 1978 911 engine. This engine lives in a PCA D-stock club racer that is used for DE / track days but is no longer raced. I did a full stock rebuild in 2004 (back when I was racing) and have since put about 15k (track) miles on the car. The engine runs strong but I made the mistake of installing ARP head studs, so after 18 years of retorquing my heads, I’m ripping them out and replacing them with Supertec, Raceware or (probably can’t afford) 993 all-thread.

The point of this post is: While I’m in there, I’m considering some upgrades. I want to keep CIS, though if I have to spend another $2k on unobtanium CIS parts again I might switch to carbs. I also want to keep the option of returning the engine to stock if I decide to sell the car. Another requirement is it needs to run on 91 octane pump gas. Finally, given the stock bottom end I mostly keep revs under 6k (and have stock rev limiter at 6.7k). The exhaust is 1 5/8" European racing headers into a very free flowing exhaust (spiral max with a crossover).

Here’s what I’m thinking in three parts:
P&C:

- Option 1: 3.2 SS. I’ve read many of the 3.2SS posts and haven’t found one specific to CIS. Based on these it seems like the Mahle PS98-015 10:1 P&C can be lowered to 9.5:1 with a 0.5mm base gasket.
- Option 2: 3.0 Euro. Same idea as above, set static CR at 9.5:1 w/0.25 base gasket.
- Option 3: 3.0 with 9.5:1 JE pistons & replated existing cyls with stock cams. Simplest and safest option. Will probably end up below 9.5:1 (prior build measured out at 8.4:1 probably due to valve job).

Cams:
From what I’ve read, it seems like 964 cams are the way to go and will reduce the dynamic compression ratio (important to the first two P&C options above). The M1 cam looks interesting but I think I need to learn a lot more about camshaft options to make this decision.

Carbs – This is the wildcard. I planned to stick with CIS but I’m now getting a little concerned about parts availability and cost for a 45 year old induction system. Is CIS dead or can I build a reliable engine with a combination of the P&C and cam options above? I’ve had great luck so far until this year when my Auxiliary Air Valve went out and later a backfire popped the whole pop-off valve out of the airbox. My “budget” for the build can handle the first two items but adding carbs will be a stretch.

Thoughts? Or pointers to other threads where this is covered?

Thank you,
-Vic

Sticking with CIS or Switching to carbs needs to be decided before making cam and piston choices if you want to get the most out of your investment.

[JackkTheMackk]

- Option 1: 3.2 SS. I’ve read many of the 3.2SS posts and haven’t found one specific to CIS. Based on these it seems like the Mahle PS98-015 10:1 P&C can be lowered to 9.5:1 with a 0.5mm base gasket.
- Option 2: 3.0 Euro. Same idea as above, set static CR at 9.5:1 w/0.25 base gasket.
- Option 3: 3.0 with 9.5:1 JE pistons & replated existing cyls with stock cams. Simplest and safest option. Will probably end up below 9.5:1 (prior build measured out at 8.4:1 probably due to valve job).

Not the way to do it. You want use whatever base gaskets thickness necessary to get a 1-1.2mm deck height. Too much deck height from too thick of base gaskets will make your engine more prone to detonation.

[Racerbvd]

Contact Fred, he knows these engines.

My new piston 3.0+

[cmovic]

All: Thanks for the replies so far. The dyno numbers look encouraging.

After writing this (I arrived at the carb question after all my P&C / cams thinking), I came to the same conclusion. I've been searching for updated carb vs CIS info but finding little. Probably time to search a little more and ask that question.

Jack - thanks for the deck height tip. Back to the books.

-Vic

[cmovic]

Originally Posted by Racerbvd

Contact Fred, he knows these engines.

My new piston 3.0+

Wow, that thread took me to some new and interesting places. Thanks for the pointer, I'll reach out to Fred.

[HaroldMHedge]

ROW 911SC pistons are know to have landing failures. Ask me how I know.

Anyone driving an SC 930-10 row?

[Racerbvd]

Originally Posted by cmovic

Wow, that thread took me to some new and interesting places. Thanks for the pointer, I'll reach out to Fred.

Glad to help.