2.7 cams and piston advice

[Myazel]

I have am working on a 75 S I purchased from a friend a couple of years ago that had been sitting for 23 years. Engine was freshly rebuilt just before the car suffered some minor damaged in a parking lot and was taken to a body shop for repair and paint. Shop owner died before the car was completed then the shop owners son was to complete the car and he was killed in an accident. So the car owner took the car home in a partially disassembled state and placed in in a building where it had set for 23 years. I had tried to buy the car over the years but he still had hopes of getting it back together but never did and finally sold me the car. He was a great engine man and had a shop for 30 years doing VW and Porsche work. He acquired this car when it lost a timing chain and the original owner sold rather than fix it. My friend went through the engine doing a full rebuild on it and was using the car when it was damaged.
The entire fuel system had gas the consistency of tar in it so the injection was in pretty bad shape and I found a good set of rebuilt Zeniths for it and some lightly used SSI’s for the exhaust side. I am looking for some advice on cams an pistons to complete the breathing side of things and get a little more out of the engine. I am not looking for a barn burner but would like to get closer to 200HP if I can. I know if I put cams in I need to change pistons for clearance but have not worked on these things in years and was wondering what might be a good setup? Thoughts? Car is still a narrow body which it will remain and has 56,000 original miles on it. I will be doing some suspension upgrades but staying with the portion bars, just building a nicer version of what this car was originally.

[Trackrash]

You have some choices, decisions to make.

IF you have Mahle cylinders, AND they are in spec, replacement pistons are readily available. 9.5 to 1 CR makes the most sense, unless you want to go crazy.

Cams could be E, S, Mod S or talk to some of the cam grinders to get their take on what your goals are.

Also to consider is your port size and what mods you intend to do to your carbs. Zeniths were designed with smog and economy in mind, so they will need to be upgraged for your purposes. Most of the 2,7s had 32mm intake ports which will put a damper on any top end power. Opening them to 35mm or so would be your best option.

Or if you have the $$$ larger bore P&C are available.

[Myazel]

I would like to try and stay with my stock P&c’s, I know this is not optimal but for now would be ok for my purposes. I am capable of opening up the heads a bit if needed, I would like to stay at 2.7 as they rev nice and my goal is a slightly modified car that stays close as possible to its main original concepts maybe just a little quicker. I have read that I could get my cams regrouped to an “SC” grind and likely get about all I can with my carbs pistons and exhaust.

[Richey]

I won't mince words , a '75 , '76 or '77 CIS engine is one of the worst engines Porsche produced because of emission requirements . If you want the car to run half decently don't even think about using the original CIS pistons , and don't use the CIS cams if you are using carburetors , the carbs alone will not change anything . Leave as is or use different pistons and cams . Any cylinder head work will be a waste of time and money with the CIS pistons and cams .

[Mark Henry]

^Agree, you won't get near 200hp with CIS pistons and cams.

Cheapest route is a set of JE 9.5:1 pistons, if you have nikasil jugs you might get lucky and be in spec, if not or alusil jugs your price of the P&C set just doubled. Plus a set of cams, headwork, carb jeting, etc.
But if you did all this the carbs should have bigger venturi's and there's not many parts available for Zeniths.
Performance costs can add up fast, known as the 'The slippery slope'.
Big thing is it's a lot of research, work and you have to do things right the first time.

The absolute cheapest route is mild cams better suited to the carbs and too be happy with 160-ish HP.

[JmuRiz]

+1 what Mark said, if you just want a small (CIS piston friendly) cam bump, I have some 3-lobe SC grind cams when I was thinking of going the same route you are leaning towards.

More looking into mine necessitated a full-full build so I changed me cams.

[Black 993]

At 56K original miles the cylinders are probably fine. I would either leave it stock (pretty easy to clean fuel varnish out of the CIS components) or go with higher-compression pistons, cams, and carbs. That will get you to 200hp. The only reason to ditch CIS is for hotter cams. And you need pistons for those. So the question you have to resolve first is whether you're buying pistons.

[Myazel]

Thanks for the advice here. I was looking at going with an SC grind on the cams as I have 35mm intake and exhaust ports in my heads and SSI exhaust to go with the carbs. My 40 series Zeniths have 32mm venturis in them which look easy enough to open up to 35 or 36mm as I have a rather complete machine shop here at the house and a background in such work. Knowing that the 2.7 RSR engine ran 8.5 to 1 pistons back in the day with similar intake and exhaust geometry (although more cam) I felt the combination I had in mind should get me somewhere north of the factory horsepower rating of between 165 and 175 depending on what you are reading. Trying to resurrect the CIS just seems like a tremendous amount of effort with unknown results as the gas in the entire fuel system had turned to the consistency of tar. The gas tank was the worst my tank guy said he had ever seen when he cleaned it and the fuel lines were the same just filled with a stiff dark goo which I am replacing through the tunnel with stainless steel. I like the look and sound of the carbureted engines and dont mind fiddling with them as I have had many cars with multiple carbs over the years. The engine seems to be just what it should be after my friend rebuilt it 25 years ago and my goal is to get it back on the road as a strong and durable runner. It will never be a concours car but it will be driven a lot whenever there is no salt on the roads as I like in snow country. Thanks again for the advice guys.

[Myazel]

One other note on the car is that it was a Midwest version originally sold in Chicago as I have all of that paperwork even the salesman’s name so it never had thermal Reactors on it which likely helped keep the engine in the condition it is in now which appears very solid with no stud issues.

[Mark Henry]

Originally Posted by Myazel

My 40 series Zeniths have 32mm venturis in them which look easy enough to open up to 35 or 36mm as I have a rather complete machine shop here at the house and a background in such work.

Knowing that the 2.7 RSR engine ran 8.5 to 1 pistons back in the day with similar intake and exhaust geometry (although more cam) I felt the combination I had in mind should get me somewhere north of the factory horsepower rating of between 165 and 175 depending on what you are reading.

You don't want to open the venturi up much past 34mm, or you will kill the venturi effect. Opening up the vents on a stock-ish engine won't do anything but make a poor running engine. With your engine I wouldn't go bigger than 32mm, in fact as is I'd likely keep the 30mm vents.
I have 34mm vents in 40mm webers on my big port 3.0SC with big cams and twinplug. IMHO if I wanted bigger vents I need to step up to the 46mm carb.

Modern cam grinds with proper carb pistons you can easily bump the CR close to 9.5 on single plug. It's the only way (plus head work, etc.) to get close to the performance numbers you want.
Once you start down the performance slippery slope the gain increments are very small, it's the sum of all the mods and parts that shows the larger gains in HP.
Porsche prices... bring money.

[911 SLANT]

I ran my 76 CIS motor with 90mm CIS pistons, 40 Zeniths, 964 grind on cams, SSI headers and M&K GT3 muffler, aluminum flywheel and clutch. Ran strong. I ended up going 2.8 with 9:5.1 J&E pistons and Mod S cams with RSR headers and 997 muffler. Love it!

[Jim Dorociak]

got a question for 911 SLANT - when you went to 2.8 were you able to bore your Mahle cylinders to 92mm and was there an issue with the CE ring on the top of the cylinder being too close to the new bore size? thanks - waiting anxiously for your reply and additional thoughts. Jim

[911 SLANT]

No, I did not bore my cylinders. I purchased the 2.8 cylinder and piston kit from EBS Racing in Reno,Nevada. The kit came with J&E 92mm 9:5.1 Pistons and cylinders. No problems with CE ring.

[911 SLANT]

[Jim Dorociak]

Perfect - thank you. Jim

[911 SLANT]

Sure, glad to help

[stownsen914]

Sounds like a cool project! Couple additional thoughts:
@Myazel you're correct that the 2.7L RS ran 8.5:1 pistons with early S spec cams. One of the challenges you'd run into is that your CIS pistons don't have the valve clearance for S cams. If you machine them, you'll lower the compression even more than they already are, and wouldn't run well with aggressive cams.
Another consideration is that your cylinders if original, they should be alusil. In case you decide to splurge for some aftermarket pistons, you may need to have your cylinders nikasil plated. I've not heard of using the alusil with aftermarket pistons. Perhaps others can chime in if they've heard differently. There are some off the shelf upgrades at places like EBS that are "reasonable" by Porsche standards but not necessarily cheap.
Sounds like you wouldn't need to tear into the bottom end, but in case you need to, do some research first. The bottom ends on the mag case motors are, ahem, not the most robust. Especially the 2.7L ones since they took some overheating abuse with the emissions equipment. When you disassemble them, the cases tend to warp and need $$$ in machining to get them right again. Just FYI.
Sounds like fun project, and welcome to the slippery slope!

[Myazel]

Thanks to everyone for the comments, I have found a set of SC grind cams and checked my Zenith carbs which have 32 mm venturis in them and a set of SSI’s I bought a while back. My car was built in late 1974 and I believe it has Niki’s on it from what I have read and comparing to mine. I am just finishing up an addition to my shop and should be back on this build shortly and will see how it goes.