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Extracting Power from CIS 911s

Pelican Technical Article:

Extracting Power from CIS 911s








Hiring a professional to regrind the cams

Applicable Models:

Porsche 911 (1965-89)
Porsche 930 Turbo (1976-89)

Parts Required:

Pay Elgin or Web-cam to regrind your stock CIS cams to the 964 profile and purchase JE Pistons with Goetze rings, wrist pins and clips

Performance Gain:

Achieving up to 240 horsepower at the engine

Complementary Modification:

Have these modifications done while you're rebuilding your engine

Those of us who own a 911 with CIS fuel injection-that is, every owner of a 911 produced during the decade starting in 1973-suffer, we are told, from the inordinate restrictions of the CIS system, also known as Bosch K-Jetronic injection. Bruce Anderson warns in his canonical Porsche 911 Performance Handbook that "if you are going to make a hot rod of the 2.7 engine, you must replace the CIS injection system with either Weber carburetors or mechanical injection." Regarding the 3.0-liter motors, Anderson states that "The next change [after backdating the exhaust] is to replace the CIS fuel injection with a pair of Weber 40IDA-3C carburetors." Most other assessments from the automotive punditocracy are similar, and CIS injection is referred to generally as yet another drawback to the congenitally defective 2.7-liter motors. In short, the accepted wisdom holds, if you want better performance from your CIS-injected 911, after backdating the exhaust the next step is an obligatory scrapping of the fuel injection in favor of carburetors or, more expensive yet, a customized fuel injection setup.

I thought that this would be my predicament, too, after a cursory investigation into modifying my 1977 Carrera 3.0. I had installed SSI heat exchangers and an early muffler, which are superb additions, but as we learned the first day of economics class, human appetites are insatiable, an observation that applies with great relevancy to the desire of Porsche owners for more power. Badly worn exhaust valve guides forced a rebuild at only 142,000 kilometers-Carrera 3.0's were built with the poor 2.7 valve guides-and so the opportunity to perform some modifications while the engine was apart presented itself.

The drawback to replacing CIS with carburetors is that after doing so, the engine internals are still untouched and the car is only negligibly faster. A set of PMO carburetors-which are extremely well-crafted, and include intake manifolds, linkages, and water shields-costs a hefty $2,600, and that's what you have to spend just to reach the point where the motor itself can be modified. But aside from cost, there are several good reasons not to want to switch to carburetion. When properly sorted, CIS is a smooth and reliable fuel injection system; it starts and runs well in cold weather, gets excellent gas mileage, and, with a pop-off valve, is very reliable. Carburetors, by contrast, run more poorly in inclement weather, can be fussy to set up and maintain, and (the main drawback) get comparably poor mileage. Paying $1,000 for used Webers or $2,600 for new PMO's-a setup that brings you down to 15 mpg and only produces a dozen or so horsepower-is not my idea of a great deal.

After talking to some knowledgeable people, most importantly Steve Weiner at Rennsport Systems in Portland, Oregon, I was surprised to learn that there are a couple of very easy bolt-on modifications one can carry out on the CIS cars that are entirely compatible with the stock injection: regrinding the camshafts to the 964 profile, and adding higher-compression pistons. First, consider the cams.

CIS is famously intolerant of the more radical (high-lift, long-duration) camshaft grinds, which is the main reason it must be discarded to build extremely powerful street and racing motors. Fuel flow in the CIS system is controlled by the rising and falling of a round sensor plate inside a conical housing, or venturi. The more the throttle butterfly valve is opened, the more air gets sucked into the airbox intake and flows up and around the sensor plate, which rises in response to the flowing air. The sensor plate acts as a lever, and the more it is forced upward by the rushing air, the more the piston housed in the fuel distributor falls, metering out precisely calibrated fuel to the fuel injectors. More radical camshaft grinds (such as those in the early 911S) produce more valve overlap than do more mild cams, and thus cause air pulsations to travel up the intake system and batter the sensor plate, interrupting the precise metering of air that allows the precise metering of fuel. "This is called intake reversion," Steve Weiner explained to me, "and is common with long-duration and narrow-lobe center cams. I've seen this in many engines since 1963 when I started [working on Porsches]. You cannot 'see' it except on the engine dyno at certain RPMs. Worst cases create a fog of fuel above the intake stacks."

It would be interesting to know who first discovered that 964 (a.k.a. 911 C2 and C4, '89-'94) camshafts are compatible with CIS injection, but produce about 10 more horsepower and a more peaky power curve. There are two ways to make the 964 setup work: Either acquire a set of actual 964 cams and have the power steering pump drive ground off to make them fit into the 2.7 and 3.0 cam housings, or send you stock CIS cams to Elgin or Web-cam to get reground in the 964 profile (I had mine done through Engine Builders Supply Co. for about $350). The effect of the 964 cams is to move the engine's power curve toward the top end of the RPM range, along with slightly increasing the engine's power. Where stock CIS engines are out of breath by 5,000 RPMs, with 964 cams they pull strongly straight up to redline.

The low-compression pistons of the CIS cars also beg for enhancement, as long as you're willing to buy high-octane gasoline. Most 2.7 motors had 8.5:1 compression (sufficient for 87 octane fuel), including the famed Carrera RS, while the '73 CIS 911T and '74-'75 911's ran only 8:1 compression. All the U.S.-market 911SC's ran 8.5:1 compression until 1980, when they were permanently upgraded to 9.3:1 compression, and the Carrera 3.0's had 8.5:1 compression. Although it may mean removing a serviceable set of pistons, you can get a respectable power increase (especially in torque) by bumping compression up to 9.5:1, or even 9.8:1 if you won't have to pull into West coast gas stations and use their "premium" 91 octane fuel. JE Pistons sells the appropriate piston sets (that include Goetze rings, wrist pins, and clips) for under $1,000, and roughly speaking, you get about a 10 percent power increase per compression point increase. The only caveat about the piston swap is that JE pistons won't work with Alusil cylinders-but those cylinders were rarely used on CIS cars.

The cams can be reground and pistons purchased for a combined total under $1,400, and if your motor is already coming apart for a rebuild, they don't take any extra time to install. The modifications were a big success: Drivability in every respect is unchanged, except that the car is very noticeably more powerful-it's considerably faster than a stock 3.2 Carrera, and there is much more high-end power than before. But as the saying goes, one run on a dyno is worth a thousand opinions. I dyno'd the car on a Dynojet and achieved 203 rear-wheel horsepower and 176 pound-feet of torque-all without doing anything to the CIS. Figuring 15-20 percent drivetrain power loss, this engine produces in the ballpark of 230-240 horsepower. Also, a few months after the dyno run, I discovered that in the federalization process my car had been outfitted with a 2.7 fuel distributor, which surely robbed my engine of at least a few ponies. Nonetheless, considering that the car only weighs a stock 2470 pounds (the 2.7's and SC's are similar), one can imagine how satisfying these modifications are.

Comments and Suggestions:
James Comments: Hi,

Super useful article, many thanks for sharing. I've ordered a set of 964 cams for my 1974 2.7CIS but am a little lost on which pistons to order from JE. The site shows four: Any thoughts much appreciated. Looking for as much gain as possible. Thanks
October 2, 2017
  Followup from the Pelican Staff: Depends on your bore and compression ratio you want. See the table data to identify what you prefer. You can also call JE and tell them about your build to get advice.
- Nick at Pelican Parts
Mike Comments: Do I need to adjust fuel mix on my 1976 911 2.7s when upgrading pistons from 8.5 to 9.5. fitted standard CIS fuel system
July 13, 2017
  Followup from the Pelican Staff: You will likely need to compensate for the engine upgrade. I would check with a Porsche engine builder for advice on the fuel tune. - Nick at Pelican Parts  
mexico356 Comments: Hello, I'm doing this mod to a 2.4t cis. I will be installing a set of 86mm je 9.7:1 pistons to make int a 2.5 liters engine, with the 964 cams, the cuestion is the port size, the stock size is I 30, E 33 wich seems to small for this upgrade, I ask the advice of the engine and cis experts to advice me on this matter

January 27, 2015
  Followup from the Pelican Staff: I am not familiar with this upgrade. Or at least not much of an expert. I will defer to the community.

I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
aircrueled Comments: Gee... I'm not sure when this article was written, but it seems as though the times have changed, a lot. I'm rebuilding my 1980 3.0 and figured that I would look into having my cams reground to 964 specification. Guess what? It is no longer $350... go ahead and add a thousand to that number. Not sure if it's going to make that much difference for the price.
December 2, 2014
  Followup from the Pelican Staff: Yeah, this article is quite old and as time goes on, so do prices. - Nick at Pelican Parts  
SpecRacer13 Comments: I am having huge CIS inj pump probs on my 1980 SC seeing the CIs post of Nov 13 2013 from Andy shows a rebuilder from NC who does this Can you lead me to him or a comparable shop?
October 27, 2014
  Followup from the Pelican Staff: We may be able to set you up with a rebuilt unit. Give our parts specialists a call at 1-888-280-7799. They can help you find the right part.
- Nick at Pelican Parts
Abaurre Comments: I´m reassembling my CIS system, checking every component. I have a 911 1980 SC stock. I saw 2 things that I can´t find an answer for:
First my CSV plastic mounting flange has a blocked hose connection. Is this right? The way I see it, that should be opened so the air from the 3 valves can reach the manifold.
Second: My auxiliary air valve doesn't seem to be original. It has a vacum conection, like the decel valve, allows air to pass but if you push the membrane from the vacum conection it blocks air. I d'like the original part number so I can purchase it, and how high the vacum has to be so I can test it.
Thank you.
June 7, 2014
  Followup from the Pelican Staff: Not sure about the blocked flange hose. Give our parts specialists a call at 1-888-280-7799 and they can help figure out which part or repair kit you need.
- Nick at Pelican Parts
willcall Comments: I have a totally stock 1976 3.0 carrera. It was originally delivered to knufsford, England and is now in Australia. The only engine work I know of is a top end rebuild about 100,000 miles ago and it has also had 2 piston shaving treatments. I believe that this would have increased the compression ratio but to what I don't know. I would really like to do te above mentioned mods ssi exhaust, je pistons and 964 cams. Can I do anything else to squeeze some more power out of the car? I am aware if the C3's rarity and would like to keep it original. Should I also look at gearbox mods or upgrades and suspension upgrades? If so what should I do?
November 22, 2013
  Followup from the Pelican Staff: Give our parts specialists a call at 1-888-280-7799 and they can help figure out which part or repair kit you need. - Nick at Pelican Parts  
andy Comments: Rebuilding CIS components? I've rebuilt, or had rebuilt by others, a few CIS Fuel Distributers and WURs. Recently I used two different 'professional rebuilders.' One is good, one is NOT. You will find one on eBay, he islocated in North Carolina. He did a fine job at a great price. The other guy specializes in Delorean cars they use Bosch CIS his shop is in Forney Texas. He is not trustworthy! He over charged and under delivered. This is just one Porsche builder's first hand experience. Make your own decision with my tale of woe in mind.
November 13, 2013
  Followup from the Pelican Staff: Thanks for the additional Info. We appreciate it.
- Nick at Pelican Parts
Erozo Comments: Hello everyone, I'm rebuilding my tired 1977 911 S 2.7 engine CIS and initially I wanted to do it with the same standard parts the car has. Someone suggested me to do an upgrade of the current cylinders and pistons 91110394901 to a "Euro" set : 2.7 L too with higher compression 911 103 928 01 which gives more power and cost just about $500 more than the original cylinders & pistons set. Will I have any trouble doing that having to do additional changes and extra costs ? Your comments will be highly appreciated because I'm ordering the parts and I live in south america where such parts are impossible to find.
September 8, 2013
  Followup from the Pelican Staff: if you increase the compression, you will want to compensate for that will additional fuel. I would look into what it will take to adapt the fuel system. For example, check into the differences between your engine and the Euro set up. - Nick at Pelican Parts  
Mario Comments: Hi i have a 1979 porsche 911 sc 3.0 with a 30 ida weber carb
Is this the correct carb for mi porsche?
Were do i get the vaccum? Do i have to drill the manifold?
August 19, 2013
  Followup from the Pelican Staff: There is a lot of great info here: - Nick at Pelican Parts  
Andy Comments: I did all the suggested mods on my '74. I also swapped in the CIS from a 911SC with larger intake runners and fuel distributer. There is a fellow who specializes in CIS rebuilds who claims that he can modify the WUR so the CIS delivers more fuel. Does this sound credible?
May 6, 2013
  Followup from the Pelican Staff: Sure, the CIS unit can be modified as well as the WUR adjusted. Just be sure it is done correctly and will match your vehicle. Over-fueling can lead to many issues. - Nick at Pelican Parts  
Charles Freeborn Comments: Is this still a viable program?
I do not see the JE pistons here on Pelican any longer, and is there additional machining / plating to be done to fit the new pistons to existing cyl's?
December 23, 2012
  Followup from the Pelican Staff: I'm having our guys look into an updated parts list if it is necessary. In the mean time, give our parts specialists a call at 1-888-280-7799. They can you figure out what is available.- Nick at Pelican Parts  
Charles Freeborn Comments: Are the pistons still available for this conversion? I have a stock '74 S with fresh valve work and turbo tensioners already done.
December 23, 2012
  Followup from the Pelican Staff: I'm having our guys look into an updated parts list if it is necessary. In the mean time, give our parts specialists a call at 1-888-280-7799. They can you figure out what is available.- Nick at Pelican Parts  
Shaky Comments: Hi Wayne, I'm a UK resident who has just purchased a 1975 911 2.7 CIS engined car, which has a broken motor. I have had an SC previously but never rooled my sleeves up witht he mechanics. I have a friend who is a motorsport technician who is willing to help me with my project and he will work with me to strip the engine. However, reading your very helpful articles, I am concerned that mods such as faster cams will be in vain due to the CIS injection system. A couple of questions, can I get some decent hp improvements together with reliability? and do you publish an engine rebuild guide for this particular engine, suggesting the best mods to extract reasonable hp increases. Also, the states seems a hotspot for air cooled specialists and as many of the engine components are expensive, do you do much trade with UK customers? Many thanks, Sean.
May 10, 2012
  Followup from the Pelican Staff: We can help you suss out the parts. Give our parts specialists a call at 1-888-280-7799.

I'd say the rebuild depends on what you want out of the vehicle. If you just want a driver, keep it stock. The car will be lively and fun to drive. If you added performance, you can have the CIS unit modified or add carburetors. - Nick at Pelican Parts
M88drvr Comments: Hello, I have a 1974 plain 911. I'm rebuilding it and was wondering what the best combo for more power without installing carbs are. Not sure what type of metal the cylinders are exactly.
April 22, 2012
  Followup from the Pelican Staff: Here are the parts. The cylinders are Nikasil. - Nick at Pelican Parts  
JFPH Comments: Hi everyone!
I'm in the process of rebuilding a 1975 2.7 CIS engine due to pulled head studs, and after having read this thread I've decided to go all the way with 964-profile cams, JE pistons and SSI heat exchangers. Should be a nice package! However, since some of you already have some experience with this, is there something special that I should look out for in the process or even consider doing instead or in addition? Also, please let me know if you can recommend someone reputable to regrind my existing cams to 964-profile. Car is in the US.

Great thread, by the way!
March 12, 2012
  Followup from the Pelican Staff: There's a good thread here with some helpful info on regrinding etc. - Nick at Pelican Parts  
Atlast Comments: I did this upgrade 36,000 miles ago with a few changes. I used web 993SS cams and a enriched CIS. Note there is no smog control in South Carolina. 208hp at the back wheels. I polished the heads with standard valves. Run 9.8 mahli's as 930/10 Euro motor with SSI and B&B muffler. 24mpg on the open road, smooth and starts easy. I had to disconnect the frequency valve and delete the DOT dox they installed. Air pump, aircon deleted too. Problem -I want more, whats is next 3.6pistons and PMO's. I think too much money for a few horse power. By the way the motor is out again as have a bad knock on left bank. What I can see lack of oil to Exhaust rockers as cam and rockers badly worn only on three exhuast ports.
December 12, 2011
  Followup from the Pelican Staff: Thanks for sharing. Great information! - Nick at Pelican Parts  
KNIGHTRACE Comments: I have in 20 years not seen any stock street motor pull better than a CIS 930/10 motor some years back. It was a gray market car that had 38mm ports. It would even outrun a 930/20. Maybe if the 930/20 had a chip it would be closer.
October 5, 2011
  Followup from the Pelican Staff: Thanks for sharing your information. - Nick at Pelican Parts  
Jim Comments: Correction my compression is 10 to 1. Sorry for the typo.
May 5, 2011
  Followup from the Pelican Staff: Thanks for the additional info. - Nick at Pelican Parts  
Jim Comments: I had a 79 3.0 rebuilt for my 69 911E and I wanted to have good bottom end power and pull strong to about 6K RPM. My rebuild is a follows. 79 3.0 built with Mahle 3.2 cylinder and J/E high compression pistons with 964 cams and 78 SC injection so I ended up with a short stroke 3.2LThe mechanic set the compression at around 100 so I need to run 91 octane pump gas. It is very quick and has lots of low end power & torque all the way to about 6,500 RPM. It is a real nice motor with the reliability of CIS Injection.
May 5, 2011
  Followup from the Pelican Staff: Thanks for sharing. This is great information. - Nick at Pelican Parts  
mich Comments: Hi
I ve a 911 1974 2.7 with the same problems or maybe more than norm! Now I've bought the 2.7 1973 rs cylinders and I want to put the best cam I can, always thinking of having more power. So my doubt is between CIS s cams or CIS SC profile cams. I've also seen there's a possibility of putting 964 cams?
Then my final question would be... What would you do?
February 3, 2011
  Followup from the Pelican Staff: The 911RS pistons are cut to be used with the RS cams (which are the same as the 911S cams), so you won't get what your looking for with the CIS system. At this point you probably want to go with a good set of Weber carbs, and then also use the 'S' cams, or maybe something even a little bit more aggressive than that. - Wayne at Pelican Parts  
Norm Comments: I have a 1974 base 911. When I bought it, the 2.7L was very tired. Valve guides were badly worn and there were a number of oil leaks. Because of this, I got the car for a very good price. I then took it to a top notch local mechanic for a full top-end job.
Along with installing all of the various upgrades such as Timeserts, Dilivar studs, and the Carrera chain tensioners, he suggested regrinding the cams to the SC spec.
What happened was incredible. My poor tired much less than stock 135 HP engine was suddenly a fire breathing monster with about 230 HP. Along with more power was much better torque that allowed me to bring the car up from nearly a standstill in 2nd gear with lots of power and very quick accelleration.
Along with the other changes were a new set of jugs and pistons. I went with the 911S set in Nikasil.
All this was done in the Spring of 1989. The car now has almost 80,000 miles on the rebuild and still runs very strong. I had it dyno'd a couple years ago and it showed 190 HP at the rear wheels. This is still better than 50 HP more than the original stock motor.
AND, the car still has the original CIS injection system.

Now, like everyone else, I still want MORE POWER, but my car is still very fast and very quick. On track days, I would chase the Turbo 911's into the first turn at the end of the straight at SIR at better than 145.
So, 230 horses in a car that weighs about 2300 with all the useless stuff spare, seats, etc... pulled out give a power to weight ratio of 10 lbs per horse.

So, if you have a tired 2.7L, this is an option.
April 23, 2010
  Followup from the Pelican Staff: Cool deal, glad it worked out well! The solution for more HP is a 3.2 punched out to 3.4 with S-style pistons and S-cams running carburetors or a dedicated, independent throttle-body engine management system. But, those are expensive! - Wayne at Pelican Parts  
Mark Comments: I did this mod to my 81 sc in 1998. I also did very mild porting and upping displacement to 3.2l. Nice improvment in power and it actually passed ca smog with the cat. Runs worse than stock in this configuration, though. I remember there being a computer controlled fuel solenoid/ignition module that could be added to k-jet cars for mapping of fuel and ignition. Think it was called k-star. I tried to obtain one but could not. The most promissing k-jet mod I've ever seen.
January 18, 2010
  Followup from the Pelican Staff: Thanks for sharing. Very good stuff. - Nick at Pelican Parts  
78SCRSMAN Comments: Hi,
I used to be into H2O cooled VW's and had a CIS car... I'm wondering if a lorger bore air sensor plate can be used on these cars as we did with the VW's. Perhaps a GTI sensor would work with the Porsche system? Also it seems, if need be, you could alter the fuel pressure by adding or subtratcting the shimms in the fuel pressure regulator to get some added power. One question I have about this article is: Is it wise or OK to reuse the OE jugs with new JE pistons assuming they are within spec? Also, is it neccessary to use JE pistons when using the 964 cams or can you use Mahle 9.3:1 pistons with this setup? Did the 1980-83 cars made for the US market have the 9.3:1 PC's? If so, why did they have the same 180 HP figure? Any answers or comments would be helpfull.
October 19, 2009
  Followup from the Pelican Staff: Simply changing the mixture by adjusting the fuel pressure won't add more horsepower - it will simply make the car run richer. Keeping the mixture regulated at the proper levels for power is the best way to go for both power and emissions, and the factory did a pretty good job of that. If you want to play around with mixture adjustment, then you'll have to go with some type of electronic fuel injection system.

As for the JE / OEM, the JE pistons are very good and make excellent replacements. They can be used with either cams.

According to my book, the Euro SCs had 9.8:1 compression, and output 204 HP versus 180 here in the states.

- Wayne at Pelican Parts
rally Comments: WOW,all the info Ive been looking for.Im working on a 74 2.7 and i have the s cams and a new set off 40ida webers in a box ,what would the approx HP be..thax ps I also have new red box of maele 90mm flat top pistons,but i dont know what commpresion they are,maybe you would know...
October 14, 2009
  Followup from the Pelican Staff: Hi there. Check out the Engine Rebuilding Book - everything you need should be listed in there, HP numbers, compression ratios, etc... - Wayne at Pelican Parts  

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