Power secrets/tips/mods for CIS?

[Patronus]

I have a '74 2.7 with CIS. No problems and no complaints. It is common knowledge that, if I want power, I have to get carbs and other cams. Everyone knows that. However, I have been wondering.
The CIS system is very simple and probably has lots of room for improvement. For those people who choose not to replace it with carbs, and who want to keep the cams, are there any effective modifications available for the CIS system? Or anything that can be done in order to make it flow better? Has anybody experimented with roundening some of the sharp edges inside the box?

I know that people have played around with intake diameters and so on, but I don't believe it made any difference.

Hoping to get some fine tips...

[Patronus]

Has nobody ever tried to improve the CIS? Ever? Anybody????

[CamB]

Hmmmm. You have a 74, so no value swapping to an early exhaust (you already have one).

Other (still expensive) options include upping the displacement (which is a bit risky on the magnesium case as well, but I believe a 2.9l option is available) and changing cams to something that still works with CIS.

Now the latter option is usually done to the SCs, using the 964 cams (or the 3.0 Carrera). I have no idea if it works on the 2.7 (but would like to know!).

There is a place in the UK which has an add-on box called the K-Star. I don't think it gives any more power, just smoother running. See here.

I think those options are it...

Cam

[This message has been edited by CamB (edited 09-24-2001).]

[This message has been edited by CamB (edited 09-24-2001).]

[RoninLB]

Patronus----finincialy speaking, if you are in for a nickel, are you in for a dime?

[Patronus]

RoninLB, it is about efficiency, not money. Of course you can bolt carbs onto it, and replace the cams, and you'll have instant power. Hell, I'm sure if you throw enough money at a 2.7 you can get 300hp from it. But that's not the point. I believe that the CIS system has some room for improvement, and if I can make it more efficient, why not?

I took mine apart and was shocked by the number of sharp edges in the path of airflow - edges that could easily have been smooth, had Bosch gone the extra mile. The underside of the aluminium housing, for example, is less than effective. The area directly beneath the intake opening has almost a 90 degree edge.
And so it continues.

In the long history of CIS, am I the only soul to notice it? Or am I missing something?

[This message has been edited by Patronus (edited 09-24-2001).]

[Wrecked944]

Has nobody ever tried to improve the CIS? Ever? Anybody????


I'm with you on this one, Patronus. I posed the very same question on this board a few weeks/months ago and got the same stunning silence. I think the short answer is "no", nobody seems to have been interested enough to improve it. But I'd be more than happy to keep this thread alive and brainstorm some McGyver-inspired upgrades. For instance, do you think boring out the throttle body would help? Seems pretty low-tech but it might help. I once cut holes in the top of the airbox on my 944 and was very happy with it. I don't think it increased HP per se, but the car leaps to higher RPM's with greater ease. I'll gladly try the same on my 911. And what about the "barn door" air sensor/flap? This seems like old technology. Some folks have tried drilling holes in the "barn door" flaps on their 944's - but I haven't tried it yet. Would a mass air flow sensor be a viable option? Heck, I have a drill and some wrenches and enough lunacy to try anything.



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Janus Cole
1980 911SC & 1987 944

[Superman]

Race pistons.

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'83 SC

[wckrause]

There is a fuel injection book by Probst that describes theory of operation for both CIS and DME systems. It also has a chapter on performance improvements for CIS. It is a VERY thin chapter, containing ideas that have been tried usually with little or no success.




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Bill Krause
'79 911SC Euro
MY PELICAN GALLERY

[Patronus]

WCKrause, thanks for the info. I'll see if I can get hold of the chapter.

Janus, I think boring out the throttle body will hurt power. It's like venturi-overkill in a carb. I think we can assume that Porsche has already done the math for us on that one. (By the way, I guess the 944 and 911 CIS look pretty much te same on the inside).

I also wouldn't touch the air sensor plate since it will affect the metering. Yes, the plate is old-tech but we have to live with it. I'm more concerned about getting air from the one side of the plate to the other, without affecting its accuracy.

The most obvious obstacle is the part directly beneath the sensor plate. Air is sucked from a large area into a small one, around a very sharp edge (the edge of the metal housing around the plate). This causes limited air flow all around the plate - the very area where air must pass through.
I was thinking about an upside down velocity stack, not very deep, just enough to minimize the turbulence around the sensor plate. It will have no effect on the accuracy of the sensor plate, I think, it will just deliver more air to it.

That's the first thing I would do, since I believe it to be the worst (or at least most obvious) bottleneck in the system.

The (plastic) intake housing also has some serious corners that air has to negotiate around. The first obvious one is directly in the line of flow between the sensor plate, its opening, and the opening of the air cleaner cover. I am still examining it to determine whether anything should be done to it. The velocity stack will probably also solve that problem.

Any comments?

[Wrecked944]

I also wouldn't touch the air sensor plate since it will affect the metering. Yes, the plate is old-tech but we have to live with it. I'm more concerned about getting air from the one side of the plate to the other, without affecting its accuracy.


My mistake. I should have mentioned that part of the mod was an adjustment to loosen the spring that holds the sensor plate closed. In theory, this might help restore the accuracy. Again, I haven't tried this. It seemed a bit...um...nutty. And yes, a larger throttle body may well reduce performance. The carb analogy makes a lot of sense. So you are thinking of improving the air air flow - makes sense as most people claim the CIS's main problem is the torturous air path from filter to cylinder. I'm going to take your list of possible improvements and compare it to the *stuff* in my engine.



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Janus Cole
1980 911SC & 1987 944

[stormcrow]

Well, if you want to make mods, you have to look at the air intake, gas flow and exhaust. All three have to be tuned to work with each other.

I've often thought of removing the CIS injection system and installing a four barrel carburator. The change over would be easy and you wouldn't have to destroy the cis system just in case you would like to reinstall it.

The only thing that would need to be done is match the carb up with the size of the engine.

Center it over the engine and you would have real good flow to each of the cylinders. Use steel railing pipe for the intake to each cylinder, a flat plate on top of the pipe bored and drilled to accept the carb.

It would be interesting to see what it would do as far as performance for the engine.

Just a thought

Steve

[RoninLB]

PATRONUS....I'll give you some comments. Intellectual discussion and reality can be very different topics. The 2.7 CIS was installed to meet air polution standards. The system, IMHO, was being used on common european cars. Performance was not the main issue with the instalation,IMHO. Hope you agree so far, if not we are both wasting our time here. IMHO the 911 becomes fun at 200 hp. Good luck getting 200 hp on a 2.7 with CIS. You can have a reliable 200+ hp that doesn't need spark plugs every 5,000 mi. Superman was being low key when he suggested "race pistons", perhaps he wanted to find out who we are talking to. I would take every word he says seriously. Also, my 2.7 started out with 2.7 RS forged pistons, 8.5 comp. ratio. Give your issue more thought before you get involved with that CIS. You may be sucessful in your journey, but sometimes the best throw of the dice is to throw them away. Good luck

[Patronus]

RoninLB, I agree with everything you say. And no, I don't want to stretch the 2.7 CIS to 200hp. But I don't believe that the CIS system is beyond improvement. And, as for getting involved with the CIS, my car has CIS injection, so I'm already involved with it.

The fact that I didn't respond to Superman's comment doesn't mean that I didn't take it seriously. I'm sure Superman will understand.

Stormcrow, I saw a 911 engine with exactly that - a large carb right in the middle, I think it was a Holley. So I guess it is possible. Couldn't find out who's engine it was, so I couldn't ask how effective it was.

Janus, my CIS has no spring holding the sensor plate closed, just a counterweight - unless I have lost the spring?? Hope not.

[This message has been edited by Patronus (edited 09-24-2001).]

[Bill Douglas]

I believe early 911 turbo's had CIS. They would have sucked a lot more gas through them than a 2.7 or an SC ever could. I would leave it alone. Some guys have troubles with their CIS's so i would count my blessings and be happy with it. Plus 2.7's arn't the most stable engines so I wouldn't try to wring too much extra power out of it.

Bill, who drives a humble 79SC

[Wrecked944]

Janus, my CIS has no spring holding the sensor plate closed, just a counterweight - unless I have lost the spring?? Hope not.,br>,br>
Good call. Your post inspired me to do some more internet research on these things and the CIS does appear to have a counterbalance. This is different from my 944 which has a spring (but again, I was highly suspicious of the whole idea of drilling holes in my air sensor plate). I fear the counterbalance will make it difficult or impossible to "straighten out" the air path - which is why I assume you were thinking of "inverted" velocity stacks. And I'm liking that idea - anything which removes the twists and bends. I also grasp what you are saying about the right angle going from a large space to a small one. I pulled off the air filter lid on my 911SC and saw it for myself.

I wonder if anyone has made a careful study of the weaknesses in the CIS system. For instance, I've read a lot about the torturous air path. But has anyone actually documented why the system is inferior to, say, carbs? That would certainly point towards some areas for research. And yes, I expect you will get a lot of comments from folks suggesting that you are beating a dead horse. And they are, of course, correct. But speculation is harmless (and cheap!). And the worst result will be a better understanding of our cars !! So I say, "Carry on!"


Disclaimer: I am not, nor have I ever been, a competent mechanic of any kind. To date, my P-Tech has made a small fortune fixing my dumb mistakes.




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Janus Cole
1980 911SC & 1987 944

[Superman]

Well sure, I do understand. And I'm not as smart or knowledgeable as some people seem to think, but I am flattered by the above remarks. for example, I do not know what shape pistons went in the '74 2.7. But most CIS cars have pistons that are not designed to get the maximum amount of exhaust gasses out of the combustion chamber. A race piston will have a taller crown and will expel exhaust gasses much more efficiently.

I'd warn against mods to the sensor plate or spring. The plate, housing, spring and fuel distributor are machined so that for a given volume of air (sensor plate movement), there will be a specific volume of fuel. If you allow the sensor plate to move further upward at a given air volume, you will be sentencing the fuel ratio to be different at some rpms, than other rpms. This will amke the car run more poorly.

A parting thought (as I dash to my next meeting): folks have wondered how to improve the performance of these engines, but they may be failing to notice that these are not like the domestic V8 underachievers. My engine is about 182 cubic inches and makes about 180 hp, stock. By any standard, this is already a high performance engine.

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'83 SC

[ChrisBennet]

Originally posted by Bill Douglas:
I believe early 911 turbo's had CIS. They would have sucked a lot more gas through them than a 2.7 or an SC ever could. I would leave it alone. Some guys have troubles with their CIS's so i would count my blessings and be happy with it. Plus 2.7's arn't the most stable engines so I wouldn't try to wring too much extra power out of it.

Bill, who drives a humble 79SC

Yes, the Turbo had CIS. What I find surprising is that they were still using it on the 1994 3.6 Turbo!

[Wrecked944]

Yes, the Turbo had CIS. What I find surprising is that they were still using it on the 1994 3.6 Turbo!


This raises an interesting question. Was the 1994 CIS system identical to the mid-70's system or had Porsche made improvements?



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Janus Cole
1980 911SC & 1987 944

[CamB]

Basically as I understand it, the sensor plate in the CIS cannot deal with pulses in the intake system, such as those caused by more agressive camshafts, when the exhaust and inlet valves are both open. In other words, if the cam has too much (any?) overlap.

So cam changes are out. As it is, CIS engines are pretty torquey (due to the cam I guess?) and semi fuel efficient.

I mean, when Porsche increased the power of the SC, they did it using higher compression (for the Euro models)...

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Cameron Baudinet
1975 911S (in bits)
1969 911T

[This message has been edited by CamB (edited 09-24-2001).]

[jorisg]

There is a nice article about the CIS system at: http://www.ncr-pca.org/Tech/tech-cis.html
by Bob Tucker


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Joris
75 911S
http://members.xoom.com/joogie/Joogie.htm