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Could someone educate me on CIS systems please?

Are the CIS systems interchangeable from engine the engine? Meaning will ACIS for a 2.7 work on a 3.0 and vice versa?

Is there a way to determine the health of A CIS systems on a nonrunning car short of taking it apart?

Is there a practical upper limit on horsepower, or do you generally run out of engine rebuild money before you outstrip the systems ability to deliver fuel?

Once installed and healthy are they pretty much set and forget?

Thank you in advance for the education.
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Old 02-21-2018, 07:00 PM
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The real experts will chime in here but:

1. Interchangeable - essentially no. The system went through a number of iterations over its lifetime as it evolved to deal with emissions controls. There were changes year to year with the same capacity engine and it's intended region of service. Some parts can be mixed and matched but it's problematic. Lots of problems are caused by people using the wrong parts from year to year.

2. Yes. Start with fuel pressures and a vacuum integrity (no leaks) test. The exhaust can be sniffed with a gas analyzer to see if it's in spec. You really have to pull all the components individually and test them to see if they are in spec.

3. There is an upper limit to what it's capable of. I don't think it's considered a tuner system at all. It served a purpose for an optimal combination of performance, reliability, fuel economy, emissions and performance at the time.

4. Yes. Now that the system is more than 30 years old most are pretty tired with failing components e.g. WUR, AAR and leaky airbox (adhesive fails) and rubber seals. A well sorted and refurbished CIS is very reliable though. The main issue is people tinkering with the fuel mixture to mask other issues and it just degrades from there over time. If the mixture leans out too much they are quite famous for causing a backfire and blowing up the airbox.

Last edited by gazzerr; 02-21-2018 at 07:32 PM..
Old 02-21-2018, 07:26 PM
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Very helpful, thank you.
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Old 02-21-2018, 07:32 PM
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Jim's Basement Workshop

Jim's website is an excellent resource for CIS information. It has helped me more than a few times.
Old 02-21-2018, 08:58 PM
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This thread has a ton of information:
CIS Troubleshooting for Dummies
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Old 02-21-2018, 10:05 PM
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Thank you gentlemen.
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Old 02-22-2018, 05:17 AM
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Im going to deep dive into the CIS on my 83 SC in a couple of months. This version has all the bells and whistles including the O2 sensor. I wont rehash what has already been covered but I will draw from all the excellent existing CIS content and document my system rebuild, including upgrades such as an aluminum airbox.
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Old 02-22-2018, 09:02 AM
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Quote:
Originally Posted by tdskip View Post
Once installed and healthy are they pretty much set and forget?
Like gazzer, I'm no expert and I apologize for speaking out of turn.

But even at the risk of this CIS discussion ending like the last one, I say the answer is an emphatic no to your question above.

There are just too many bells and whistles which can go wrong and cause a problem.
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Old 02-22-2018, 09:10 AM
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they ARE pretty much set and forget. that's one of the things I love about CIS. it has one adjustment. set it for spec and all other running conditions are taken care of based on the design of the system.
given, its not a great performance system by any means.
the 76-79 systems were the best made and are extremely reliable. before and after there just band aides on the system.

not sure what pmax considers too many things that can go wrong. sure, if the car has been sitting in the woods for 30yrs, yea its gonna have issues.
you go thru the system, replace the old rubber and seals, test the vacuum devices, rebuild the FD and WUR and new injectors and you have a trouble free system for another 30yrs+.
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Old 02-22-2018, 09:24 AM
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#2 is a loaded question with an answer that depends on how long the car has been sitting dead. CIS components do not like to sit idle, bad things happen. If you know that the fuel system in your car is clean and has fresh fuel you can test the system in situ with a properly functioning fuel pump. If not then the components may need to be removed and bench tested. I cannot overstate how important clean fuel and regular use is to the life of these systems.
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Old 02-22-2018, 09:38 AM
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Quote:
Originally Posted by RarlyL8 View Post
#2 is a loaded question with an answer that depends on how long the car has been sitting dead. CIS components do not like to sit idle, bad things happen. If you know that the fuel system in your car is clean and has fresh fuel you can test the system in situ with a properly functioning fuel pump. If not then the components may need to be removed and bench tested. I cannot overstate how important clean fuel and regular use is to the life of these systems.
This is very true. They hate to sit. They become garbage when they do.

But if you run them and it's setup correctly - it will be no problem. I really like CIS and it's always worked well for me on everything I have ever owned with it - as long as I drove it regularly.
Old 02-22-2018, 11:06 AM
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Are the CIS systems interchangeable from engine the engine? Meaning will ACIS for a 2.7 work on a 3.0 and vice versa?
The basic components are the same. I have swapped parts around. Even ran a 2.7 CIS on a RS spec motor (this required the idle to be around 1200RPM). I have modified warm up regulator to get different fuel curves, etc. A lot is possible but you have to know what you are doing. Wide band 02, dyno, fuel pressure gauge etc needed for tweaking.


Is there a way to determine the health of A CIS systems on a nonrunning car short of taking it apart?

No, unless you took all the parts off and had a means to test them individually

Is there a practical upper limit on horsepower, or do you generally run out of engine rebuild money before you outstrip the systems ability to deliver fuel?

CIS systems have lots of restrictions, probably the lowest performing fuel system on any 911 engine. It is not a good solution if your goal is more power.

Once installed and healthy are they pretty much set and forget?
yes pretty much a mechanical system that measures air mass and fuels accordingly. So they can compensate for altitude and air temperature inherently in the design.

john
Old 02-22-2018, 05:32 PM
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It really isn't a good idea to mix and match CIS components. There are specific component specifications for not only each engine size but each engine type number. The CIS system designed for a 2.4L 911/92 will not properly fuel a 3.0L 930/04 for example. The operating ranges of many components do overlap somewhat allowing for some cross reference, however the results would not be optimal unless the component was tuned to the proper spec for the application. It is a mechanical system so mechanical tuning can be done. Tuning for engine modifications is common, to a point. Unfortunately 911 CIS systems do not have MAF capability so cannot compensate for air temperature or altitude. The air meter plate mechanically correlates the air used by the engine to the fuel needed. The plate has no way to gage air density due to altitude or temperature/humidity. It is common to make small seasonal adjustments to the idle mixture to compensate, or on cars that live in the mountains versus sea level. The 964T WUR -160 and -165 from 1990-1994 do have altitude compensation via a 3rd diaphragm.
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Old 02-22-2018, 06:48 PM
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CIS function by design, moving a plate with air (as well as later flapper box air flow meters) are mechanical mass air flow meters. This is the reason neither of these systems have altitude compensation (like earlier MFI or later EFI systems). Now that doesn't mean they were highly accurate at this function. This is the reason these systems had adjustments and later systems incorporated more accurate methods of measuring the amount of air entering the engine.

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Old 02-22-2018, 08:32 PM
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CIS or ITB?

Before




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Old 02-22-2018, 09:36 PM
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Old 02-23-2018, 03:37 AM
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Old 02-23-2018, 06:06 AM
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Quote:
Originally Posted by tdskip View Post
Is there a practical upper limit on horsepower, or do you generally run out of engine rebuild money before you outstrip the systems ability to deliver fuel?
There is not a lot of data on this, because generally speaking the first thing you do when you start to modify a 911 for more power is scrap the CIS system for Webers or now ITBs.

If you look beyond Porches, lots of people have found that Bosch built a lot of headroom into CIS. The commonly accepted figure for the Rabbit GTI is ~140 HP before you have fueling problems. The stock HP for that engine is 90.

Only person I know of who tried to go as far as possible with CIS with a 911 was "rdane" who got ~223hp to the wheels with a 3.4 with early SC CIS. Details here Assuming ~15% driveline loss, that is around 250HP for an engine that came stock with ~180.

The problem with CIS is that because of the single plenum, it doesn't tolerate a very hot cam.
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Old 02-23-2018, 08:06 AM
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Quote:
Originally Posted by emcon5 View Post
The problem with CIS is that because of the single plenum, it doesn't tolerate a very hot cam.
This PLUS the flapper door air flow meter for fuel pressure control. A hot cam results in very unstable/low manifold vacuum and causes the door to oscillate and makes the fueling very unstable, particularly at idle.
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Old 02-23-2018, 09:17 AM
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Going down a different path..............

When I built my 3.0 into a 3.3SS, I decided to keep the CIS and see how much performance I could get out of it. However, what I built was a Frankencis system using a 1978 large port airbox and hardware from a 1980 system. I plumbed it to be as much like the 1973 1/2 system as possible with the addition of the AAV. Much to my surprise and relief, the system runs great! It took some tweaking here and there but now it starts easily even after sitting for a couple of weeks, idles well and pulls like a freight train up to 7500 rpms where I have the ignition interrupt set. I added a little trick by adding a contact switch on the throttle linkage so that the cold start valve is activated at full throttle (only). This drops the A/F ratio from about 14.5 down to the low 13s to help prevent burning a valve and giving it a bit more fuel on the top end. The cold start valve has its own little distribution box that makes sure each cylinder gets some extra fuel. The only changes I make now are in the spring and fall when the weather changes. So, are all CIS systems trouble free? No. Can they be made to be reliable? Yes. And, there is a lot more performance potential than most people give credit for!
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