Technical CIS operation & Cam Timing w/ CIS Discussion

[911sTarga]

Hello all.

I've read many posts on this subject, but I have yet to find a detailed thread that explains exactly how a CIS system meters fuel and what inputs is uses to do so. I believe it soley uses vacuum, but I'm unsure.

I'd also like to talk about camshafts while using the CIS system, as it appears most people think that there isn't much of a choice here. I think I can give a lot of suggestions and even test quite a few profiles on my engine, but I'm limited on actual CIS "function" knowledge right now.

I have a feeling that the CIS unit uses vacuum for the majority of it's input. I believe this because I have read that you can't use "s" cams with the CIS system. I bet the "s" cam has a tight LC (lobe center) with more duration then a non-"s" cam. What this does is drop the vacuum signal at idle and off idle running....which "might" hurt CIS performance. Again, I don't have full CIS knowledge yet.

Anyway, I'll go into this more in a bit. Can someone chime in on the operation of the CIS and anything they know about camshaft profiles while using the CIS system?

Thanks!

Jay

[MotoSook]

You could say that the air meter works on vacuum (differential pressure). Flow through the air meter body causes the air plate to rise which in turn raises the plunger in the fuel distributor. The plunger position w/in the fuel distributor and the fuel control pressure determines how much fuel is injected into the engine.

Reasons why folks have used conservative cams on a CIS engine:

-piston/valve clearance

-intake pulses (reversion) that will result in an oscillating air plate...(see above interaction as it relates to fuel).

I love to see folks wanting to experiment, but I think you should research CIS cam timing advance, and 964 camshaft installation first.

[klaucke]

I would venture that the CIS meters fuel according to the mass (inertia) of the air entering the engine and not vacuum. There are numerous sites on the internet dedicated to CIS, read around some are quite in depth. I printed a whole CIS manual that I found on the web in .pdf form.

[911sTarga]

So, vacuum signal does play an important role with CIS? I've been thinking about a custom ground "s" cam, but with a wider LC to increase the vacuum signal at idle and off idle. I've used this trick on many other EFI engines, especially on MAP equipped vehicles, but I'm still learning about the operation of the CIS.

I truly think that aggressive cams can be used (without low RPM problems), if the right combination of compression/ignition timing/and cam timing is selected.

I admit that I need more knowledge on the CIS system, but it is hard for me to believe that it is limited to just above stock HP numbers. Maybe I just "want" to believe that it could work well in a "hot" engine build, but as of right now I can't see why it couldn't.

Please note that I'm not trying to argue with anyone, but merely trying to gain CIS knowledge and discuss CIS vs Cam profiles with everyone.

Thanks!

Jay

[911sTarga]

Originally posted by klaucke
I would venture that the CIS meters fuel according to the mass (inertia) of the air entering the engine and not vacuum. There are numerous sites on the internet dedicated to CIS, read around some are quite in depth. I printed a whole CIS manual that I found on the web in .pdf form.

I just found a very cool tech diagram on how the CIS meters fuel.





It's just like you said....metered off of the amount of incoming air and how far up it "pushes" the plate/arm.

I bet you could made enrichment "lobes" that basically built up the range of motion for the plungeron the metering port. Either that or we can move the sensor plate further away from the arm? Who knows, maybe it is as simple as changing spring rates in the pressure regulator. This way you can keep low end fuel delivery AND have greater fuel delivery at full throttle. I'm betting the spring rate change is the way to go.

Again, I'm just thinking out loud here.

Thoughts anyone?

[randywebb]

the mass of the plate and its associated parts + friction will always limit the responsiveness of these cars

I would NOT try to experiment with hotter cams, etc. -- unless the engine belonged to an enemy...

FWIW - I am a known CIS-hater. Death to the CIS!

[911sTarga]

Originally posted by randywebb
the mass of the plate and its associated parts + friction will always limit the responsiveness of these cars

I have to agree with you on that point. That doesn't mean it can't be modified for less then a full injection swap. Again, just brainstorming. It sounds like everyone is against the CIS though.

[randywebb]

It is the work of the Devil

Get your pitchfork and torch and meet me outside the moat...

[randywebb]

I thought the Motronics had hot wire air velocity sensors -- like in an anemometer?

[stormcrow]

Fuel Distributor

With regard to the CIS system - it is limited in the amount of fuel that it delivers. There are openings in the body of the fuel distributor housing alongside the the plunger to allow fuel to reach the injectors when the plunger is raised. The plunger covers these openings when it is at rest (car not running)

When the engine is started, the sensor plate lifts up the plunger and the narrow part of the plunber is exposed to the incoming fuel flow and at the same time to the openings which provide fuel to the injectors. As the sensor plate rises, more of the narrow part of the plunger is exposed to allow for more fuel to flow.

Maximum fuel flow is reached when the full lenght of the narrow part of the plunger is exposed to the fuel coming in at the bottom and the fuel leaving the top of the fuel distributor (both dark and grey areas on the above diagram).

Now, if you continue to raise the plunger, the bottom of the plunger which is the same width as the top of the plunger will begin to restrict the flow of the incoming fuel by closing off the bottom opening (the light grey area on the above diagram).

The diagram shows the maximum output when the engine is running under full load. (both dark grey/light grey areas are exposed to the narrow part of the plunger)

Control Pressure Regulator

Now, if we reduce the pressure on the top of the plunger by allowing more fuel to flow through the control pressure regulator, we would then increase the fuel flow through the fuel distributor. (cold start and run condition)

On the other hand, if we put more pressure on the top of the plunger by restricting the flow through the control pressure regulator, we would then reduce the fuel flow through the fuel distributor. (warm start and run condition)

The control pressure regulator affects how high the air flow sensor lifts.


Hope this info helps

Steve

"A Porsche does more then just go fast in a straight line"

[rs911t]

I picked up this book at my local Borders. It covers all the various CIS systems.

[MotoSook]

Guys, the air plate is counter balanced. It's balance enough that if there were no fuel pressure, you could blow on it, and it will move.

Flow through the throat of the meter creates a "differential" across the plate...that's what causes it to lift! Flow and differential pressure...internal flow...crack a book. No need to beat this one...concentrate on the valves to piston clearance and overlap. You don't want parts hitting and you want to minmize reversion.

Randy, the DME AFM is trap door type meter..not a hot wire.

Getting fuel to the injectors is not a big hurdle. I can run rich enough to wash down the cylinders! And if that's not enough, you can add extra injectors. Fuel and air through the air meter are not your problems.

Getting the engine to breath (at the valve) is your big hurdle. If you are going to go custom grind, start with a 964 cam. Then find out what your clearance is over the range of cam rotation. You might be able to add a little more lift and maybe add some duration until you start to stutter when running because your air plate is dancing!

I've heard that the AFM meter is less affected by reversion than the CIS air plate....but that's not going to help Jay any.

If you can make a custom grind that is an improvement over the 964 grind, you'll have a nice product for the CIS and 3.2 engines!. But be wary of kissing the valves. Has anyone ever checked the piston to valve clearance on with a 964 cam on a 3.0 or 3.2 across the rotation? What is that number? (John - Camgrinder?)

Take a look at the top of a CIS or 3.2 piston. Someone posted a picture in the picture of a CIS/3.2 piston in the engine forum with valve pockets cut into it...seem it might have been less effort to get JE pistons and build from that.

If the ultimate goal is to make a better CIS engine...concentrate on the cams/vavles...use the 964 as your baseline and build from that...until you start hitting parts. Air and fuel can be tweeked easily.

[MotoSook]

If you really want to take this to the next level, design a pulsation bottle of sort to cancel or dampen the reversion! Put it between the air meter and the throttle body. Frequency range would be limited unless you build a dynamic bottle to keep up with the change in frequency. One drawback that comes to mind quickly is lag...thottle lag due to the volume in the bottle.

I like my CIS engine..it runs well and I can tune it with my eyes closed...but I understand it's limitations.

If you want more....pay up. There are plenty of engine combinations...just have a fat wallet.

[ianc]

It sounds like everyone is against the CIS though.

Not at all, some reactionaries aside...

The CIS is a relatively simple, very robust system that offers excellent driveability and is a HUGE advance over carbs. It can't stack up with later Motronic systems, but it is pretty good at what it does and is easy to troubleshoot once you understand the principles of operation,

ianc

[randywebb]

I agree it is a huge advance over carbs in terms of emissions and fuel economy - maybe warmup too.

It is simply inappropriate for a sports car.

Troubleshooting is not too horrible once you (re) educate yourself on how it works (tho it is still harder than carbs). Unfortunately, one troubleshoots CIS by replacing components, each of which is usually a few hundered dollars in cost. After doing 3 or 4 replacements, tho, you will eventually find the problem. Enjoy!

I do think Souk has some good ideas. Everyone should go back and print out his last sentence and staple it to your chest...

[ianc]

It is simply inappropriate for a sports car.

Hmmm... My car seems to like it fine, and so do I.

Troubleshooting is not too horrible once you (re) educate yourself on how it works (tho it is still harder than carbs).

Definitely not. Leaky throttle bushings? Trying to jet correctly for different engines when you've modified? Byzantine linkage mechansims? Trying to synchronize 6 carb throats? Troubleshooting poor running conditions with carbs is an absolute nightmare, even with only ONE!

Unfortunately, one troubleshoots CIS by replacing components, each of which is usually a few hundered dollars in cost.

I suppose one could approach problems that way, if one didn't really understand how the system worked...

ianc

[911sTarga]

Souk,

I appreciate the input and suggestions. They are truly appreciated and provided me with some great information!

I'll be the first to admit that I am new to Porsche engines, but I am also well versed in engine design, theory, and assembly. I know that you didn’t imply that I lacked knowledge or experience, but I wanted you to know that I am not green when it comes to performance engineering. I have the engineering and hands-on background to discuss this CIS/CAM "problem" in full detail, plus possibly sort a solution with other members.

I understand differential pressure and its effects on airflow (don’t forget intake pulses, etc…). I design and engineer cold air intakes for many major manufacturers on a daily basis, plus I work with Honeywell on filter design for the Fram “AirHog” line of filters. I've spent about the last 11 years designing and producing parts that directly rely on thermal dynamics.

I’m not trying to say “look what I’ve done”, but more to point out that this was a mere attempt for a discussion from a fellow user of this board to fully dissect the CIS limitations and possibly raise the performance bar a bit. I just needed a little bit of background on the basic function of the CIS system first before the discussion went totally in-depth.

I’ve done some searching on the net and these links might help others who want a little more knowledge about the CIS system and how it works.
How the Porsche CIS system works
CIS Troubleshooting Chart
Adjusting Your Warm-Up Regulator

Now, 99% of the threads that I have read on this board say that the horsepower limit is about 210HP (N/A) for a CIS equipped 2.7L engine. I can't pick up a book and read about the "why's" and "how's” of this because there hasn't been a book written concerning this directly (That I know of). If you know of a book that explains in detail why not to use an "s" cam with the CIS unit or why a 2.7L is limited to 210hp with the CIS, then please let me know and I'll buy it tomorrow! That information has come from users of this and other forums and I (and possibly others) would just like to discuss it instead of just passing it off as truth.

While we’re discussing this, I’ll be tearing down my 2.7L engine at the same time. I have the equipment to flow test everything and I plan on doing just that. Once I have all of my flow data, I’ll post all of the data for everyone to compare. I will then design a few camshaft profiles around that airflow data and set the compression accordingly. This is so far ahead of where we’re at in the discussion though.

Along with trying to get a little background on the CIS system, I want to discuss real world experience concerning the CIS unit and large cams from other users on the board. Trust me, I’ve searched and have yet to find a clear explanation as to why it has these “limits” besides reversion and vibrations. Depending on the setup, some reversion can be eliminated using the proper cam profile and compression ratio. Again, I think this topic needs more detailed information before we dive into new cam profiles.

Thanks and I look forward to reading and providing some great information on this subject!

Jay



PS – Souk & everyone else, please do not take this as an attack or as a harsh reply directed at you. I am merely trying to explain myself and my writing style has a tendency to come off as aggressive or harsh. This was not my intent at all with this reply.

[MotoSook]

Jay,

You'll find that my posts are all harsh! Well they come off like that anyhow. Really, you have to wear your "thick-skin suit" when you start discussing technical matters on this board. Not because folks are going to jump down your throat, but because you don't get the facal expressions and body language through the screen. You know how it is when you start discussing tech topics with a bunch of engineers....oh boy...get a big cup of coffee! Heck I've even managed to piss off Randy a few times before.

Like I stated in my first post here, I love to see folks going down paths never taken. Engineering can be a real bore if all we do is piece together parts that we already know will work. It's like building a lego project by copying the picture on the box! Woohoo!

I have not found one source that lays everything out for CIS. Everything I know about CIS comes from several books and other resources...and from frustrating work on CIS. Heck I should quite my day job and write a book Yep...that'll be a hot seller.

Anyhow, I like that you are chasing the hard truth...and armed with an engineering background! If you were standing here I'd hug you man!

Now, anything you build is not going to be limited like the factory was. Remember they were building cars for the general public..well, the sports car loving portion. So going to more aggressive cams may result in a car that we all like, and the car may behave in such a way that we can endure. Compromises that the fancy Long Island lawyer would hate...we'd all love! (sorry all you lawyers ... I'm not sorry about the Long Island part though ) If you are going to use new pistons, great! That may take car of one of the biggest concerns (valve contact). Now, there is considerable damping in the induction, but I've never seen anyone publish information/data on the use of aggressive cams. I would live to see where your cam project goes!

If you are building a mean street engine....some of the what you take away from a CIS' driveability is probably fine. Think of all the lumpy engines out there...if you have the time and resources...GO FOR IT! Now that I know you are going to open the engine up and do all the stuff....go wild! Then back down...changing cams ain't that difficult.

Something else that occured to me while I was typing this post...

If you want to go another step further...build a damper for the air meter plate/lever. Dampen it just enough to handle the pulses, but still allow it to move freely when it has to as more fuel is needed. That will not be as difficult as the pulsation bottle idea.

[911sTarga]

Souk,

Thanks for the reply and for understanding my writing style! I didn't want anyone to think I was being harsh, hence my many "disclaimers"

As far as the engine build goes, I just want to build power out of the 2.7L CIS engine and I don't understand why it has such a low HP "limit". A switch to carbs or a different EFI removes said limits, so I'm just curious as to why. From the way this thread has gone so far, it looks like it might be a pretty heated topic too. Should prove to be fun!

Originally posted by Souk
If you want to go another step further...build a damper for the air meter plate/lever. Dampen it just enough to handle the pulses, but still allow it to move freely when it has to as more fuel is needed. That will not be as difficult as the pulsation bottle idea.

I'm concerned with weight and its affects on the balance of the lever. I'm thinking a different sensor plate (lip lower then center of plate) or a different arm might just be the answer to the majority of the problem.

Since this isn't a vacuum problem associated with the CIS (MAP systems), then cam choice should be larger then what people think. It sounds like it is more fuel control at different RPM and cylinder pressure levels then anything else.

The biggest hurdle will be fine tuning the CIS at all RPM points. Just like a carb, this is a mechanical induction, so hard parts will need to be changed/modified to tune the beast. This might mean new springs for the regulator, maybe a new plate/lever, and who knows what else? I think if we can fully understand how small changes in the lever/springs/plate/fuel pressure affect the tune, then we might be able to see much larger HP numbers with the CIS system as well.

Well, this is just my .02 anyway!

Thanks!

Jay

[MotoSook]

I would not mess with modifying the air plate lever. Like I said, it’s balanced, and it’s designed to react such that air and fuel are within acceptable ratios. Altering the level will require that you calibrate it with the air plate (and air meter throat) and the fuel plunger travel.

You could space the plunger (fuel dist.) closer to the air plate to increase the plungers travel relative to the plate’s travel, but even that will hit a limit…(the plunger/fuel dist limit). That would require a modified lever so that the roller on the lever moves with the plunger. You’ll also have to work out the force that’s required to overcome the fuel pressure force on the plunger. Bosch spent a lot of R&D money to develop the system. Even re-engineering will take considerable effort and resource. (If you have both..spread it around man…I’d love to quit my day job and play with stuff like this!)

A little CIS trick that I’ve yet to quantify (although my 3.0 runs exceptionally well, if I can boast a bit) is to lower the air plate by lowering the stop under the air plate, the idle mixture then has to be adjusted by raising the plunger back to the “before” position. You are essentially lowering the air plate w/o lowering the plunger. The result is a slightly quicker/and higher rising rate of the plunger. Like I said, I have yet to quantify it, but with all the things I’ve done to my CIS engine, I’m not aching for carbs or EFI just yet. (This is the first time I posted this...so if anyone wants to do this, and has free dyno time I would love to get some data! Unfortunately, my track car has been out of commmision for so long..and I've not have any time to play on a dyno )

Again, fuel quantity will not be your problem…dynamic fuel control (quality w/r to AFR) will be a problem as you go toward more aggressive cams. The big hurdle is designing a cam that creates just enough intake pulses yet will still allow for a smooth and “linear” air plate operation. You can alter the fuel control pressure with a modified warm up regulator (WUR). Unless you change the air intake quantity, you’ll run rich all the time. We know running rich is not the answer to more power. I’ve been playing with a dynamic regulator idea that can be set for an “ideal” idle control pressure and as the RPM’s change or as boost changes, so does the control pressure. A lower control pressure will get more fuel into the cylinders. Get the idea? The stock WUR is too simple to work the way I want it to work with a highly modified engine…or a boosted engine. You’ll need feedback.

While you are at it, you’ll also want to massage the heads, change the intake runner ID…the list goes on.

I admire your out of the box thinking, but if you are going to spend all the effort and resources…why not go with EFI? OK…I didn’t say that…wait! I did say that. I also said that there are limitations to an engine running with CIS. That’s why MotoMotions is developing carburetors and EFI. Search ITB EFI. The EFI kit will surprise you from a performance and cost perspective.