Technical CIS operation & Cam Timing w/ CIS Discussion

[911sTarga]

Souk,

That is basically what I'm thiking as well. We have the same idea, but discussed making it happen using two different methods.

We might be able to machine down the "stop", but we'll have to figure out how much we can remove and what angle to cut at (the plate moves on an arm, so the lower the plate goes the angle of the plate will contiue to change. That plate to stop seal is important for idle quality).

Another option might be a different sensor plate like I said. The weight would be the same, but maybe it'll have a "step" on the lip to allow the arm to sit lower? I'll have a better idea on which way to go once I tear into the system and actually hold the pieces in my hand.

I've designed EFI systems for vehicles before, especially the inline 6. I usually run with the Fel-Pro unit (F.A.S.T. now) for control and then one-off everything else. I'd really like to avoid swaping systems and I don't mind "tweaking" the CIS. Actually, I think it's just my sick mind wanting to tinker and make the CIS work where everyone says it can't.



Thanks!

Jay

[kstylianos]

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?)

83 SC (Alucil p/c's) with 964 cams, .25 milled off the heads and running 1.0mm deck height measured about .25mm over min p/v clearance.

[911sTarga]

After looking at the diagram, I think the easiest option might be the "cam" idea on the arm. Take a look at the diagram below and how the idle adjustment arm has that round stand-off on it. By changing the height of this stand-off (smaller), we can actually get more range of motion out of the arm. This might be easier then a new sensor plate and/or cutting down the plate "stop".

It is either that or maybe a new plunger? I'm thinking the "ports" in the plunger might make all of the mods useless....now that I look at it. A new plunger might be needed for these mods to work.

Thoughts?

[MotoSook]

That's tight Charlie...1.25mm. So I guess, CIS pistons with 964 cams is the limit w/r to lift?

[MotoSook]

Jay, the roller under the plunger allows for movement w/o off-axis forces. A cam solidly mounted on the lever will induce some off-axis force on the plunger. Will it hurt? maybe not, but the plunger-fuel dist. tolerance is tight...tight enough to prevent fuel from coming out of the plunger.

Like I side...it's not fuel quantity you need to worry about.

[911sTarga]

Has anyone ever flow tested the heads? Is more lift really needed? If the heads don't flow more with higher lift, then we shouldn't run more lift. We can change the ramps, nose, and profile of the lobes to help flow.

[kstylianos]

Min intake is 1.50mm, min exhaust is 2.00mm. The 964 cams timed at 1.8mm measured about 1.75mm intake and 2.25mm exhaust. Still some room to play, especially if you want to play with cam timing as well to make more room for lift.

EDIT: FYI the webcam 20/21 grind and Elgin SuperC2 (both very similar grinds) both have a tad more lift and duration than the 964 grind. These things scream in a 3.2 and also work well in a 3.0. Don't have clearance figures for these though.

[911sTarga]

Originally posted by Souk
Jay, the roller under the plunger allows for movement w/o off-axis forces. A cam solidly mounted on the lever will induce some off-axis force on the plunger. Will it hurt? maybe not, but the plunger-fuel dist. tolerance is tight...tight enough to prevent fuel from coming out of the plunger.

Like I side...it's not fuel quantity you need to worry about.

We can still make it a roller, but with a smaller O.D. then. Again, these are just thoughts.

I'm not so worried about fuel quantity, but more so about fuel control over a wider range. The CIS has a set "range" it works in and you can adjust within that range for rich/lean conditions. If we can open that range up a little then we might see the improvement that we need/desire?

The more I look the picture, the more I'm starting to think the answer is in the plunger/spring.

[MotoSook]

Charlie...I just got some 964 cams in...maybe I should have gone with 20/21..but would the 20/21 pass emissions? I still have another set of cams I can send out for regrind.

Quick Jay...get to work on the cam! The fuel is simple...just get me some wild cams to work with my DME 3.2 Carrera! And still pass emissions!

[kstylianos]

Charlie...I just got some 964 cams in...maybe I should have gone with 20/21..but would the 20/21 pass emissions? I still have another set of cams I can send out for regrind.

What kind of exhaust are you planning on using (SSI's, OEM w/cat)? Cats can hide ALOT and you can most probably pass smog with either cam, a healthy cat, lean idle co% and stock timing (25 degrees btdc). With SSI's its a toss up with either cam, but I wouldnt expect it to pass. The 20/21's are a bit more emissions unfriendly.

BTW: If you want hotter cams, I'd send the cores to Camgrinder and have him grind you the similar 20/21 grind for $500 cheaper.

[MotoSook]

Yep...John did all my valve train work. He made the 964 cams for me. I did ask him about my options for cams with emissions being a factor. I guess I have to build a spare engine and swap it in every two years

[928ram]

FWIW, most CIS systems have the capacity to handle more to much more air/fuel than their orig. installation. As an example, there was a fellow running 7psi supercharged on his 4.5L 928 using the stock injection system; I believe he even sells a kit for this application. The guy who used to care for the injection on my 928 before I took that chore over was racing some VW Rabbit/Golf with CIS - same thing, highly modded, but stock injection components.

Now whether this relates to 911 apps or not..

[patkeefe]

I'm a little dumb in my insistence to hot rod my CIS ( "it can't be done!!") The crux of it is the atomization due to the fuel hanging around the port for a while, compounded by the cam overlap.

Pat

[911sTarga]

I don't doubt that at all, 928ram. In fact, it is what I'm hoping for! Basically, I'm trying to get to the bottom of why larger cams can't be used with the CIS and why the 2.7L CIS engine is limited to about 200hp.

I think I understand the "s" cam statement now, as it would induce reversion in the intake system. I'm curious if anyone has tried to increase exhaust duration and leave the intake side alone? (I'm not forgetting overlap, etc...). Remember, this cam was designed to promote high port signals for the carbs. Not a good idea when running a single plenum....especially the small one found on the CIS system.

Reversion can be avoided by changing cam timing, increasing plenum size, improving exhaust flow, etc... Obviously, anything you can do to scavenge the exhaust and reduce overlap will aide in stopping reversion. There are side effects though, so modifications must be done wisely.

Does anyone know the exact specs on the "s" cam and the "964" cams?

Opening/closing events -
Intake/exhaust duration -
Intake/Exhaust Lift -
LSA -

Also, has anyone every tried using a higher ratio rocker on the exhaust side (if even possible)?

Maybe this whole CIS thing is WAY off and it isn't the actual system that is bad! Maybe some fine tuning to reduce reversion while using larger cams is the actual answer. Is reversion the major problem then?

Again, I'm still trying to have someone step up and say EXACTLY what is limiting the power output the CIS system? Why is it well known that 200hp is about all you'll get from a 2.7L engine with a CIS, but you can make almost 300hp by changing the induction? After more research, I'm guessing it is a reversion problem only and the camshaft design was never addressed.

[patkeefe]

I'd guess limiting HP is around 500...the turbos ran CIS

[911sTarga]

Here are a few quotes that I found -

I think valve overlap or cam overlap may mean different things to different people. For example, my S cam allows the intake valve to be open at top dead center. Most high-perfomance cams have this characteritic. Some even have both valves open at top dead,hence the strange, lumpy idle many muscle cars have. It's not so great for idle characteristics but is really nice at high rpm.
(It's also why it's a bit harder to get a MFI "S" car to idle really nicely that a "T", the camshaft profile favors idle conditions a bit more than the S.) This is produced by a single camshaft, it just depends on where the lobes are.

A CIS car cannot tolerate this condition because as the piston comes up to top dead, if the intake valve is open it causes the
sensor plate to go crazy, or flutter. This just does not work with
a CIS system. So, what I'm referring to when I say valve overlap is very possible with a single cam system, it's just a matter of where the lobes of the camshaft are. Take a look at some 911 cams and you'll see quite a variation in profile from,say an early 6 cam to a later CIS cam.

So, for a CIS car, trying to change cam timing can be somewhat counter-productive. CIS cars like to be set pretty much at stock cam timing.
S cams also have a fairly narrow range of settings as they are opening the valves so far that interferance can occur.

Anyway, all of this discussion is good because it gets people thinking. If I'm wrong, correct me. I'm glad to learn-that's why I spend time on a site as good as this one.

and another -

You are absolutely correct about CIS motors not tolerating overlap, but the overlap you are refering to is a function of cam design, not the advance setting for that particular cam. Porsche used the same cams from 1978 to 1989 in the 911 with 3 different cam timing specs. Changing the timing does not affect the overlap- it has to do with the spacing of intake and exhaust lobes to one another.

In '78-'79, intake began to open @ 1deg. BTDC, for '80-'83 it was changed to 7deg. BTDC, (with all corresponding openings/closings moving 6deg., of course), for 3.2 motor they split the difference. This was done in 1980 to improve low-end grunt, but motors lost a little in upper range. Anyone who wants to can change theirs in either direction within limited range, but performance characteristic change is not dramatic, and power output does NOT change. Changing to 964 or 20/21 grind is another story, however, same overlap but different lift/duration. That will add horsies, especially w/ exhaust change,(SSI's), and other upgrades- higher CR, etc.

Good finds! So, we're on the right track here with overlap/reversion. The direction now slight mods to the plenum volume/exhaust flow/and cam profile.

I think that 200hp limit is going to go bye-bye real soon!

[klatinn]

Hi,

I haven't seen anybody mention the biggest performance problem of CIS (I have 2 CIS cars and I like CIS for its simplicity and robustness):
It meters fuel more or less by air mass. It measures air mass by putting a circular plate square in the air stream. The aerodynamic resistance of that plate is (more or less) proportional to the air mass flow and used to move the plunger. But that plate is also a big intake restriction. Basically the volumetric efficiency of the engine is limited by it, no matter how well you make it breathe downstream. And as the plate has basically the same aerodynamic resistance in either flow direction (unlike a flapper style MAF sensor) it is sensitive to intake reversion. Porsche (and other CIS users) use the rubber boot between throttle body and CIS plate throat as pulse damper, but that works only so far. I actually measured the effects of that rubber boot on AFR when switching between a new one and an old and hardend one, and they were measurable at certain RPMs. The old boot was still airtight, so it was not leaks but more of the intake pulses getting to the plate.

Regards,
Klaus

[safe]

Originally posted by 911sTarga
...why the 2.7L CIS engine is limited to about 200hp.

I would be happy with 200 Hp! How do I get that with CIS?

[911sTarga]

Originally posted by klatinn
Hi,

I haven't seen anybody mention the biggest performance problem of CIS (I have 2 CIS cars and I like CIS for its simplicity and robustness):
It meters fuel more or less by air mass. It measures air mass by putting a circular plate square in the air stream. The aerodynamic resistance of that plate is (more or less) proportional to the air mass flow and used to move the plunger. But that plate is also a big intake restriction. Basically the volumetric efficiency of the engine is limited by it, no matter how well you make it breathe downstream. And as the plate has basically the same aerodynamic resistance in either flow direction (unlike a flapper style MAF sensor) it is sensitive to intake reversion. Porsche (and other CIS users) use the rubber boot between throttle body and CIS plate throat as pulse damper, but that works only so far. I actually measured the effects of that rubber boot on AFR when switching between a new one and an old and hardend one, and they were measurable at certain RPMs. The old boot was still airtight, so it was not leaks but more of the intake pulses getting to the plate.

Regards,
Klaus

I can actually flow the entire system and I plan on doing that rather quickly. Once that is done, we'll be able to see the power numbers the system could "support".

So your Air/Fuel ratio changed quite a bit after seitching the boot alone? How did you measure the change?

Also - Thanks for the info!

[911sTarga]

Originally posted by safe
I would be happy with 200 Hp! How do I get that with CIS?

I'm still working on that myself. Those numbers are quoted from other posts on this forum.

If we can get enough data, then we'll be able to see what we can and can't do before we spend the money on modifications. That is the point here!

Jay