Question on CIS AFR at Full Throttle and Low RPMs

[David Borden]

I recently hooked up my LM1 Wideband O2 sensor to my 74 911S with CIS.

It runs a around 14.5:1 AF during cruise/idle and around 12.5 at full throttle near redline.

However, full throttle at lower RPMs starts around ~14:1 and makes it way to 12.5 at redline.

My concern, is that it is running a bit lean at full throttle at lower RPMs. Is that normal for CIS?

Is part throttle AFR adjustable on CIS? If so, what is involved?

David

[911st]

My best understanding is most the US CIS Porsche WUR's did not have provision for riching the AFR's with acceleration.

Some other makers and some of the Porsche euro CIS cars had a vacuum hose to the WUR and when intake vacuum lowered they would lower control pressure which would then richen the AFR for acceleration.

Many CIS track cars set the AFR a bit fat as there is more power at about 13/1 AFR. The down side is less MPG.

Probably all you can do with your car is adjust the base AFR, play with the spring tension in the WUR to effect control pressure so it gets progressively richer, leaner,or more stable with changes in air flow. You can also play with the system pressure which is determined by shiming the regulator at the exit of the fuel distributor head. Higher system pressure will increase AFR's with increases in air flow, lower the opposite.

You might be able to source a WUR from a euro Porsche or some other CIS car like the Audi and set it up to do as you wish. The advantage here is you can maintain AFR's in the 14's at idle and cruse for good MPG, then with acceleration the AFR's should go to about 13/1 for better acceleration. Also, the lowering of the control pressure with acceleration reduces the restriction at the metering plate for better throttle response.

The hot set up now for CIS is a digital WUR where you can program and adjust you AFR's at all rpm and load points. I built my own programable WUR for a 91 turbo I had and it made a significant differance in throttle response when I lowered control pressure quickely and significantly with acceleration to get the metereing plate moving then would bring CP back to keep my AFR's where I wanted them.

[911st]

PS, if you are interested in going to a WUR that responds to acceleration you might check with Larry at CIS Flow Tec. He might be able to suggest a WUR and set it up with control pressure settings that might work for you.

I would also look into if any of the euro 2.7 WUR's had idle and acceleration control pressure function.

I think CIS can be a great fuel delivery system with some tinkering.

[lucittm]

David,
You have a 1974 car and you hooked up a digital air/fuel ratio computer that the engineers could not have dreamed about in 1974. It reads perfectly at idle and maybe .1 too high at full throttle. I say count your blessings. Your CIS, WUR, MAF, and hundreds of other minor parts are working as designed on a 36 year old car. Sheesh.

I had a 1976 911S and the car ran great as I assume your does. Drive it and don't worry about your AFRs being "a bit lean at full throttle at lower RPMs." If it bothers you don't go to full throttle until the RPMs come up a little.

Mark

[Glen Pettigrew]

Isn't 12.5 RICH, rather than LEAN?

The 74 CIS system has an adjustable "throttle position regulator"
that richens the mixture in the mid-range.
You might want to try adjusting it to get a leaner
mixture at full throttle.
However, as Mark said, if it runs well, LEAVE it alone!

Glen Pettigrew

[911st]

Running in the 14's at idle through 3k or so would be fine for mileage and emissions.

Max power is reported to be at about 13 to 13.2/1 on a 911 motor.

However for max power is would be nice of after 4k rpm if one could get to near 13/1.if possible if power is important to you.

Peak efficiency is at TQ peak as is sensitivity to detonation. It is ok to be a little fat at TQ peak but at HP we want to be at about 13/1 for max HP if possible.

Still, CIS gives better and more stable AFR's through the rpm range than most carb or MFI motors. Just depends on your goals.

I am not sure how sensitive we really are to changes in AFR.

Looking at two of my old 3.2 Carrera dyno runs, my AFR's and TQ at 5500rpm were:

13.7/1 AFR = 192.6 ft lbs (lean)

12.4/1 AFR = 190.4 ft lbs (rich)

I have to say both were at less than ideal AFR's. I wish I had nailed 13.1/1. I wonder if I left much on the table or not.

There is a significant amount of opportunity on the ignition side and often there is more power and response to be found there.

[T77911S]

Originally Posted by Glen Pettigrew

Isn't 12.5 RICH, rather than LEAN?

The 74 CIS system has an adjustable "throttle position regulator"
that richens the mixture in the mid-range.
You might want to try adjusting it to get a leaner
mixture at full throttle.
However, as Mark said, if it runs well, LEAVE it alone!

Glen Pettigrew

how does this throttle position regulator work?


my thoughts exactly. it runs well, leave it alone. you will never feel or see any difference in making it richer at low rpm's on a stock 2.7. even if you did, the cost would not be worth it. as long as you are not going lean and damaging the engine, you are getting the best out the cis you have.

i did not know the 74 cis had anything to make it run richer. i thought the shape of the AFM provided for extra enrichment at hight rpm's.

[Glen Pettigrew]

"Throttle Position Regulator" is actually called:

Control Pressure Regulator
(Throttle Valve Position)

Per book:
"The control pressure regulator for throttle valvle
position is basically similar to the regulator for
warm running compensation (WUR). It is attached to
the throttle vavle housing.

A cam plate is attached to the throttle valve
shaft and rides along with it, compressing the
regulator spring to a greater or lesser degree.

This changes the control pressure along with
changed throttle vavle position, adjusting
fuel/air mixture as required."

The specs show that it leans out the mixture
(raises the control pressure)
in the mid-range position. (my previous post
was wrong when I said it richens the mixture
in the mid-range)

Glen Pettigrew

[911st]

Interesting device.

Is it plumbed in line between the WUR and top of the Fuel Distributor? Maybe between the WUR and return line?

Dose the cam action increase control pressure only in the mid range (cruse) of the throttle angle and cease it's function at WOT?

Can anyone post a digram.

Thx!

I love CIS.

[ossiblue]

Originally Posted by 911st

Interesting device.

Is it plumbed in line between the WUR and top of the Fuel Distributor? Maybe between the WUR and return line?

Dose the cam action increase control pressure only in the mid range (cruse) of the throttle angle and cease it's function at WOT?

Can anyone post a digram.

Thx!

I love CIS.

It is plumbed in line between the top of the Fuel Distributor and the WUR, as you speculated. There are actually eight lines coming from the 73/74 Fuel distributors--six to the injectors, one to the throttle position regulator, and one to the WUR. The line to the throttle regulator shares the same port as the line to the WUR.

edit: Corrected by Glen, plumbed between FD and return--I was just thinking of the inlet and totally forgot about the outlet! Brain freeze. Thanks Glen.

[Glen Pettigrew]

Plumbed between Fuel Distributer and return line,
Just like WUR.
Increases control pressure at mid-range.
Does little to nothing at idle or WOT.

See pictures.

Glen Pettigrew

[911st]

Glen,

Thank you for the great info and digram!!!

Way cool stuff.

First the “Vacuum Controlled Version ":

This basically lowers control pressure with acceleration from about 3.6 bar at warm idle and cruse to 2.9 with acceleration.

This keeps the AFR’s at a more efficient level at idle and cruse and richens them to better AFR’s in support of acceleration.

In my opinion this is the better system and what I was trying to describe above.


The “Throttle Position Version” just lowers control pressure at part throttle.

Looks like it acts more like an ‘accelerator pump’ on a carb to reduce ‘lean surge’ off idle.

However, it does nothing to support better AFR’s beyond part throttle like the vacuum sensing WUR noted above.

The “Throttle Actuated Valve” is basically a bypass valve that passes pressure around the WUR and lowers control pressure temporarily.

In principle I did the same thing electronically some 8 years ago before there was a Digital WUR using a frequency valve (basically a fuel injector) and a custom controller I had made for my CIS turbo so I could tune my AFR’s at all RPM and load (manfold vac/pressure) points. Here Porsche is just doing it mechanically and just off idle.

Adjusting the TAV could help off idle throttle response but not much else.

One could in theory reprofile the cam on the valve. If someone tries this they would want to try to make sure they keep cruse throttle angle part of the cam in tact if they want to maintain good MPG's.

After about 50% of throttle angle I suspect one could reprofile the cam if more fuel is desirable for acceleration assunming one's wide band determins this is needed and one can not adjust to one's needs in the other ways.


Way cool, something new to learn about CIS.

[Jay Laifman]

I was just reading this, in search of other info, and thought I'd add a bit. The Throttle Valve Position Sensor/regulator (aka TPS) does indeed have a cam in it. As indicated, the cam has three zones - the idle/low rpm zone, the mid range zone, and WOT zone. The idle and WOT zones are narrower than the mid range, but they are not just "only" idle and WOT. So at light throttle, you'd still be in the richer zone.

Another thing to note is that the TPS mounts on slotted holes. So it can rotate a little bit. I have never seen exact instructions on just how to set it. Presumably, if you want the idle zone to be shorter or longer, there is a way to do it. Rotating it CCW would shorten the idle range, and bring on the leaner conditions sooner.

But, it might take a wide band O2 sensor and driving around with adjustments to see whether it has a meaningful impact. Guess what I just ordered ...

[eastbay]

Pretty easy to go through the throttle valve, probably a good idea after 50 years.

pic from DDK, I didn't take pics when I went through mine



From the factory manual

[Jay Laifman]

FWIW, I actually purchased a new TPS from Porsche. It did make a difference. Yes, they still have them.