Need help/info. on running Lambda system

[tuned6]

Thanks in advance for looking this over.
I have a 1974 911 that was upgraded to a 1980 3.o from the original owner before I bought the car. The lambda unit was not transferred over to the '74 and the exhaust was backdated, kept original (early '74 car).
The car really runs well once warm but I keep wondering what I may be missing out on by not running the lambda.
I see that there is a difference in the '80 and 81,83 units and was wondering if I would want to upgrade to the later unit with the cold enrichment, etc. or would I need to or want to stick with the '80 lamda?
Should I run lambda at all?
Thanks Pelicanites for your help!
Christopher

[psalt]

Hello Chris,

What you are missing is a ability to tune the cold running and WOT performance mixture and still have the system correct to stoich for good mpg at cruise. Do you have the 80 CIS with the frequency valve still connected ? The system was designed to have the frequency valve pulsing in the middle of the range, with the O2 sensor signal trimming the mixture back to stoich under 35% throttle. If you still have the FV and it is not being pulsed by the ECU, the mixture will be very lean, and the mixture adjustment will have to be altered to compensate. This shifts the entire fuel curve. The best advice would be to test you AFR at cruise and see how far from 14.7:1 it runs. If you do not have a cat and do not care about mpg, you might not be missing much if you are happy with how it runs.

[longhornchris04]

First off, nice set of wheels you have there - the car they are riding on ain't too bad either

Now, on to your questions. Paul covered most of it.

CIS once warm uses 2 things to control the amount of fuel going to the engine. (During warm-up there's a few other things in play) The more air into the engine, the higher the plate. The higher the plate, the more fuel goes into the engine. The air-meter is funnel calibrated to ensure that when set properly at idle with the right control pressure the proper air-fuel mix is provided for the various operating condition (this is why you don't want to swap the air-meter/fuel regulator without checking first).

The other side is the control pressure (CPs) which counter-balances the air metering plate. Higher CPs result in leaner mixtures because the counter-force on the plate is higher. The lower the CP, the richer the mix. The CP is controlled by the Warm-up Regulator (also called CP regulator in some Bosch material). The WUR provides warm-up enrichment and then maintains the proper pressures once warm. In pre-80 CIS, the WUR also has a vacuum adjust that provides a leaner mix at partial load (fuel economy & emissions), and a richer mix for higher loads (WOT).

The later CIS used a Lambda system (yours included). There are 4 main components - an exhaust gas O2 sensor, a computer, a 35% throttle position switch, and a frequency valve (FV). The frequency valve reduced the control pressure. The Lambda had two modes - open loop and closed-loop. The open-loop system ran the FV at about 70%, providing enrichment for either increased load (throttle > 35%) or warm-up. In this mode it ignored the O2 sensor. In closed-loop, the FV is pulsed as necessary to achieve proper AFR using the O2 sensor for feedback (generally 50% duty cycle). This functioned much like the vacuum-adjust WUR but more accurately, especially if out-of-tune.

As for warm-up, your WUR was calibrated to have the additional enrichment provided by the Lambda, but if your car is working now w/o the lambda hooked up then someone has already modified your WUR (or replaced it with an earlier model). Additionally, the extra cold-start mods in the later CIS systems weren't tied to the Lambda, but rather the air box assembly, so you would have to change that too.

So, what does this all mean to you. Well, without the vacuum adjust WUR or Lambda, you won't get the fuel economy benefits at partial load w/o risking being lean at the top end. If this is a weekend fun car then no big deal... if its a daily driver then the decision might be harder. Additionally, if you want to re-add the lambda, match the years.

Just note that you won't be able to use the Bentley guide to set your control pressures as they will list the CIS + lambda specs.

Personally, I wouldn't mess with it. I have the pre-lambda CIS and it works well enough. Your vehicle should be emissions exempt pretty much everywhere, and it won't increase performance or drivability in any meaningful way.

[psalt]

The later CIS used a Lambda system (yours included). There are 4 main components - an exhaust gas O2 sensor, a computer, a 35% throttle position switch, and a frequency valve (FV). The frequency valve reduced the control pressure.


This is incorrect, the frequency valve (FV) does not effect control pressure, it adjusts the mixture in an entirely different manner. The FV is a solenoid valve, like a pulsed EFI injector, that acts as a variable pressure restrictor to control the fuel pressure in the lower chamber of the fuel distributor. It alters the pressure differential across the metering slits and adjusts injector delivery for any given plunger lift. The control pressure remains constant and the airflow sensor does not move. The lambda fuel trim is more subtle, and it controls part throttle, low load cruise mixture with more accuracy. A correctly functioning 81-83 system, once you understand it and have a some experience tuning the open loop mixture, works remarkable well. And it will talk to you.

[longhornchris04]

I stand corrected. Thanks Paul.

[tuned6]

Thanks guys for helping me out. So, the way I am understanding it is the lambda really controls the cis fuel distributor starting with my '80 engine and this distributor is recalibrated from the earlier type to only be used with the Lambda. I must not be experiencing the potential of this engine that I've invested in, and now, thanks, I know I want to put a cherry on top and do the lambda.

May I benefit from using a 81-83 setup on my car? I have an '80 engine and I have read that they incorporated a few different components on the later system. I would imagine these would be an upgrade if I can install them on my engine, or were they to satisfy emissions requirements?

I do have the '80 Frequency valve within the fuel loop, the black throttle switch which comes out of the throttle shaft, and I have a short part of the harness which connects the switch, fv, and I think lambda 8 pin connector for the main Lambda harness, which I am missing, down to the control unit.
The Wur I have is the vacuum type. Is this correct for '80? Is there a reference for part numbers that anyone knows of so I can match this guy up?
Were these warmed sensors already, or is this a doable upgrade when I install?
Thanks so much! A few of these questions answered and I'm off to the used parts forum to find a unit and main Lambda harness.

Thanks Longhorn C for noticing my Fuchs… I feel very fortunate to have found them, finally! I am really loving the car. A few details here and there and I'm ready to start fixing up the next one .

[psalt]

The Wur I have is the vacuum type. Is this correct for '80? Is there a reference for part numbers that anyone knows of so I can match this guy up?

Were these warmed sensors already, or is this a doable upgrade when I install?


My advice is to first determine what was done to the engine during the conversion. There is a part number on the WUR and you can find the correct number on a BOSCH website and by searching here. Also verify the engine number, fuel distributor, ignition distributor and fuel pump type. A hose to the WUR does not always mean vacuum, it is probably a vent line, unless it was changed. The vacuum lines are often mixed up on later cars, it is best to know what the device does and why it needs ported or manifold vacuum or a vent, before working on the engine .

After you determine what you have, you can figure out what it would take to put it back to the way it was designed to be. If you want closed loop feedback control, you will have to drill a hole in the exhaust and fit a threaded bung for the O2 sensor. If you need to buy new parts, this may be an expensive proposition, you may be better off buying a complete used system with ECU and harness from a carb swap owner. The 81-83 system is better, but the main difference was to reduce the EPA warm up cycle emissions and the add ons help cover up the leaner calibration. You are already without a cat and the benefits would be small.

Paying for a single dyno run with an AFR curve would tell you how well your current setup is working.

I don't know what you mean by "warmed sensors"

[tuned6]

Thanks again for the info. By warmed I meant heated o2 sensor. I know that Bosch went to a heated o2 sensor eventually for regained accuracy with cooled down exhaust gas at long idle, and also, systems with them would regain accuracy sooner after start up.
I know that I've seen aftermarket 2 1/2" air fuel gauges that come with it as an option.

[psalt]

The CIS lambda uses a single wire, unheated O2 sensor, which is all you need, unless you are are trying to pass an OEM emission test. AFR gauges that use a narrow band sensor, even a heated sensor, are only accurate around stoich and are not accurate enough to tell what is going on at WOT. Borrowing a wide band meter would be ideal for your situation.