DME Motronic - O2 sensor questions

[hcoles]

I hope this is the right forum for these questions or do I need to send them to Steve Wong? Anyway here goes....

O2 Sensor - I assume the way this works is that the O2 sensor reads zero at 14.7AF and the DME adjusts the PWM for the injectors up and down to get the O2 voltage to read around zero. This happens on most of the map until the WOT switch goes closed and then ?? happens?

As I understand it the O2 sensor is then (when WOT is positive) disregarded and the PWM is calculated from the map and at some point in the rpm range the PWM goes to full rich (the actual PWM for that is ?)

When the DME box gas quality adjustment switch is set to say position 2 (3% rich) or position 3 (6% rich), how does the O2 sensor still work? It would then be running at say 13AF so it would not be able to swing back and forth across zero to control the PWM because its reading would be off scale. How does this work?

Thanks,

-Henry

[ChrisBennet]

What a great question!
-Chris

[hcoles]

I hope some guys will weigh in on this...not the usual rod bolt upgrade discussion.... that's why I suggested we have a CSI/Motronic section on this board....we'll see...

[hcoles]

I went and re read descriptions in 911Chips (Steve Wong).
It looks like the voltage "trim" is applied to the O2 sensor to make the sensor act like it is trying to "control" to a new or different AF. E.g. add 1 volt of trim and the "zero" for the O2 is 13AF and not 14.7AF.

When you activate the WOT switch the DME goes to a completely different map and doesn't use the O2 signal and some other sensors.

What is not clear is that Steve states that while on this WOT map at some point no more fuel is added and you can run lean. What I want to know is when does this happen? Do you run out of PWM or ? Why would the map stop being in play?

-h

[PPilot]

This is a good discussion! Am I correct in thinking the Euro-RoW 84-89 engines do not use the O2 sensor?? What changes did they require to legalize them when brought to the U.S.??

[EfiOz]

The O2 sensor reads roughly 450mv at stoich (lambda 1). The sensor output scales from 0-1V with 1V being as far as they'll read rich (lambda 0.70 I think) and 0 being full lean (lambda 1.25 from memory).

Here's the catch. Around lambda 1, the output changes very suddenly for every point of lambda. So the effect is that it outputs a continuosly swinging voltage of between 600 and 300 mv. The ECU reads this as being lambda 1 and uses it to correct idle mixtures and sometimes steady state cruise mixtures. It's pretty inaccurate but it works close enough to fool the authorities.

Bear in mind that most smog tests are only at idle or steady state cruise, and very,very rarely in a transient (accelerating) or WOT mode.

The ECU normally kicks itself out of closed loop control and into open loop and some preset amount of throttle opening, or a switch by the sound of it in this case.

"Full rich" injeciton pulse width would be around 80-85% duty cycle which could be as much as 32msec depending on the rpm your at. Injector control is infinite between the low end being as small as the ECU can control and the high end whichis dictated by total cycle time available (i.e. 20ms @ 6K rpm, etc)

[Steve W]

Review the following chart on how a healthy narrow band O2 sensor operates:



As you can see, a narrow band sensor is most sensitive at the stoichiometric ratio of 14.7:1 (lambda=1), ratio for best overall emissions. This is a voltage between 0.2 and 0.7 volts and if the voltage exceeds this range, the DME will attempt to adjust the fuel to bring the voltage back into the correct range. The sensor only has influence at idle and part throttle, it is ignored at full throttle.

Now, depending on the program, the O2 sensor has a significant, but not complete influence on the air/fuel ratio at part throttle (no matter what LorenFb says - who is an idiot btw ) Transient enrichment can be momentarily provided where the O2 sensor's influence is overridden for 2-2.5 seconds on a throttle transient, by increasing the quantity/pressure of the fuel injected either by software or hardware. This provides a richer mixture momentarily for power and throttle response. If you disconnect the O2 sensor, the lambda mixture correction will not function, same as open loop, thereby keeping the injector pulses to where they are mapped to. The following chart is an AFR vs. power curve for a standard internal combustion motor – although not necessarily Porsche:



At full throttle, as the O2 sensor is ignored, fuel quantity is referenced directly from the full throttle fuel map, multiplied by a factor of the output voltage from the air flow sensor. However, after 5000 rpm the flapper door has bottomed out and can supply no further voltage increase, so the DME’s injector pulses are solely determined by the map after this point. They maps are always in play and the injectors never stop fueling, no matter how high an rpm you go. So to clarify the statement you read, if you need more (or less) fuel at WOT for modifications or displacement increases, it must be done by reworking the WOT maps, as there’s no other way for the DME to know what the additional air flow increases and fuel requirements are.

[CBRacerX]

Good discussion, I am going to need Steve's help with my 3.8 conversion once it is back together and broken in. I've been suspect as to whether the O2 sensor on my install is even working, I have made sure the connection from the DME to the O2 sensor is intact, but I still seem to run rich (as a 3.6 with a stock map). If the WOT performance doesn't use the O2 (which is what I have always read) then tuning this on the chassis dyno seems quite feasible for a man of Steve's talents.

[hcoles]

Steve,
Thank you very much for your explaination.
This is all making much better sense now.
I need some time to digest your writings and then attempt to produce some intelligent questions if needed.
My focus right now is to confirm that my 89 3.2 is all working properly to start with and then look at making modifications, e.g. one of your chips.
Thanks again for you time and thoughts.
-Henry

[hcoles]

Another thought now after rereading what Steve explains....

After 5000 rpm when on the WOT map or when the air flow pins the door the DME is operating "blind" as there is no direct way for the DME to know the amount of air going in the engine.

In these cases the DME uses the maps developed at Porsche with I guess the primary input being RPM to come up with the PWM values.

This seems sort of scary, because this is where the engine is running much of the time on the track. Steve, I guess this issue is eliminated with your air flow assembly using a hot wire type technology which I assume can still measure the air flow up to at least the rpm limit and if modifications are installed that allow pumping more air it will read that also.

Correct?

Thanks again,

-Henry

[hcoles]

Here is a simple diagram of inputs and outputs to the DME for off idle operation.
I contend that the DME doesn't know anything else than this at a given time and possibly past history. Steve, is this correct?

This will be my starting diagram.
Next will be how I think the maps work and when the DME is calculating the PWM without enough information as Steve suggests. So we will all understand and see the benefits of a different chip. I will appriciate very much if Steve can check as I go.

Also.. Steve, Do I understand that the voltage trim you talk about is a "reverse" EMF applied to the O2 sensor so it will read a lamda of 1 at different than 14.72AF ?

[Steve W]

So far, that is correct, except that the AFM voltage and O2 voltage have no correlation to each other. The standard Motronic system, is only accurate compensating within a narrow boundary of where it is mapped. Modifications such as exhausts and displacement increases will throw it off at both high and low rpms. Our HFM system continues to supply metered airflow increases to the DME beyond the rpm and flow capacity of the most demanding normally aspirated motor, and does it so accurately that almost no changes are needed in the fuel maps when the engine is modified, whether the system is placed on a stock 3.2 motor, or installed a displacement modified 3.4 or 3.5.

The fact is, both part throttle and WOT of the factory chip is not programmed for best power, but for best emissions, and since the AFM can't meter past 5000, the air fuel ratios in the upper rpm zone are more of a WAG for the lowest common denominator. This means low and mid rpm is on the lean side for lowest emissions, and the upper rpm range is way fat to keep the ceramic monoliths of the catalytic converter from melting down under hours of sustained WOT operation on the autobahn at 150 mph, and for the factory to cover their butt. The key here is not as simple as just turning up the fuel, but to provide the correct amount of fuel where it is needed to achieve the optimum AFR. Look at the following remap of a 3.2 motor at WOT as an illustrated example. The engine is a fresh built 3.2 on Superflow engine dyno measured with an industrial grade wideband system, so the results are very accurate:

[kuehl]

So to clarify the statement you read, if you need more (xx) fuel at WOT for modifications or displacement increases, it must be done by reworking the WOT maps, as there�s no other way for the DME to know what the additional air flow increases and fuel requirements are.

The key here is not as simple as just turning up the fuel, but to provide the correct amount of fuel where it is needed to achieve the optimum AFR

Ok. Sounds logical.

So if you wanted to adjust your AFR richer at WOT .....
you can monitor your AFR, say with a wide band display, and if you have enough fuel psi, and a rising rate fuel regulator, you might be able to achieve your objective provided the fuel demand is within the limits of the present system ?

Steve?

[Steve W]

Yes.

[kuehl]

Thank you Steve.

Does anyone know the limitations of the stock 3.2's fuel system in terms of maximum flow (per cylinder).

[hcoles]

Steve,
Thanks very much.
What is disturbing and appaling is that the air flow meter "pegs" at considerably below the air processing capability of the stock motor.

IMHO - this is an unforgiveable product development decision on the part of Porsche - certainly they knew the AFM was limited.

What "saves" the situation is that the situation is "fixed" by setting the maps to run rich so that the motors don't self destruct.

Likely, I guess, is that for minor mods... e.g. cat out and 964 cam the engine will still be fat above 5000 and while leaving hp on the table I have a motor that still doesn't run lean causing the balance in my account transfering to the PelicanParts account.

Another thing that occured to me, trying to get more power by adjusting the fuel quality switch to a richer setting may in fact cause the reverse affect by going from rich to way too rich. I think I will check today to see that I'm at the zero setting.

Steve, also...what brand model or attributes should I look for if I want to get a wide band AF reader considering not wanting to spend a lot and also I don't think I need laboratory accuracy. I would be curious to find out how rich/lean I am at different operating points.

Thanks again for your time.

-Henry

[kuehl]

Steve,
Just looking at your AFR graph shown in the bottom of your
dyno results for the 3.2 87:

I have a 3.2 87 that is basically stock we will say for the moment, and observed my wide band readings (from WMS). My readings seem to be different than yours. For instance, in loop it is typically 14.5 nominal and out of loop never below 12.
Any comments?

[Steve W]

For a wideband AF reader, I'd recommend the Innovate Motorsports LM-1, as it is very easy to use, accurate, and expandable if you ever desire more data capabilities. There is also a less expensive LC-1, but for a display, you would either need a separate gauge or laptop. Wideband systems aren't usually inexpensive because they use a special type of sensor that requires a separate controller to operate it and provide a output.

There can be several reasons for AFR variations among different engines, even of the same configuration. First is the accuracy of the wideband you are using, which we shall assume for purposes of this post to be accurate. Having logged and examined dyno charts of many 911 engines, it never ceases to surprise me the amount of variation I see, even among stock to almost stock motors. It is usually the better maintained motors that are more consistent and read a richer AFR curve, while the poorly maintained ones read leaner (such as some Motormeister rebuilds I experienced). The standard deviation is as much as 2 full AFR points. This can be attrubuted to many factors:

1. Wear and inaccuracy of the AFM sensor. Wear on the resistance track of the AFM will cause dropouts and a reduced output voltage, leaning out the overall mixture. A loose spring tension from heat or age on the other hand will cause a richer low and mid rpm mixture. Renewing the meter by moving the wiper arm up or down will restore the resistance track accuracy.

2. Reduced fuel output from clogged injectors, fuel filter, or weak fuel pump.

3. Low system voltage thus not supplying enough power to open the injectors properly and drive the fuel pump

4. Deviations in the fuel pressure. While the standard fuel pressure of a 3.2 is supposed to be 2.5 bar, they can vary as much as 3 psi from norm. A new one a customer purchased from the dealer measured out at 2.72 bar, resulting in a richer than expected mixture.

5. Intake system air leaks: air leaks past the manifold to head gaskets, injector O-rings, vacuum hose leaks, and rubber manifold sleeves will all contribute to air not passing through the AFM sensor, thus the barn door not opening up as far as it should, resulting in a lean mixture.

6. variations in the intake air temp sensor or head temp sensor from spec

7. quality and amount of alcohol in the fuel used: oxygenated gasolines today have as much as 10% ethanol in them - mixture varies by season and region of the U.S. or world you are in. As alcohol has only about half of the specific output as pure gasoline, it requires roughly about twice the amount alcohol per c.f. of air to produce the same lambda. Thus a 10% ethanol mix is only supplying 95% of the fuel of pure gasoline, with the result being much leaner mixture than intended. Cars using race fuel or from European countries such as the U.K. often log richer than American cars because I believe their fuel does not contain oxygenates.

8. Cam timing

9. Engine manufacturing tolerance that account for flow deviations from motor to motor. The greatest deviation on Porsche motors are in the 5000+ rpm region. There are many duds out there, and occasionally you get a ringer that just jets.

10. Fuel quality switch setting

11. WOT switch activation. I see a high percentage of cars where their WOT switch does not activate either because the throttle linkage is not pulling it open all the way, or the switch is misadjusted - even on cars that are owned by Porsche mechanics that should know better. I used to get dyno runs where as many as a third showed the car never got out of part throttle mode, result being a 20+ hp loss.

12. Engine and exhaust modifications obviously.

It is because of all of this that for peak system performance, all of the above must be maintained properly. They are also part of the reason the factory fuel ratios are programmed as they are - insurance for all the potential deviations. That is why for maximum performance, tuners choose to have their chip or EFI systems custom tuned to their dyno and AFR plots, as it is the only equalizer for all of the above and ensures maximum power and reliability.

[kuehl]

Steve,
I appreciate your detail of what can raise or lower AFR.
I wonder what Porsche's spec is for AFR in the loop? I would presume something to keep "everyone" happy: the fuel miser's, the liberals and naturally their performance minded enginears.

[Steve W]

Closed loop AFR is always 14.7:1, or Lambda=1