DME Motronic - O2 sensor questions

[kuehl]

Originally posted by Steve W
Closed loop AFR is always 14.7:1, or Lambda=1

Reference your AFR graph shown in the bottom of your
dyno results for the 3.2 87:

was from closed loop or WOT?

[Steve W]

Dyno runs to measure for full power are always done at WOT, and so is open loop.

[kuehl]

Oh. You were doing pedal to the metal.

I think I would would want to see what the readings (for various items) would be in closed loop, a full transition up to WOT, and then WOT.

OK, so your graph was entirely WOT.
Thank you.

[2.7RACER]

Henry,
Chris is right, Great question. From Steve Wong, even better answers.
Thanks Guy's,

[hcoles]

Steve,
just to clarify...

you said a few posts back that the lamda was always 1 and the AF was 14.7... this I guess assumes the O2 sensor is in play and we are not on the WOT map.

can the AFM door hit the stop and still be on the "regular" map...and then it could use the O2 sensor to know if it is getting too rich... does this happen? Maybe this would be a good reason to disconnect the WOT switch and make some use of the O2 signal to not run way rich or way lean... why doesn't this work?

I thought that the AF could be adjusted up and down a bit using the "trim" of voltage against the O2 EMF...I think you confirmed that this is how it works but now not sure again.

you also say that the AF can't be altered to run very far off the map.. so I guess that is what you mean by the AF is always 14.7 when not on the WOT map? or it is close to that.

That may not be a very clear set of questions. Sorry, I'm trying to understand a bit better and then make another diagram.

When you install your high tech. air flow meter do you use the O2 sensor and trimming to get the motor to run at 12.6 or there abouts on the new "Steve" WOT map?

My breakout connector and box should be here in a week or so. I'm planning to hook it up and view the behavior of some signals e.g. O2 voltage and I have a connector set coming to make a breakout for the injector ground wire. I'll attempt to see the PWM changing on an oscilloscope while driving around. I looked for a chip that can look at a signal and out put PWM value but didn't find one yet... would be a handy gage to look at to confirm how all this is working. I find chips to generate PWM but not report PWM.

Thanks,
-Henry

[jimbauman]

So hcoles and I just played around with aigel's LM-1. Last time I borrowed it, the best I could get with the Wong chip in my '86 Carrera was a peak low AFR of 13.5 at WOT Looked at Steve's recent list of things that can cause this .... decided to replace the two simplest things that I knew needed replacement - fuel pump and filter.

Today, after replacing those items, we did some more testing .. good results. On my car, I got a consistent 13.0 at WOT from 2500 RPM up to about 5200 RPM. At that point it leaned out a bit to between 13.5 and 13.8 all the way to 6000. MUCH better!! I'd like to get 13.0 all the way to redline - but based on what Henry thinks, I don't know if that is possible. I could send the injectors - they've never been serviced after 20 years and 160K miles. Should I even be concerned about this variance?

On hcoles '89 Carrera with the stock chip, the AFR graph looks just like it should. At WOT, AFR starts off at 13.0 at 3000 RPM and peaks down to 10.89 at 6000 RPM!

JB

[hcoles]

another thing we noticed...on my car with the stock O2 sensor out and the wide band sensor in and just connected to the reader...the car runs very good and the AF is very close to 14.7 without the use of O2 sensor. I think it was even running a bit richer which made the idle smoother. Steve Wong's reports of the stock chip running very rich at WOT and above 5k, or maybe starting a bit lower, are correct.... I do sort of like the feeling that at least I'm not running lean....but I'm throwing away power and gas. I would like to see the car run at about 12.5 or so...just a bit rich... I think Steve says max. power on these cars is at 13AF.

[Steve W]

At steady state loads, the O2 sensor tries to keep the mixture at a lambda of 1. Stock part throttle mapping for the most part is designed to keep the O2 sensor happy at lambda 1. At higher loads and rpms, it is not, nor do you necessarily want the mixture at lambda 1, as it would be too lean, potentially causing damage, and not the optimum ratio for maximum power. Thus after a certain rpm and load point in part throttle operation, the O2 sensor is ignored, however the part throttle mapping still remains in effect. So disconnecting the WOT switch and using O2 trimming will not work. You will also loose at least 20 hp, as part throttle timing maps are not optimized for maximum power, and the part throttle mapping under high load is optimized for acceleration enrichment, and not full power. Part throttle mapping also looses resolution after 6000 rpm, so the AFRs after 6000 rpm will not be optimum and power will fall off a cliff.

On the new carbon fiber HFM5 system, just as the stock system, the O2 sensor is ignored at WOT. WOT mapping is determined either by past statistical data, or ideally from fine tuning off a subsequent dyno or LM-1 AFR chart.

Jim, the 13.0 WOT you achieved is very much where I target for, however the 13.5-13.8 you're getting from 5200-6000 is leaner than what it should be. There is probably another factor or two limiting your fuel output. Whatever you try, if you're not able to correct this, you can always have your chip retuned to provide the additional fuel where you need it.

[hcoles]

Steve,

Thanks for the explanations.
With your new carbon fiber assembly assuming it can read air flow as high as needed...doesn't the DME then have all the info. it needs to know the mass of air coming in and therefore not need "tuning" as it could have e.g. 12.8AF on the WOT map for a given rpm. ?

-Henry

[klatinn]

Hi,

Just as additional info about O2 sensor operation:
The sharp "jump" in the voltage output of an O2 sensor is not actually a property of the sensor itself, but is an effect of the chemistry of the combustion process.
The voltage output of an O2 sensor is inversely proportional to the partial pressure of oxygen in the exhaust and to the sensor temperature. At stoichiometric (Lambda 1.0 or 14.7 AFR for standard gasoline) this oxygen partial pressure jumps from a few pascals to something as low as 10^-15 pascal over a span few hundredth of AFR (going from lean to rich). This causes the jump. The middle of the "jump" is at about 0.45V O2 sensor output (about 10^-5 pa O2 partial pressure). The ECU just looks at that voltage in closed loop and gradually leans out when higher, and riches up when lower. The result is that the measured AFR oscillates around 14.7 2-4 times/second. This oscillation is also needed for correct cat operation. The cat oxydizes CO and HCs to CO2, H2O and H2. For that oxydation it needs additional O2. This additional O2 comes from the reduction of NOx to N2 and O2 and from the excess O2 in the lean phase of the oscillations.
Older 2 way cats (Thermal reactor cats, no NOx reduction) required additional O2 to be pumped into the exhaust via an air pump as on 80-83 CIS.
Because of the sensitivity of O2 sensors to exhaust Oxygen, even a small, pinprick exhaust leak before the O2 sensor can cause the ECU to run the engine way too rich in closed loop, resulting in failed smog tests and lousy milage. The additional O2 from the leak causes the O2 sensor to read excess O2 (lean). This causes the ECU to richen up, trying to overwhelm the O2 leakage. I have seen engines run actual 9:1 AFR in idle closed loop (measured before the leak), smoking black, while the O2 sensor (behind leak) indicated stoichiometric.
That's why many ECUs have a limit to how much they correct by O2 sensor, otherwise even small exhaust leaks could possibly cause engine flooding.

Regards,
Klaus

[Steve W]

Henry, yes the HFM system does a good job of measuring for changes and variations in your motor beyond the flow capacity of a 911 motor. The HFM sensor is much more consistent than the mechanical flow meter which have lots of variation from car to car. However it can't compensate to variations in the system such as fuel pressure or injector flow variances because of the open loop zero feedback condition, so optionally fine tuning from AFR data is the cherry on the pie to ensure that everything is running in as optimal tune as possible.

Klaus - thanks for that very valuable bit of information. Just before your post, I could not figure out why a customer's car idled at what was around 13:1 closed loop according to the LM-1 no matter how much I attempted to lean out the base idle mixture in the flow meter. Open loop, the car's O2 sensor measured out at 0.9 volts and after my feeble attempt to lean it out, it read around 15.7 at idle, open loop. Under load, it appeared to be fine. It never smelled rich and all sensors were checked. I assume that any exhaust leaks will also affect the wideband sensor of the LM-1 and fool it into reading a richer than actual mixture, would that be correct?

[ischmitz]

Originally posted by Steve W
I assume that any exhaust leaks will also affect the wideband sensor of the LM-1 and fool it into reading a richer than actual mixture, would that be correct?

It will affect the LM-1 wideband for sure. An exhaust leak will make the wideband read lean (it detects higher partial pressure of O2 introduced by the leak). In closed loop the leak will cause the ECU to richen the mixture. The net result is a rich-running car...

[scarceller]

I have been playing with my 84 Carrera DME and tuning it. While learning about how it works I decided to log (real time) the AFM signal VS RPM at WOT.

The run was done in 2nd gear starting at 2000RPM and going WOT up to 6000RPMs. I also know I'm in WOT mode because I also log and record the WOT switch and it showed closed/grounded durning the entire run.

In this chart I plotted the RPMs against AirFlow, air flow was measured by recording the AFM barn door position (0-5volts) and then converting the value to a % door opened. 0-100% 0 being closed and 100 being fully opened.



I wanted to share this because this thread (as well as other threads) say that the AFM bottoms out at 4000RPM while in WOT mode. I mean no disrespect to any of the many smart folks on this board but my realtime testing shows diffrent.

So, is my car not working correctly? or maybe we don't understand how the AFM works 100%?

Just thought I'd share my findings.

EDIT: new information learned - it seems that some basic Motronic manuals state that AFM bottoms out at 4000RPM and this may be the case for some Motronic cars but not our 84-89 Carreras. For these cars the bottom out point for the AFM is around 5500RPMs as you can see from the graph above. This means that after 5500RPMs the DME will no longer utilize the AFM signal to calculate fuel, it will rely on RPM, WOT switch and intake air temp to calculate fuel.

[hcoles]

the door doesn' look like it is opening to much more above 5500
I think this is what Steve Wong has said all along - when on WOT and above 5500 the injectors go full rich...I think that is what has been said and is the reason we run too rich
also the volts from the door to actual mass airflow....the algorithm for this is somewhere in the EMU.. likely it is not % of 5 volts.
the interesting thing is if you can record the pwm percentage for the injectors along with this...but I think that is hard to do.. I looked for a chip that would "listen" to a pwm signal and output the percent open time.. but couldn't find that animal

[silverc4s]

Great interchange of information! Thanks to Steve and all for sharing your experience with us.

[Fast951]

If the VAF (AFM/Barn Door) is maxing out over X RPM, why not upgrade to a MAF system with the proper software that understands Air Mass?

[hcoles]

yes, by all means... I think Steve W. sells a system that has all that figured out. Actually measures the maf up to the limits of what the engine can eat and adjusts the AF to what you want... there's a concept... that apparently was missed back in 1983 or so by Porsche.

[Fast951]

Originally Posted by hcoles

yes, by all means... I think Steve W. sells a system that has all that figured out. Actually measures the maf up to the limits of what the engine can eat and adjusts the AF to what you want... there's a concept... that apparently was missed back in 1983 or so by Porsche.

I don't think I understand what you mean by "measures the maf up to the limits of "?

If the transfer function inside the DME software is rewritten to understand the MAF signal there should be no limit on the MAF signal.. We have used the same MAF sensor with the appropriate DME software, on a 400rwhp turbo engine, as well as a stock 911.
I'm not a regular on this board, feel free to email me directly if you have any questions.

[hcoles]

on e.g. a 3.2 with Motronic... after 5500 rpm at WOT there is no new maf information from the maf device (the maf device is pegged)...it can only be infered using rpm and other factors, e.g. air temp. This is what I mean... and I think what Steve's product does is properly measure the maf up to the limits of what the engine can eat using the new and improved maf device. To put it in other words the system is not a great design at the higher rpm and power levels. This is why the Steve Wong chips work.. he tunes out the richness, amoung other things but I think that is the major one.

[hcoles]

when I say Steve's product I refer to
http://www.911chips.com/HFM1.htm
This is what the factory should have done IMHO but the technology may not have been there quite yet.... If I had an extrie $2,700 I would buy this in a second.