tici

iTrader: (1)

I was talking to a local tuning shop (they tune only European and ricer's cars).
He explained that he usually changes the AFR ratio in closed loop
Normally there is a table with AFR vs. mV.
In another table you can set a target AFR (like a VE table), The PCM then changes the fuel settings so to reach the set mV.
In this way he can set a richer idle, a lean cruise, and all sort of AFR depending on the load.

I explained that in our cars, because of the narrow band O2's the closed loop AFR is fixed at 14.7. no way to change it. No matter what you do the PCM will change the trims to get back to this value.
The only chance we have is to tune WOT (PE) or stay in open loop all the time and use the VE to determinate the AFR.

Am I right? Or not?

I think other cars (like VW, Audi and Porsche) have wideband O2's. This would explain the possibility to choose a different AFR.

Steve Bryant

iTrader: (1)

This question really belongs in the tuning forum. However, that being said, you are basically correct. The narrow band oxygen sensors used closed loop operation in the LS series engines monitor whether the AFR is above stoichiometric (14.68 AFR for gasoline) or below. If the left or right bank is lean the short term fuel trim (STFT) changes the duty cycles of the injectors on that bank of cylinders to enrichen the mixture quickly. The opposite course of action is taken for a rich mixture. Over time, the PCM/ECM monitors the value of STFT and changes another correction factor called long term fuel trim (LTFT) that biases the duty cycle to lean or enrichen the mixture for a given operating region of RPM and manifold absolute pressure (MAP). The computer automatically fills out the various values for the left and right bank LTFT tables.

Some OEM PCM/ECM systems use wide band AFR Sensors and that's why we have fairly affordable sensors like the Bosch sensor used in most AFR meters. Also, some aftermarket fuel injection controllers use only a wide band for feedback.

I hope that this helps.

Steve

tici

iTrader: (1)

soo... this means that with our stock PCM and stock O2 sensors closed loop = 14.7 period? right?
Is there a way to recalibrate the stock PCM to accept a WB and have different AFR settings depending on load?

stevieturbo

Im pretty sure HPTuners and EFILive both offer a version of autotune, that allows the pcm to target AFR's via a wideband.

gametech

iTrader: (1)

The only reasonable way I see to keep closed loop at non stoich is as follows.

Use two wideband o2 sensors, and wire one of the aux outputs on each into the narrow band wiring to the computer.

Set one of the aux outputs on each to simulate a narrowband signal, but set it to do so in a more linear fashion between 0-1 volt.

Program stoich in the factory computer as whatever you want it at.

Set your o2 rich/lean vs airflow voltage to correspond with the output you programmed into the wideband controller for you desired afr. This may also involve customizing the general mode vs airflow table.

I'm sure there are a few codes that would have to be dealt with, and there may be a simpler way to accomplish non-stoich closed loop, but I'm fairly certain this way would work.

Steve Bryant

iTrader: (1)

The target AFR for Closed Loop is 14.7 by design for emissions compliance. The production of environmentally harmful emissions is minimized at stoichiometric combustion. The narrow band sensors are designed to switch above and below stoic and nowhere else.

Now, with tuning software, there are other things you can do. For instance, I have a custom operating system with EFILive that I have flashed into the PCM. I am running a quasi-closed loop (after warm up) where short term fuel trim is functional but long term is not. Also, I have my AFR migrate to from stoic at 60 kPa to about 12.8:1 by 90 kPa (although I've had to set a lower value on the table, the range is from about 12.8 to 13.1 on various days at different temperatures and atmospheric conditions).

Steve

tici

iTrader: (1)

But this using wideband sensors instead of the stockers, right?

I still tune using LS1 Edit and I begin to see it's limit...
My PCM is a 98... I'm not even sure EFILive or other "modern" programs can work with it

12secSS

iTrader: (30)

Both HPTuners and EFI Live have support for 1998, that shouldn't stop you from switching.

Anyway, most wideband output a 0v-1v (or in stock ECM terms: 0mV-1000mV) along with the standard 0V-5V output for logging software. You could use this to signal the PCM what AFR you want. Using the O2 mV switch points (which are about .431 - .450 mV), you can do just what those european calibrations are doing. Down side, you will need a wideband (and controller) for each bank. This is very doable with the stock ECM.

Steve Bryant

iTrader: (1)

Tici,
Yes, the information that the AFR is going to 13.0 or 14.7 or anything else is being monitored and logged via a wideband AFR that is output to my EFILive Software. Without a wideband, I have no way to monitor what the AFR is doing.

12secSS,
You are right that most wide band controllers have a separate narrow band emulator output to feed into the stock PCM if you are only using the wideband in the exhaust bung. However, the design of the NB emulator is to mimic the switch-over point of a standard narrow band sensor and that 0 to 1 V output you are talking about is set for stoic.

Steve

12secSS

iTrader: (30)

Steve,

Well, the voltage is set to switch around stoich, but you can change it to whatever "ratio" you want. GM has the stock cal set at 341mV at 0 airflow mode, then upping to 430mV and finally to 451mV at about airflow mode 16. Trucks have it flatlined at 481mV from airflow modes 1 to 16. The stock ECM will go to the out edge of the rich/lean limitations, set forth in the calibration, and try to average the swing to equal the target switch point mV. I am going to have to give it another shot with mine and really play around with it.

Steve Bryant

iTrader: (1)

So are you saying that the software in your WBAFR controller lets you adjust the AFR switching point? If so, how would you use this? Unless this is a race only vehicle, you sure don't want to trick the PCM into running closed loop at an AFR of something line 12.8 all the time. Your gas consumption will be terrible and you'll have fouled plugs, etc.

Steve

tici

iTrader: (1)

Steve,

Your screenshot of EFILive is exactly what the guys over here are doing with European cars. What they told me is that every (?) PCM has this capability, but not every editing software is able to change the specific tables.

Mayby every car over here has already a wideband... I know for sure that cars of the VW groupe have it (the same sold in the States as a tuning device). The target is usually set at lambda = 1, but for "fun" purpose it can be set at a lower value for better acceleration (and better pollution too )

Strange: over here it's usual to talk "lambda values" and not AFR

Steve Bryant

iTrader: (1)

tici,
EFILive (more specifically FlashScan, the tuning sofware portion of the logging/tuning suite) has the capability of displaying AFR or Lambda or Equivalent Ratio (I believe). You can have a mixture of metric and English units according to the user's preferences. HPT is probably similar in this sense although I am not certain about this.

Steve

PS
I believe that the Cadillac Northstar PCM has dual wideband sensors and controllers like VW and others.