I was wondering what controls commanded AFR in open loop other than the tables "Loop Open % Change AFR vs MAP" and "Loop Open % Change AFR vs Temp"

The target AFR that I am logging seems to be off from what I would expect the tables to command.

Example @ 50KPa and 92C and forced open loop.

14.7/(1+.0391+.0977)=12.93

Data logging shows it to be commanding close to 14.7.

Maybe I am not understanding the tables correctly.

 

What mask ID are you referring to?

RBob.

 

It's a 7730 using 8D.

 

In $8D the AFR is really FAR (fuel to air ratio). So the math is working on the FAR, which makes it turn out different. I thought there was a thread about this in the tuning guide book sticky, but I don't see it.

RBob.

 

The power enrich tables work the way I would expect.


Example @ 4000RPM and 80C.

14.7/(1+.2266-.0430)=12.42

The target AFR from the data log is right in line with this.

I don't quite understand what you mean by Fuel to Air Ratio.

Does the computer calculate the target AFR differently in open loop than it does in power enrich?

 

Check this link:

https://www.thirdgen.org/techbb2/sho...hreadid=214156

This shows the math as the ECM does it for PE mode. Which is basically the same for open loop AFR.

The modifier tables are a percent change to the FAR value. For 14.7:1 the ECM uses 445. The conversion to AFR is: AFR = 6553.6 / FAR (445)

Note that the FAR isn't the true FAR, but one that has been scaled for use by the ECM.

RBob.

 

Lately there has been a number of requests about how $8D calculates open loop AFR. So... put this together:

The code in the $8D mask works with a fuel to air ratio (FAR) value. This makes the math easier for the ECM to run. This FAR can be converted to AFR by dividing 6553.6 by the ECM's FAR value.

This why stoich is 445 in the ECM: 6553.6 / 445 = 14.7 AFR. Also note how the math is done using binary increments. Such as: 6553.6 * 10 = 65536 in decimal, which is $10000 (hex).

In open loop the ECM uses six parameters to create the FAR. Note that this is standard mode, not in PE mode.

There is:

Stoich FAR
% change versus in idle
% change versus MAP
% change versus CTS
% change versus after-start enrichment
scalar for % change versus CTS

The scaling of the % change values is: % of change = value / 2.56

With the value being the raw BIN value. So a value of 36 is a 14% change to the FAR. And an increase at that, as that is a lower (richer) AFR.

All of the % change values are added together (it is a 16-bit value). The stoich FAR is then multiplied by the total % change and then divided by 256. The result is the commanded FAR.

So:

FAR = (Stoich_FAR * (%_idle + %_MAP + %_CTS + %_AfterStart + CTS_Scalar) ) / 256

Using values from AXCN, at 56* C (25 for 9.8%) and 90 KPa (36 for 14%), afterstart already decayed out. Note that the % change for idle is 0 in this calibration.

FAR = (445 * (0 + 36 + 25 + 0 + 221) ) / 256
FAR = (445 * 282) / 256
FAR = (125490) / 256
FAR = 490

The value of 490 is used by the ECM in the injector PW calculation.

For the AFR:

AFR = 6553.6 / 490
AFR = 13.4

RBob.

 

Does this hold true for your P4 EBL also?

 

Thanks RBob.

This clears things up a lot.

The only thing I cant locate in the tuning software is "scalar for % change versus CTS".

Is this some sort of constant that cant be changed?

Thanks again.

 

It is right before the table:


RBob.

 

The EBL P4 is done differently, in some areas similar.

RBob.

 

How can I view the code? With Tunerpro, Excel?

Thanks

 

IIRC, the original hac of this mask is a pdf file.

{edit: can see/modify the BIN file with the hex editor built into Tuner Pro}

RBob.