Commanded AFR ≠ {B3601}

[mr.prick]

gm.eqivratio = 1.000000
{b3601} = 14.628573
gm.afr = 14.627930
diff = 0.000643

I know the difference is minuscule but could this be a bug in the software?

[TFZ_Z06]

gm.eqivratio = 1.000000
{b3601} = 14.628573
gm.afr = 14.627930
diff = 0.000643

I know the difference is minuscule but could this be a bug in the software?

I would think this would be affected by the source of GM.AFR. Startup, Cold, warm run, PE vs RPM. You didn't mention the conditions of the occurence. Maybe some interpolation going on..??

Could it also be since the pc stores this data in 2 byte unsigned format (believe this is a 2 byte datalog param), vs floating point, I would wonder if the data stored and used for internal RAM calculations, is actually ever what we display in floating point and view for editing purposes.

I checked a recent log I did and noticed a similar discrepancy, however I rarely use that value for tuning due to cam overlap.

[mr.prick]

I'm not sure what gets lost here. :sly:

When {GM.EQIVRATIO}=1, {GM.AFR}≠{B3601} it is a little richer. :confused:
{GM.AFR} should be exactly the same as the value in {B3601} when {GM.EQIVRATIO}=1 :doh2:

In short logged {GM.AFR} values are not what they should be even tho
logged {GM.EQIVRATIO} values are.

{GM.AFR} should be {B3601}/{GM.EQIVRATIO}
Something is throwing it off and I see nothing in the .tun that would do that

[joecar]

I think GM.AFR and B3601 are represented differently (i.e. their binary representation) so you will get that small discrepancy.

[joecar]

I would ignore GM.AFR and instead pay attention to GM.EQIVRATIO (and calculate BEN's on EQIVRATIO and WO2EQR1).

[mr.prick]

Yes Joe I know :doh:
I use my own EQ BEN and custom WBO2 AFR for reference,
but this has always bothered me.

It would not be an issue if commanded EQ was higher than 1.00
that would explain why commanded AFR is a tick richer.

Combine this with an improper WBO2 slope and it will cause a headache. :throw:

[TFZ_Z06]

I found this post interesting and of some relation to yours:
http://forum.efilive.com/showthread.php?t=13094

The dyno datalog posted shows a steady GM.AFR of
12.23. When you look at this PE vs RPM table, it "appears" the processor is actually adding the last different AFR to the current one and averaging the two. I would have thought that anything >= 2000 RPM would have been 12.19 AFR according to this PE VS RPM. Somehow this doesn't make sense to me,...:shock:

0 13.037127
400 13.003176
800 12.946982
1200 12.514335
1600 12.268353
2000 12.198419
2400 12.198419
2800 12.198419
3200 12.198419
3600 12.198419
4000 12.198419
4400 12.198419
4800 12.198419
5200 12.198419
5600 12.198419
6000 12.198419
6400 12.198419
6800 12.198419
7200 12.198419

[mr.prick]

IMO something is wrong with the {GM.AFR} PID.
EQ1 should match {B3601}.

[Blacky]

gm.eqivratio = 1.000000
{b3601} = 14.628573
gm.afr = 14.627930
diff = 0.000643

I know the difference is minuscule but could this be a bug in the software?

The math done in the PCM is performed using 16 bit fixed point precision. Some values in the above are only accurate to 1/1024.

If a value (for example AFR) is represented as a value from 0 AFR to 63.999 AFR but is stored in the PCM as a 16 bit integer then each of the 65536 possible combination that the 16 bit value can have will map to a discrete AFR value.

I.e.
0 = 0
1 = 0.0009765625
2 = 0.001953125
3 = 0.0029296875
...
15052 = 14.69921875
15053 = 14.7001953125
...
65534 = 63.998046875
65535 = 63.9990234375

So you can see not only are the values discreetly quantized, but they cannot even accurately store 14.7 since there is no integer value that can exist between 15052 and 15053

Given the limitations of the CPU in the PCM, the GM engineers/programmers traded accuracy for speed. Obviously they figured 3 decimal places was enough accuracy for AFR computations.

Regards
Paul

[mr.prick]

Thanks Paul.
Comparing 2 logs I have;
one with 24 channels selected and has the "off" values for GM.AFR
and the other has 23 channels selected and GM.AFR is right.

[Steve Bryant]

I have a related question about {B3601} values. Some stock tunes use 14.63 and some use 14.68. Why the difference? Most truck tunes use 14.68 and most Car tunes use 14.63. However, if you'll look at the attached image from a 2002 ZO6 Corvette (downloaded from HoldenCrazy.com), it's 14.68.

[Steve Bryant]

TTT, I'm still curious. Does anyone have an idea about the 14.68 versus 14.63 that I posed above?

Thanks,

Steve

[WeathermanShawn]

Steve:

I caroused the Internet for about an hour after you posted your question. I was curious too.

I could not find a definitive answer. You had some difference with emission qualifications that year..No EGR, Corvette-pup cats were moved..but no real answer.

Its interesting looking at a 2002 Camaro vs 2000 Corvette ZO6. You will see subtle differences in O2 switch-points and Commanded fuel. Essentially everything is a little 'leaner' in the ZO6 vs Camaro..

Whether that was an EPA difference or the ZO6 H/C combo..it would be nice to know.

So, I could not find any definite information either. But, I have noticed the same thing. Perhaps one of our GM engineers can help us out.

[mr.prick]

Maybe because the fbody and ybody are supposed to use 93 octane
and trucks use 87 octane. :nixweiss:

It doesn't matter what {B3601} is, all it does is set an AFR value to EQ1.
As far as the PCM is concerned it only knows EQ.

{B3601} should match your WBO2's Stoich AFR if you are using AFR & BEN as a tuning reference.
See this thread. (http://forum.efilive.com/showthread.php?p=117265#post117265)

[TFZ_Z06]

TTT, I'm still curious. Does anyone have an idea about the 14.68 versus 14.63 that I posed above?

Thanks,

Steve

Just a guess, but in 2004, the Very Low Emissions standard was phased in. Some cars already had this status and I believe the 2002 Z06 was one of them.
Maybe this leaner mix was part of meeting that requirement??

[mr.prick]

Lean AFR=higher emissions
One of the reasons we don't have lean cruise in the US.

[joecar]

I have a related question about {B3601} values. Some stock tunes use 14.63 and some use 14.68. Why the difference? Most truck tunes use 14.68 and most Car tunes use 14.63. However, if you'll look at the attached image from a 2002 ZO6 Corvette (downloaded from HoldenCrazy.com), it's 14.68.
I have been curious about this for a long time and have never found an answer...

[joecar]

Lean AFR=higher emissions
One of the reasons we don't have lean cruise in the US.More specifically: leaner AFR typically means higher NOx emissions and lower HC/CO emissions...

The EPA is willing to raise HC/CO levels a little to keep NOx levels low.

:)

[Steve Bryant]

Thanks for the responses guys. Most citations of AFR for gasoline/petrol round to 14.7:1. It appears to me that {B3601} as well as some parameters like O2 Switch Points are trying to model Stoic for the PCM to properly reflect the O2 sensors so that various AFR/Lambda/EQ values are in harmony with the O2 sensor. It's always been a point of confusion for me and evidently for others too.

Thanks,

Steve

[TFZ_Z06]

Lean AFR=higher emissions
One of the reasons we don't have lean cruise in the US.

leaner mixtures increase gas mileage. As you approach stoich, the cats are supposed to work their best. Who said GM creates cars to reduce greenhouse gases? Are we trying to use less fossil fuels??

[mr.prick]

The EPA/GOV wants both but it's a little unrealistic.
We're also better off without foreign oil but we won't produce more of our own.
Besides, increased gas mileage=less gas sales who wants that. :hihi:

[TFZ_Z06]

The EPA/GOV wants both but it's a little unrealistic.
We're also better off without foreign oil but we won't produce more of our own.
Besides, increased gas mileage=less gas sales who wants that. :hihi:

I would love to line up a few of the cars on this site and watch the EPA guys as we cranked them off. The look on their faces - > :shock:

[joecar]

Actually, all of them crank up and run clean. :good: