why is stoich for gas vehicles slightly different in different os's??

[S2H]

is it because of the o2 being used possibly being slightly different??
or is GM trying to get a slightly richer/leaner than stoich so that they can play with fuel economy/power ever so slightly?
is it that way because of pushing emmissions slightly one way or the other??

why is stoich..not the same in every car??
isnt stoich an exact calculation based on remaining oxygen or lack there of in teh exhaust gasses??

I know the diferences arent huge..14.63 in one 14.68 in another..
just trying to understand why GM does some things...LOL

[chrisj6]

Not sure GM knows why GM does things lol

[MeentSS02]

Mine was 14.63, but I changed it to 14.68 after I saw that my LC-1's setting for gasoline was 14.7:1.

[xonelith]

Mine's 14.7002. I was wondering this as well. I just assumed that this is the value, based on the narrowband, that it was trying to adjust to (hence wonky LTFT's and such). But I was told not to touch this to get rid of my rich condition (was going to increase it by a few tenths).

So, who knows

[S2H]

yeah I was asking as I want it to be true stoich..not a GM wanna be for whatever reason they did it...

[PurplePiss]

Maybe it has to do with vehicle weight?

[horist]

only thing I can think of is maybe depends where the vehicle was slated for? like midwest where most of the gas is 10% ethanol or other areas that don't use ethanol?

I've noticed this but never looked into it... I wonder if there's just a difference between OS's? Meaning an early 99 would have one value, but the next OS update from GM would have a different value that they found works better over real world driving situations?

[MeentSS02]

I'd like to know why I can type in 14.7 all day long, and it'll fill the square with 14.681 instead, or type in 14.71, and it'll fill the box with 14.73. Just can't get a 14.7 no way no how

[Chris@HPTuners]

its the resolution of the PCM. PCM stores vaues with a limited decimal precision. When you hit save we round the value to the nearest acceptable value in the PCM.

Chris...

[MeentSS02]

What kinda resolution are we talkin here? I don't quite understand how values of 14.68 or 14.73 are acceptable to the PCM, whereas 14.70 is not

[Shifted]

A lot of values that the PCM derives are products of formula's...

Like timing advance is (.5 * DataA) - 64

So obviously there is a certain resolution there, like if you had 1 for DataA, then timing advance would come out to -63.5, and if DataA was 2, then timing advance would be -63, so there is no possible way to get -63.75...

If that makes any sense...

[Shifted]

Btw, some values can only use integers, like DataA can be a whole number value between 0 and 255

[Chris@HPTuners]

the PCM stores this on actually as Fuel/Air Ratio with a precision of 0.0002441 (ie. 1/4096)

we display it as AFR for your convenience. The LS1 PCM can only store integer values, so you can see:

a PCM value of 279 gives 4096/279 = 14.681 AFR

and the next higher legal value

280 gives 4096/280 = 14.6286 AFR

and the next lower legal value:

278 gives 4096/278 = 14.7338 AFR

Hope that helps,

Chris...

[xonelith]

Actually, clears up some. Thanks Chris.

So, what is affected if I increase the value? Will it change what the computer is trying to obtain as stoich?

[Chris@HPTuners]

this value is used by the fuel code to work out the base fuel mass required for a given amount of air. it has nothing to do with the O2 side of things.

the PCM fuel calc works like this (high level):

1. Calculate airmass (from MAF or VE)
2. Calculate desired EQ ratio (from Openloop, PE, COT or 1.0)
3. Convert EQ ratio to AFR (using the stoich value)
4. Calculate Fuel Mass (Airmass / AFR)
5. Calculate injector pulse width (Fuel Mass / Inj Flow Rate)

From a units perspective its easy to follow:

grams (air) / AFR = grams (fuel)

then to get pulse width in seconds:

grams (fuel) / (grams/sec) = sec

Chris...

[S2H]

my curiosity is more about teh stock computer looking for o2 volts that = approx.450mv..equaling lambada of 1.0(or perfect stoich)
obviously this is all done in the o2 switchpoints tables...
I'm just trying to understand where the value of stoich in our computers really comes into play and what changes when we change it to a different value...

[S2H]

this value is used by the fuel code to work out the base fuel mass required for a given amount of air. it has nothing to do with the O2 side of things.

the PCM fuel calc works like this (high level):

1. Calculate airmass (from MAF or VE)
2. Calculate desired EQ ratio (from Openloop, PE, COT or 1.0)
3. Convert EQ ratio to AFR (using the stoich value)
4. Calculate Fuel Mass (Airmass / AFR)
5. Calculate injector pulse width (Fuel Mass / Inj Flow Rate)

From a units perspective its easy to follow:

grams (air) / AFR = grams (fuel)

then to get pulse width in seconds:

grams (fuel) / (grams/sec) = sec

Chris...

so according to this..if we change the "stoich" value...lets say upwards..like my 00 camaro has a value of 14.62587
If I up it to 14.68001(the next step up)
and I change nothing else...
then I will be making my injector pulse width will go down..leaning out my current mixture...
so IF I did this then my whole VE table would need to go up to compensate?? correct?

[Chris@HPTuners]

yeah, its an attempt to make the fuel code independant of fuel type. Eg. those who have 2004+ cals will notice the Stoich value is a table that varies with alcohol %. The stoich value for alcohol being around 8.5:1

[MeentSS02]

That makes a whole lotta sense. Thanks for all the clarification.

[xonelith]

Thanks Chris. Very clear now.

Curious, when reviewing LTFT's and STFT's, what is it trying to correct to? There must be some baseline. I had originally thought it was this stoichemetric value, but it appears not.

Thanks !