2023 Hemi FA Stoich rich?

[Slorango]

Looking through my stock tune I noticed that the FA Stoich is set to .0720. I’ve noticed some other Hemi’s set at .0688 which seems more realistic. Are these actually targeting an AF of .0720, or are there other tables I’m not seeing that bring it closer to 14.7:1 at idle and light throttle?

[DodgeBoy_8HP]

Which vehicle do you have?

Engine -> Fuel -> O2 Sensor -> Upstream

That would give you a good indication of FAR

[Slorango]

2023 Durango RT. What I was referring to was Fuel -> General -> FA Stoich

[Stoopalini]

2023 Durango RT. What I was referring to was Fuel -> General -> FA Stoich

So those Stoich fields are not used during closed loop. They are used in Open loop though.

The narrow band O2 sensors are measuring oxygen content of the exhaust after combustion, with Lambda 1.0 being the perfect stoich burn. The narrow band O2s are only effective within a narrow band of measurement, right around 1.0 lambda (ie: perfect stoich burn of whatever fuel you're using), hence when the PCM is wanting to add fuel for PE, or if the O2 sensors are not ready yet ... the PCM won't use them to feedback the signal (ie: will run in Open Loop). .

If you get E0, or you get E10, or you get some other gasoline with additives which affect the burn efficiency, or contaminated fuel ... then it's not a problem as the narrow band O2 sensors will measure the resulting burn and feed the data back into the PCM, which will in turn adjust the LTFT and STFT to compensate. That's why the stoich values really don't matter for closed loop. The PCM is using the narrow band O2 sensors to measure the burn, looking for the perfect 1.0 lambda (ie: stoich), and adjusting the trims to make sure it is being hit.

The fields in the tune are to tell the PCM what the stoich of the fuel being used is, so when you go into open loop (WOT or other scenario), where the narrow band O2s cannot measure the oxygen content effectively ... the PCM then takes the Stoich value, and adds in any FA adder being applied (including FA Power Enrich added from WOT triggers), to come up with the fuel component of the FAR calculation.

The PCM is not targeting 14.7, or any other ratio ... it is targeting a complete burn of the fuel when in closed loop using the narrow band O2s as a feedback mechanism to make adjustments. Yes, the ideal gasoline with 0% ethanol achieves a stoich reaction with 14.7 parts air to 1 part gasoline. That's why people use 14.7:1 as "stoich" for calculations; But if you have E10, then perfect stoich is roughly 14.1:1 ratio. The resulting burn is still a stoich result though (ie: Lambda 1.0), so that's what the narrow band O2's are measuring.

Look at your PE tables in the Power Enrich tab. Take the stoich and add the values found in this table, and you'll arrive at the FAR ratio for WOT. The factory calibrations will be suitable for a wide variety of gasoline types, but you won't know how close to the FAR target the PCM is hitting during WOT without putting a wide band into the system.

For closed loop, you can log the LTFT + STFT and see how much % the PCM is needing to adjust to hit Lambda 1.0, in order to determine how effective your FAR mixture is during idle and light cruise. But the Stoich values entered into the tune are not being referenced during closed loop, so changing those won't have an impact on the LTFT or STFT.

[HaasExp]

But the Stoich values entered into the tune are not being referenced during closed loop, so changing those won't have an impact on the LTFT or STFT.

The PCM still needs to calculate the required mass of fuel to inject for the measured/calculated airmass to target a commanded EQ. To tie this all together it needs to know the stoichiometric AF or FA of the fuel being used.
Therefore, the FA stoich parameters are used in both closed and open loop. The only difference here is the commanded EQ.
In closed loop it will always target an EQ ratio of 1.0 (with narrowband o2 sensor feedback for correction) but the PCM still needs to calculate the required fuel mass to meet this commanded EQ ratio.

For example: if flex fuel is not active and you run E85 without changing the stoich FA to tell the PCM that you are running a fuel that has a different stochiometric FA ratio, then you will find that your LTFT+STFT fuel trims in closed loop will go +40%. Well at least on the GPEC2/2A's

[Stoopalini]

The PCM still needs to calculate the required mass of fuel to inject for the measured/calculated airmass to target a commanded EQ. To tie this all together it needs to know the stoichiometric AF or FA of the fuel being used.
Therefore, the FA stoich parameters are used in both closed and open loop. The only difference here is the commanded EQ.
In closed loop it will always target an EQ ratio of 1.0 (with narrowband o2 sensor feedback for correction) but the PCM still needs to calculate the required fuel mass to meet this commanded EQ ratio.

For example: if flex fuel is not active and you run E85 without changing the stoich FA to tell the PCM that you are running a fuel that has a different stochiometric FA ratio, then you will find that your LTFT+STFT fuel trims in closed loop will go +40%. Well at least on the GPEC2/2A's

Ahh, that makes sense. In the E85 scenario, the trims would keep closed loop hitting 1.0 if they could, with a high % adjustment … but when open loop was entered (ie: WOT), then it would blindly use stoich + PE adder and would be way off without the feedback and trims being available.

But for standard pump gas, I’d think using 0.0688 or 0.0720, or in my case 0.0709, the trims in closed loop would vary by just a few %, right?

I changed mine to 0.0709 because that aligns to 14.1:1, which is what I think E10 is …. And that’s commonly what I find with the 93 octane I have locally. My closed loop trims run in the 0-3% range with this config as well.

[HaasExp]

Ahh, that makes sense. In the E85 scenario, the trims would keep closed loop hitting 1.0 if they could, with a high % adjustment … but when open loop was entered (ie: WOT), then it would blindly use stoich + PE adder and would be way off without the feedback and trims being available.

Correct.
E85 is obviously an extreme case to E0 (0.0688 vs 0.1035) but I was just merely pointing out that the FA stoich parameter is used as the fuel mass conversion factor. Thus, it is used all the time for the base required fuel mass calcs.

[Stoopalini]

Correct.
E85 is obviously an extreme case to E0 (0.0688 vs 0.1035) but I was just merely pointing out that the FA stoich parameter is used as the fuel mass conversion factor. Thus, it is used all the time for the base required fuel mass calcs.

Makes sense. So it’s the base value, and then the trims adjust during closed loop to hit 1.0 lambda, and PE tables used during open loop to blindly target whatever the summed value is without any feedback correction mechanism (except for the 6.2 motor that is, which has dual wide bands from the factory)

[HaasExp]

Makes sense. So it’s the base value, and then the trims adjust during closed loop to hit 1.0 lambda, and PE tables used during open loop to blindly target whatever the summed value is without any feedback correction mechanism (except for the 6.2 motor that is, which has dual wide bands from the factory)

Thats it!

[Slorango]

Great information, thanks for explaining everything.

[LilSick]

Makes sense. So it’s the base value, and then the trims adjust during closed loop to hit 1.0 lambda, and...

i know daniel said yeah, but i do not agree. Closed loop is adjusting to hit a target, it is not adjusting to hit lambda 1.0

lambda 1.0 is stoich of whatever fuel is in the tank

narrow band sensors can't sense where stoich is (lambda 1.0) and how far off you are like a wide band does

you gotta give a narrow band sensor a target

once you give it a target it becomes a switch stating that you are either above, or below, the target

gpec2 and gpec2a differ greatly in that part of the tune but those targets must be changed if you change the stoich of your fuel

otherwise the narrow band sensor is not gonna know that you did and you will never get the rest of the tune right : )

[Stoopalini]

i know daniel said yeah, but i do not agree. Closed loop is adjusting to hit a target, it is not adjusting to hit lambda 1.0

lambda 1.0 is stoich of whatever fuel is in the tank

narrow band sensors can't sense where stoich is (lambda 1.0) and how far off you are like a wide band does

you gotta give a narrow band sensor a target

once you give it a target it becomes a switch stating that you are either above, or below, the target

gpec2 and gpec2a differ greatly in that part of the tune but those targets must be changed if you change the stoich of your fuel

otherwise the narrow band sensor is not gonna know that you did and you will never get the rest of the tune right : )

It's my understanding narrow band O2 sensors measure O2 content in the exhaust and feed the result as a voltage to the PCM. They are only effective at measuring this with accuracy around combustion stoichiometry though, and can only tell if the resulting combustion gasses are rich or lean of stoich. They do this by comparing the O2 content of the exhaust with the O2 content of the ambient air.

The NB sensors essentially produce a boolean output of "rich or lean", and that's it. They can't tell by how much rich or lean of stoich though.

You can tell the PCM the stoich of your fuel using those fields, and it'll use this info as an input to the base fueling calc it determines is needed to achieve stoich without trims .... but it doesn't set a different target for the NB O2 measurements. They will always only report back rich or lean of stoich, regardless of what you enter in these fields and regardless of what fuel you have in the tank.

What do you mean by "Once you give it a target..."? Since NB sensors can only measure rich or lean of perfect combustion stoich, I don't know how you could target anything other than lambda 1.0 with a narrow band. This is a limitation of the sensor itself, not the fuel type or PCM being used.

Here's another thread where I discussed this a bit when I was trying to get my head around it .... https://forum.hptuners.com/showthrea...l=1#post798238

And for a good read on stoich itself, I found this helpful: https://ftyracing.com/tech/afr-of-any-hydrocarbon/

[Stoopalini]

Looking through my stock tune I noticed that the FA Stoich is set to .0720. I’ve noticed some other Hemi’s set at .0688 which seems more realistic. Are these actually targeting an AF of .0720, or are there other tables I’m not seeing that bring it closer to 14.7:1 at idle and light throttle?

Here is another thread I created last year when I was 1st jumping in to all of this. It has good detail on how the PCM is managing things: https://forum.hptuners.com/showthrea...bles-on-GPEC2A