Stoich A/F Modification and closed loop effect

[LSChev]

Hey all,

All of the 2015 trucks I have tuned so far have the stoich AF set to 14.1 from the factory. I did some reading and it seems that this is because GM expects low octane fuel to be used, which is normally around E10.

My question is, since premium fuel will be used in these trucks now, is there any reason I can't set stoich to 14.7? My main concern is closed loop operation, are the factory O2s still considered narrowband or can they adjust for the different stoich?

Thanks in advance,
Ryan

[Crawler]

I don't know about the part of the country that you live in, but here (DFW TX) the Premium is also E10 in many cases.

[5FDP]

It really wouldnt make a huge difference if you switch it back to 14.7, nothing that you would notice.

[ElecTech]

It really wouldnt make a huge difference if you switch it back to 14.7, nothing that you would notice.

Except it will shift your fuel trims by the percent difference in stoich and completely effect you WOT fuel calculation by that same amount.

You should try to keep your stoich as close to the fuel makeup you can or invest in a flex fuel conversion. Most fuels of all grades have between 5-10% ethanol, unless otherwise specified.

[LSChev]

Thanks for the responses.

I have a 2014 Sierra with flexfuel that I used to verify that shell nitro has 0% ethanol. I live in Canada so perhaps it is different in the states. Shell advertises 91 with 0% ethanol. But, Husky and petro have 94 but advertise that those are both E10. I have found that shell is the most consistent so that is why we use them.

I went ahead and made the change a while ago, it does seem to be running closer to 14.7 now but definitely still seems a tad rich. Which was why I thought maybe the O2s were unable to adjust. I had also heard rumors that these new O2 sensors are more like widebands, can anyone confirm?

[ElecTech]

Thanks for the responses.

I have a 2014 Sierra with flexfuel that I used to verify that shell nitro has 0% ethanol. I live in Canada so perhaps it is different in the states. Shell advertises 91 with 0% ethanol. But, Husky and petro have 94 but advertise that those are both E10. I have found that shell is the most consistent so that is why we use them.

I went ahead and made the change a while ago, it does seem to be running closer to 14.7 now but definitely still seems a tad rich. Which was why I thought maybe the O2s were unable to adjust. I had also heard rumors that these new O2 sensors are more like widebands, can anyone confirm?

If you have flex fuel already, then the stoich should be a table based on the percentage of alcohol content. No need to change it if it is already populated.

The factory O2's are narrowband "lambda" only sensors and will report lambda for any fuel makeup. An aftermarket WB will likely show "14.7" if not specifically able to calibrate the stoich value to display. Lambda is always lambda, regardless of stoich.

[Higgs Boson]

All pump gas in the states except for literally a handful of stations run E10 for low grade and premium.

The O2 sensors are not widebands.

The only way to tell if you made the right change are the fuel trims. If they are closer to 0 at 14.7 or 14.1.....saying it runs closer to 14.7 now doesn't mean anything and neither will saying it runs closer to 14.1 otherwise.

14.7 and 14.1 are both Lambda 1.0 and that is what your fuel trims are trying to achieve, depending on the fuel in the tank.

[LSChev]

If you have flex fuel already, then the stoich should be a table based on the percentage of alcohol content. No need to change it if it is already populated.

The factory O2's are narrowband "lambda" only sensors and will report lambda for any fuel makeup. An aftermarket WB will likely show "14.7" if not specifically able to calibrate the stoich value to display. Lambda is always lambda, regardless of stoich.

The Denali doesn't have the sensor but another truck does and it consistently shows 0% so I believe it.

All pump gas in the states except for literally a handful of stations run E10 for low grade and premium.

The O2 sensors are not widebands.

The only way to tell if you made the right change are the fuel trims. If they are closer to 0 at 14.7 or 14.1.....saying it runs closer to 14.7 now doesn't mean anything and neither will saying it runs closer to 14.1 otherwise.

14.7 and 14.1 are both Lambda 1.0 and that is what your fuel trims are trying to achieve, depending on the fuel in the tank.

Okay good to know, I figured GM wouldn't put actual WBO2s in a factory vehicle. Not sure where I heard that from.

Right! I am familiar with Lambda. I didn't realize that narrowbands function based on Lambda though.. Well I guess in a way I did, just connecting the dots haha. That makes sense now. Thank you for your insight.

[ElecTech]

14.67822265625    14.322265625    13.9658203125    13.609375    13.25244140625    12.896484375    12.54052734375    12.18359375    11.82763671875    11.47119140625    11.1142578125    10.7578125    10.40185546875    10.0458984375    9.68896484375    9.33251953125    8.9765625

Use the values above in those cells. These are from a factory flex fuel calibration.

[LSChev]

Thank you ElecTech!

[LSxpwrdZ]

Changing the stoich affects fueling yes but it is just the ratio of fuel the PCM uses to calculate fuel demands based off of the airflow the engine is ingesting. In the end you could put 9.0afr in the stoich table and make the truck run pig rich but if the fuel trims have enough leeway they'll correct the physical pulsewidth back to the same exact pulsewidth required to achieve stoich in the exhaust.

Point being you don't "tune" the stoich ratio. It is a fixed parameter based on the data of the fuel being used in the tank. Stoich ratio for E10 is 14.1afr hence the reason the non-flex vehicles come with that stoich setting.

[Redline MS]

The target stoich should match the stoich of the fuel used. Its chemistry. For starters AFR is an acronym we shouldn't be using....The stoich is the balance point of the fuel and has nothing to do with the octane technically. The 14.1 per GM is because the OEM have realized that the majority of fuel out there is cut "up to 10%" with ethanol. Mind as get a close to the chemistry as you can from jump to keep the trim controls from going crazy.

We use racing gases that can vary from 14.7 to 12.4 stoich so its not about the octane. This is again another reason to calibrate with lambda as its always 1 when you nail it no matter of what it is.

[Crawler]

The target stoich should match the stoich of the fuel used. Its chemistry. For starters AFR is an acronym we shouldn't be using....The stoich is the balance point of the fuel and has nothing to do with the octane technically. The 14.1 per GM is because the OEM have realized that the majority of fuel out there is cut "up to 10%" with ethanol. Mind as get a close to the chemistry as you can from jump to keep the trim controls from going crazy.

We use racing gases that can vary from 14.7 to 12.4 stoich so its not about the octane. This is again another reason to calibrate with lambda as its always 1 when you nail it no matter of what it is.

Well put.

[schpenxel]

Indeed. Pretty much everything is E10 around here.. doesn't matter if it's 87 or 93.