Stoich AFR Question

[2002bird]

I was wondering if someone could give me their option on this. I read in a tuning publication that when you are calibrating in an area that uses 10%ethenal fuel you should change your Stoich AFR in the tune to 14.08 as opposed to the 14.68 in the stock tune.I did this and then obviously it richened everything up.No Problem I recalibrated the VE and MAF accordingly. When my table display is commanding 1.0 lambda I am +- 4% car runs good when warming up and when warm. The problem comes on cold start up. The pcm is commanding 0.77 Lambda because when divided by the richer 14.08 stoich in the tune the colder points in the open loop eq ratio is commanding a richer mixture. I started to adjust the eq ratio then I thought this seems like chasing my tail.Why not go back to the stock Stoich,get to 1.0 lambda at idle and part throttle again and then I wont have to mess with the eq ratio. Because if you do the math at 14.68 the pcm goes back to commanding a fuel ratio that make more sence at 0.83-0.85(12.3-12.5)when it is cold. Any thought on this subject would be great.

[GHuggins]

A lot of people don't realize this... It actually changes all of the fueling tables in open loop including wot.

This is how I've always looked at the subject. Unless your tuning with "confirmed" non ethanol fuel, what ever gas is in the vehicle all ready has up to 10% ethanol and since your dialing in the MAF and VE tables with this ethanol fuel, your kind of all ready compensating for it, right? I'll get criticized for this statement, but ethanol leans out the fuel mix making it add fuel via fuel trims. Your compensating for this in your MAF and VE tuning. SO your all ready changing things for the ethanol whether you realize it or not?

[2002bird]

Yes sir. I thought the same thing. If I am adjusting the Maf And Ve to the ethanol fuel why do I need to change the Stoich.Further more you have to now jump thru hoops and reinvent the wheel to get the car to run good thru tables you should not have to worry about once you get the Maf and Ve adjusted. Thanks for the input on this subject.

[JamesLinder]

I also tried changing the stoichiometric AFR parameter to match that of 10% ethanol fuel after I purchased Don LaSota's Ultimate GM EFI Tuning Manual, however I came to the conclusion that this makes dialing in the tune much more complicated
than just leaving it at the GM OEM setting for my 2000 SS Camaro. Other professional tuners have stated that as long as you are using the OEM oxygen sensor, or equivalent replacement, you should not change the AFR parameter. Changing this
will definitely mess up all of the cold start fueling settings.

[2002bird]

I got the info from the same book. Crazy small world. Anyhow I also changed it back and except a little tweaking in the colder areas of the Open EQ table I got the cold start pretty much fixed. Now also the commanded Fuel in the cold area makes more sense(.83-.85).There is some good info in that book but that part just never made sense to me.Anyhow thanks for your input.

[Higgs Boson]

narrowbands trim to the fuel in the tank. if you are running E0 gasoline at 14.7 and tune the trims to 0% in a perfect world and then change the stoich value in the tune to 14.1 for E10, the trims will be off, because the fuel in the tank is still E0.

If you fill up with E10 and have 14.1 in the stoich value but tuned the car to E0 with a stoich value of 14.7 then the trims will still be 0%.

The Stoich value in the tune should match whatever is in your tank. If you are running E10, change it to 14.08 (or 14.1). That is the right thing to do.

If your cold start fueling is off then your airflow tables (MAF and/or VE) are off. They are off because you are tuning with fuel trims instead of a wideband and lambda.

Put the Stoich value to match whats in the tank (probably E10) and turn off closed loop, install the wideband, tune to lambda 1.0 (and PE to whatever you set). When it's perfect, turn closed loop back on and your trims will also be 0% if you did it right.

[schpenxel]

Fuel trims drive towards stoich, no matter what you have in the AFR table and no matter what fuel is in the tank. It doesn't give a damn what fuel is in the tank.. Not saying it's a good idea to leave it wrong on purpose.. but if you've already (knowingly or unknowingly) factored the ~5% difference between E10 and E0 in all the other tables, i.e. VE/MAF/open loop/whatever else, and it's working.. then leave it

The stoich value is used for base fuel calculations. I.e. airflow / stoich AFR = fuel needed. O2 sensors / fuel trims adjust from there so that it goes just above and just under stoich to make the NB O2's cycle

(obviously this applies to closed loop only--changing stoich AFR will affect PE fueling since obviously fuel trims don't work there)

[HeavyChevy305]

The only way to make sure the PCMs O2 sensors are actually reading the correct commanded stoich AFR would be by calibrating the O2 voltage lean/rich threshold with a wideband. Please correct me if i'm wrong?

[schpenxel]

On gen 4's the rich/lean thresholds don't do much of anything. The window for what makes NB O2's switch is so narrow that even a few % leaner or richer than stoich will make them shoot up to 800mV or down to 100mV, so it really doesn't matter that much. I've heard that on gen 3's they have a greater effect though, but I've never tried messing with it as I don't think it really matters.

Regardless, everyone should understand that NB and WB Oxygen sensor all read LAMBDA!!! They do not read AFR. WB's just display lambda x 14.7 no matter what. You can put a wideband on a car running E85 and it will still show it's running 14.7:1 when it's at stoich for that fuel unless you've gone in and told the gauge that you're running E85.

[HeavyChevy305]

How is it that the WB will read lambda when running E85 if lambda for E85 is 9.4:xxx. Wouldn't the WB think the mix is rich? Or is it because stoich regardless of fuel is a specific mV it reads?

BTW, if not a WB, what gauge/sensor reads AFR?

[schpenxel]

Lambda is lambda. Lambda=1 is stoich. Doesn't matter if you're burning gasoline or hydrogen or cat piss

I should said NB "measurements" are based on lambda. Obviously a NB sensor can't give you a precise afr reading away from stoich

Google afr vs lambda and see what you find. Lambda is so much easier.

[Higgs Boson]

How is it that the WB will read lambda when running E85 if lambda for E85 is 9.4:xxx. Wouldn't the WB think the mix is rich? Or is it because stoich regardless of fuel is a specific mV it reads?

BTW, if not a WB, what gauge/sensor reads AFR?

AFR is just a calculation. Lambda is the real deal.

Torque is what an engine produces and horsepower is just a calculation based on torque and RPM.

Oxygen sensors do not read AFR ever. The little box might display and AFR value but it is based on an internal conversion from lambda. In this case, whatever AFR the box displays as lambda 1.0 (stoich) will be the same for E85 or gasoline (for example, it will always read 14.7 no matter what fuel because it is really just displaying lambda 1.0).

Forget about AFR. Lambda (and EQ Ratio) are what is measured and calculated with. AFR is fine as long as everyone and their widebands and laptops and ECMs and fuel types are all EXACTLY the same, which they never are.....if a tank of gas is E10 and next week it is E7, stoich lambda is still 1.0 even though AFR is slightly different.

[HeavyChevy305]

You guys should have PHDs in tuning lol Thanks for the detailed explanations

So the only way to know if your o2s are reading lambda correctly is to hook up another lambda sensor and compare?

[schpenxel]

Any wideband is a lambda sensor. Lambda of 1 will show up as 14.7 on most. Doesn't matter if you're running E0, E10, E85, E100

[Eaglegoat]

So the only way to know if your o2s are reading lambda correctly is to hook up another lambda sensor and compare?

IMHO this is correct. I've tuned vehicles where the stock narrowbands add tons of fuel once in open loop. I've tuned others that pulled enough fuel to cause low speed drivability issues and cause a surging during highway cruising. Once you recalibrate the switching voltage, and sometimes the P/I fueling, it clears things up. This has been on e40 and e38 computers. Both cars had wrapped headers. That's the only similarities.

[HeavyChevy305]

IMHO this is correct. I've tuned vehicles where the stock narrowbands add tons of fuel once in open loop. I've tuned others that pulled enough fuel to cause low speed drivability issues and cause a surging during highway cruising. Once you recalibrate the switching voltage, and sometimes the P/I fueling, it clears things up. This has been on e40 and e38 computers. Both cars had wrapped headers. That's the only similarities.

This is what I figured. The only way I think one goes about calibrating a false stoich reading which is the most common problem LT's cause, is to mess with the o2s switching voltage and have a WB plugged in at the same time till true stoich is read by the o2s using the WB as reference. Some say that messing with the mVs makes little to no difference though.

[schpenxel]

To be clear.. I did say it doesn't make much difference on gen 4's that I've tried it on. I've always been told it makes a bigger difference on gen 3's for some reason

Here's a good graphic to help explain.. you can see that basically from 200mV to 700mV output on a NB it's like a 2% change in AFR/lambda total.. so not exactly much adjustment. Not to say you can't get some but I don't think you're going to get 10% changes or anything like that

[Eaglegoat]

Dunno. I've seen 25+30% total change. I've not messed with it on a gen.

Go experiment, set it at 50mV and see what it does to fueling, then set it 50mV below the top of your range and see what that does.
This is no different than seeing your WB calibration in HPT.

[schpenxel]

A change to that extreme an end of the spectrum probably will make a noticeable change. Trying to go from 300 to 500mv or something like that isn't going to do much though

[Eaglegoat]

I just put it where it needs to go. ;-) Dunno.