Closed Loop STFT with Lower Base AFR
 

I All,

I am sure this is a relatively simple question but I can't seem to figure it out. I am wanting to run 14.42 base AFR due to the 10% ethanol in fuel these days. So I tuned the VE table and MAF with my AEM wide band and now want to go back into closed loop with the STFT but they say I am running rich and pull fuel to get my AFR back to 14.7ish. This makes sense to me since I have read the narrow band o2 only read around stoich 14.7 AFR. So if I am commanding a base of 14.42 does that mean I can't use my narrow bands and run in closed loop because they will always show an error and pull fuel?

Thanks for the help!

Save yourself some headaches. It won't make a measurable difference.

Thanks for the response! I take that to mean I should go back to 14.7 and redo the VE and MAF tuning? Is there anyway to "tune" the o2s if I do this and they are still showing a bit off? Or would you recommend tuning with STFT instead of the wide band? Thanks!

O2s will switch around stoich which is when lambda = 1. Whatever AFR that is doesnt matter as that depends on the fuel. Stoich AFR of gas is 14.7, e85 is 9.something and methanol is 6.something, but all are equal to lambda=1.

You can set the "stoich afr" to whatever you want in HPTuners, the o2s will still switch around lambda = 1. The only reason to do what you are wanting is if you want your wideband to be inline with your fuel trims (or just tune your wideband display lambda also).

Hi Richard, I was hoping you would comment.....

I have been reading up on lambda but don't fully grasp the concept yet. If the O2s will switch around stoich no matter what then why did I need to tune the VE and MAF tables? Or did I not really need to unless I was going to run SD or OL? Maybe I did a bunch of work for no reason.....

With regard to lambda I have been trying to read up on using it to tune instead of AFR so I may give that a whirl....I think my AEM wide band will display lambda as well as AFR.

So where should I go from here? Just let the O2s do their thing? Should I go back and redo the VE and MAF at 14.7 or is it a waste of time?

Thanks again!

The O2s give you feedback on if the amount of air the computer thinks is going into the engine is the amount that is actually going in. The difference is seen as a fueling error. Controlling precise amount of fuel is easy (and why injector data is important), but calibrating for air is more difficult and changes with your mods. This is why you tune the VE and MAF, to make sure the air model is correct. The o2 sensors are part of a closed loop control system, meaning it uses feedback to adjust (vs open loop that does not use anything for feedback). The closer the estimated and actual fuel and air values are, the smaller the error the O2s will report.

Once that is done, you can tell the computer to do any lambda (afr) and it should go to exactly that. Say you want 0.75 lambda under boost, you put that value in the enrichment table (actually they use equivalence ratio which is 1/lambda) and it should go exactly there. This is important because lambda = AFR/AFR_stoich, AFR_stoich depends on the fuel you are using. So if you put in 10.0 in your stoich table it will be very rich.

This is important because in closed loop, the o2 sensors will always force it back since there is feedback, but in open loop, there is no feedback. So I should probably clarify my earlier statement, it does matter in open loop for correctness, and also matters in closed loop to reduce error.