Using ST fuel trim to determine AFR???

JBushman73

Founding Member
Aug 29, 2001
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Roanoke, VA
For those of us who don't have widebands, we look at MAF counts, fuel pressure, etc.

From what I have googleing, I read that if you take your short term fuel trim value at WOT, you can multiply that by "lambda" 14.64.

So my STFT is .82 X 14.64 = 12.0048:1 AFR

Anybody else heard of this? :shrug:

Thanks!
 
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That just tells you what is commanded at WOT, not what you are actually getting. Actually the calculation is correct, except it's not short term trim times lambda value, it's stoich point of the fuel times lambda - but in your calculation it is correct because you used the stoich point of gasoline: 14.64. That's really what tuning is partially about, getting your actual A/F to be the same as commanded A/F. You can sometimes extrapolate what the short term trims are doing at idle and part throttle to WOT IF the MAF you are usuing has a fairly linear output, but certainly not always. Nine times out of ten, corrections must be made at WOT even if short term trims are in line.
Don
 
Don,
Thanks for the response. I still need to buy a MAF.ia from you. I bought the XCal2 from you almost a year ago. It took this long just to figure out how to data log using my WINDOWS VISTA laptop.:mad:
After I get the MAF.ia I'll invest in a wide band.

Thanks for the thorough reply! :nice: