How do i 'read' / interpret the STFT logs?
- Thread starter TopherSho
- Start date
Here is the fuel equation from the manual...
The adaptive fuel strategy uses O2 sensors for fuel feedback. The fuel equation includes short and long term fuel trim modifiers:
FUEL MASS = AIR MASS * SHRTFT * LONGFT/EQUIV_RATIO * 14.64
Where:
Fuel Mass = desired fuel mass Air Mass = measured air mass, from MAF sensor
SHRTFT = Short Term Fuel Trim, calculated
LONGFT = Long Term Fuel Trim, learned table value, stored in Keep Alive Memory EQUIV_RATIO = Desired equivalence ratio, 1.0 = stoich, > 1.0 is lean, < 1.0 is rich 14.64 = Stoichiometric ratio for gasoline
A conventional O2 sensor (not a wide-range sensor) can only indicate if the mixture is richer or leaner than stoichiometric. During closed loop operation, short term fuel trim values are calculated by the PCM using oxygen sensor inputs in order to maintain a stoichiometric air/fuel ratio. The PCM is constantly making adjustments to the short term fuel trim, which causes the oxygen sensor voltage to switch from rich to lean around the stoichiometric point. As long as the short term fuel trim is able to cause the oxygen sensor voltage to switch, a stoichiometric air/fuel ratio is maintained.
When initially entering closed loop fuel, SHRTFT starts 1.0 and begins adding or subtracting fuel in order to make the oxygen sensor switch from its current state. If the oxygen sensor signal sent to the PCM is greater than 0.45 volts, the PCM considers the mixture rich and SHRTFT shortens the injector pulse width. When the cylinder fires using the new injector pulse width, the exhaust contains more oxygen. Now when the exhaust passes the oxygen sensor, it causes the voltage to switch below 0.45 volts, the PCM considers the mixture lean, and SHRTFT lengthens the injector pulse width. This cycle continues as long as the fuel system is in closed loop operation.
The adaptive fuel strategy uses O2 sensors for fuel feedback. The fuel equation includes short and long term fuel trim modifiers:
FUEL MASS = AIR MASS * SHRTFT * LONGFT/EQUIV_RATIO * 14.64
Where:
Fuel Mass = desired fuel mass Air Mass = measured air mass, from MAF sensor
SHRTFT = Short Term Fuel Trim, calculated
LONGFT = Long Term Fuel Trim, learned table value, stored in Keep Alive Memory EQUIV_RATIO = Desired equivalence ratio, 1.0 = stoich, > 1.0 is lean, < 1.0 is rich 14.64 = Stoichiometric ratio for gasoline
A conventional O2 sensor (not a wide-range sensor) can only indicate if the mixture is richer or leaner than stoichiometric. During closed loop operation, short term fuel trim values are calculated by the PCM using oxygen sensor inputs in order to maintain a stoichiometric air/fuel ratio. The PCM is constantly making adjustments to the short term fuel trim, which causes the oxygen sensor voltage to switch from rich to lean around the stoichiometric point. As long as the short term fuel trim is able to cause the oxygen sensor voltage to switch, a stoichiometric air/fuel ratio is maintained.
When initially entering closed loop fuel, SHRTFT starts 1.0 and begins adding or subtracting fuel in order to make the oxygen sensor switch from its current state. If the oxygen sensor signal sent to the PCM is greater than 0.45 volts, the PCM considers the mixture rich and SHRTFT shortens the injector pulse width. When the cylinder fires using the new injector pulse width, the exhaust contains more oxygen. Now when the exhaust passes the oxygen sensor, it causes the voltage to switch below 0.45 volts, the PCM considers the mixture lean, and SHRTFT lengthens the injector pulse width. This cycle continues as long as the fuel system is in closed loop operation.
lamrith said:
In a perfect world yes, but with density changes in fuel and air and temps it wont be ''that'' perfect .. some folks here have -20 STFT like i just did and it seems that while not ideal is ok for runs?
I am curious if AJP or someone else could advise on what a BAD LOW STFT would look like ...
TopherSho said:
In a normal application (no meth) it's considered best to keep within 7% either way (adding or removing) on the trims.
You can max out your trims too, so if the total added trims is 1.25 then the ECU will close the throttle to prevent you from going to lean.. the car can no longer add fuel to keep your AFRs where they are demanded. I ran into this when I was Torrie tuned a lot...
If you go .75 or lower meaning the ECU has pulled all the fuel it can, then your AFRs will go too rich. This is bad too as it can lead to bogging out, misfires, etc...
Generally with a standard tune like I said.. plus or minus 7 is what most people want... gives the ECU plenty of room to make corrections for air density changes.
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TopherSho said:
No, you add the trims together... LTFT is derived from readings of your STFT... basically it takes the historical data from your STFT to create a LTFT value that is used for a given load... or throttle position.. not sure which or if it's queued of another value...
In any even, if your LTFT at WOT is 1.07 which is technically adding +7% more fuel but at that moment your STFT is at .79 which is technically pulling -21% fuel then your fuel trim at that
moment is 7 + -21 which equals -14.... which means technically your ECU is pulling 14% fuel to achieve desired AFR. Hope that makes sense... I'm only about 5 sips into my coffee....
My fuel trims at WOT sit around .86 or so... Brad can adjust the fueling which brought my fuel trims to around .96...
Throttle closures directly effect your AFR when spraying... so if your tune uses throttle to control boost spikes then you will go rich when the throttle closes for a brief second...
hawkeye93 said:
It's the way the data is being presented... you do add up the fuel trims for LTFT and STFT to identify the total fueling picture....
As your LTFT even out your STFT will move closer to 1.00....
Example: lean condition is triggered when the ECU sees a total of 25% added fuel to maintain AFR....
So if your LTFT is 1.10 (or +10%) and you have a spike on your STFT of 1.15(or +15%) the ECU will read that as +25% fuel added which is the maximum it can add... then close the throttle to keep from going even leaner on the AFRs.
If your LTFT is 1.00 and your STFT spikes to 1.25 the ECU sees it the same way... +25% and closes the throttle for the same reason as above...
I have logs showing this... the point is, you need to account for LTFT and STFT to understand how the system is running... it's not one or the other...
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