Greg E
04-06-2012, 10:00 PM
Nothing really new to report about this. Most of all this we already know.
Basically, fuel trims are the percentage of change in fuel over time. For the engine to operate properly the air/fuel ratio needs to stay within a small window of 14.7:1. It has to remain in this zone under all the various conditions an engine encounters every day. So, the ECU is trying to maintain this proper air/fuel ratio by fine-tuning the amount of fuel going into the engine. As it adds or takes away fuel, the narrow bands monitor how much oxygen is in the exhaust and reporting it to the ECU. The ECU monitors this input from the continuously when in closed-loop. If the o2 sensors inform the engine computer that the exhaust mixture is lean, the computer adds fuel by lengthening IPW to compensate. Conversely, if the o2 sensors report a rich condition, shortens IPW.
This change in fuel being added or taken away is called Fuel Trim. Changes in O2 sensor voltages cause a direct change in fuel. The short term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. The long term fuel trims (LTFT) are driven by the short term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.
A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second. Usually they'll stay around positive or negative 5%, but they may occasionally go up towards 8 or 9% depending on the efficiency of the engine, age of the components, and other factors. A normal long term fuel trim reading will appear to stay the same, giving a long term average of fuel added. It should be close to zero, positive or negative single digits under normal circumstances. It will fluctuate much slower, possibly appearing static.
The LTFT is divided only by the MAF sensor readings. This table was included in Chrome V1 but I don't think I ever explained this very well.
http://i.imgur.com/6tvHN.jpg
Anything under 94hz is usually idle and I almost never see the High LTFT change as usually we're in open loop by that range. You can adjust this if you'd like but I never had any reason to. Maybe if you want a very high idle with cams and ported heads, I could see raising the low -> mid & mid -> low.
The other table you see is the NB voltage the ECU considers to be 14.7. This needs to be tested, but I believe raising this value will also change the target AFR the ECU uses to cycle the NBs thus improving gas mileage.
There's a trade off though... A leaner AFR will also raise your EGTs so it could have some serious long term effects on the life expectancy of your motor so be careful.
ALSO! I have also patched Chrome V2 so that the rear fuel trims won't be disabled anymore when Periphery0 Bit2 is disabled (you need this when the secondary O2 sensors are removed). This should help keep the motor running smoother when you have a crappy tune. :)
Basically, fuel trims are the percentage of change in fuel over time. For the engine to operate properly the air/fuel ratio needs to stay within a small window of 14.7:1. It has to remain in this zone under all the various conditions an engine encounters every day. So, the ECU is trying to maintain this proper air/fuel ratio by fine-tuning the amount of fuel going into the engine. As it adds or takes away fuel, the narrow bands monitor how much oxygen is in the exhaust and reporting it to the ECU. The ECU monitors this input from the continuously when in closed-loop. If the o2 sensors inform the engine computer that the exhaust mixture is lean, the computer adds fuel by lengthening IPW to compensate. Conversely, if the o2 sensors report a rich condition, shortens IPW.
This change in fuel being added or taken away is called Fuel Trim. Changes in O2 sensor voltages cause a direct change in fuel. The short term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. The long term fuel trims (LTFT) are driven by the short term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.
A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second. Usually they'll stay around positive or negative 5%, but they may occasionally go up towards 8 or 9% depending on the efficiency of the engine, age of the components, and other factors. A normal long term fuel trim reading will appear to stay the same, giving a long term average of fuel added. It should be close to zero, positive or negative single digits under normal circumstances. It will fluctuate much slower, possibly appearing static.
The LTFT is divided only by the MAF sensor readings. This table was included in Chrome V1 but I don't think I ever explained this very well.
http://i.imgur.com/6tvHN.jpg
Anything under 94hz is usually idle and I almost never see the High LTFT change as usually we're in open loop by that range. You can adjust this if you'd like but I never had any reason to. Maybe if you want a very high idle with cams and ported heads, I could see raising the low -> mid & mid -> low.
The other table you see is the NB voltage the ECU considers to be 14.7. This needs to be tested, but I believe raising this value will also change the target AFR the ECU uses to cycle the NBs thus improving gas mileage.
There's a trade off though... A leaner AFR will also raise your EGTs so it could have some serious long term effects on the life expectancy of your motor so be careful.
ALSO! I have also patched Chrome V2 so that the rear fuel trims won't be disabled anymore when Periphery0 Bit2 is disabled (you need this when the secondary O2 sensors are removed). This should help keep the motor running smoother when you have a crappy tune. :)