How can you actually modify the fuel trim since the car's computer can adjust up to 25% on the ltft and 25% on the stft while the safc only allows for 50% of adjustment.
Everytime I try leaning out the fuel trim I just see the long term increase more, and say I took it all the way to -25% on my safc, the short term would just start increasing wouldn't it?
Joel Baldridge, ASE Certified Master Technician, Audi Certified Expert Technician
#78883, "RE: Simple SAFC Tuning Question" In response to Reply # 1
The way I understand it is this. The AFC is fooling the computer by showing that there is actually more/less air coming into the engine than there actually is. So let's say you adjust it to show there is more air coming in than there actually is, the ECU will adjust fuel accordingly and add more fuel than you really need. Conversely if you show the ECU that there is less air coming in than actually is, the ECU should pull fuel so that the engine runs leaner than it normally would. Understanding this, I would think that if you were leaning the engine out, you would see the LTFT and STFT go further into the negative since the ECU would be pulling fuel to accomadate the "less air" you are showing it. I could be completely off base though.
Lets just say this... I'm over 100whp and soon to cross 200whp. Thanx 2GNT! Now if I could only learn to prove it.
#78963, "RE: Simple SAFC Tuning Question" In response to Reply # 2
The problem is that that it "corrects" what it thinks is right using the o2 sensor, so even if it thinks it's getting a certain amount of air in, the o2 sensor says the mixture isn't correct and adjusts it accordingly.
Joel Baldridge, ASE Certified Master Technician, Audi Certified Expert Technician
#79383, "RE: Simple SAFC Tuning Question" In response to Reply # 4
bump again
Maybe the question isn't clear?
Basically what I wrote is that every % I change my fuel trim on my SAFC, the computer readjusts so that it's back to stock. The SAFC is capable of changing up to +/-50% and the ecu is capable of changing up to +/-50% (long term 25, short term 25). So what is the point of it?
Joel Baldridge, ASE Certified Master Technician, Audi Certified Expert Technician
#79447, "RE: Simple SAFC Tuning Question" In response to Reply # 6
Since your talking about STFT and LFTF I'm guessing you're not talking about open loop (WOT) but closed loop (part throttle). First remember that your ECU will trim fuel to reach 14.7:1 AFR under cruise. Proven on a wideband. Now say you've dumped 440cc/min injectors and still run stock FP. The ECU can only trim so much to make the car have the desired 14.7:1 AFR. Now what the AFC does is that it modifies the MAP signal so the ECU looks at a different cell in it's fuel table. This cell should have a value that would accomdate the larger injectors. The ECU will still modify it's STFT and LTFT to reach the 14.7:1 AFR it desires but it's now starting from a different point (a much leaner point) than it did without the AFC correct.
In open loop the STFT and LTFT mean nothing. The ECU goes off a preset fuel map. The AFC corrections in this case just modify the MAP value so the ECU looks at a different cell in the open loop fuel map.
#79455, "RE: Simple SAFC Tuning Question" In response to Reply # 8
Well, WOT would mean 100% throttle but I think you meant when is the car in open loop. Throttle position is part of the equation but there are other factors that determine when the car is in open loop such as MAP. But as a general case when you are at 100% throttle you will be on open loop, the rest I can only take assumptions at.
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