Discussion on how bypass valves can affect LTFTs.

[08_ms3_gt]

Originally Posted by Sacrilicious +/- 8%? the LTFT should stay about constant after your ECU learns it. since there's 4 different LTFTs, you might see different LTFTs flash by on the screen as you vary your throttle position, but your heavy throttle LTFTs should stay about constant after it's been fully learned. if your heavy throttle LTFTs are varying from -8% to 8%, then something whack is going on with your HKS SRI. usually, it means that the air is too turbulent over the MAF sensor and is confusing the hell out of your ECU, because the thing it's counting on is that the fuel trims will stay about constant under similar throttle conditions. your WOT AFRs look correct for an AP...have you checked your fuel pressures?

well, the LTFT does seem constant for a given operating condition, but those separate values are different. i'll have to log to figure out specific values, but i saw 8% when off the throttle/engine braking.

i've been wondering about the air turbulence with my SRI - BUT supposedly the air filter itself acts as a straightener. if my LTFT values remain steady then i won't worry as much about it.

i've looked a bit at the fuel pressure and it looks about right, holding above 1600PSI and hovering around 1700PSI at WOT. no decreases upon running WOT for a couple thousand RPM's. but i haven't done any logs yet, which i would probably need to diagnose the cut - boost, fuel pressure, possibly IAT, etc.

[DaleNixon]

Sacrilicious, when Christian helped you was this via the Cobb AP? I am on the stock ECU and do not own an Accessport.

If I didn't have the Dashhawk, I wouldn't even be concerned with this problem as the car runs fantastics. Chris @ Cobb did say that my LTFT's are higher than he'd like to see though.

[Sacrilicious]

Originally Posted by 08_ms3_gt well, the LTFT does seem constant for a given operating condition, but those separate values are different. i'll have to log to figure out specific values, but i saw 8% when off the throttle/engine braking. i've been wondering about the air turbulence with my SRI - BUT supposedly the air filter itself acts as a straightener. if my LTFT values remain steady then i won't worry as much about it. i've looked a bit at the fuel pressure and it looks about right, holding above 1600PSI and hovering around 1700PSI at WOT. no decreases upon running WOT for a couple thousand RPM's. but i haven't done any logs yet, which i would probably need to diagnose the cut - boost, fuel pressure, possibly IAT, etc.

ok, it's perfectly normal to have 4 distinctly different LTFTs for the 4 operating ranges. as long as each individual LTFT is pretty steady, then that's pretty darn normal. as long as your LTFTs are below 15%, you should be good to go.

here's a common misunderstanding: my LTFTs are 10.0%...that seems high...i must have a problem!

the truth to this matter is that all the LTFT means to you is that due to the way the air flows in your intake, the way fuel injectors respond to your ECU's command to inject X amount of fuel; the system has to compensate by Y% after all these sensor/actuator inaccuracies are taken into account to achieve the AFR that it wanted to get. as long as you allow the LTFTs to develop normally after an ECU reset, you will be able to safely do WOT pulls.

where a large baseline LTFT at WOT can be scary: if you have a baseline LTFT of 15%, and you go and reset your ECU and then you don't bother taking the time to relearning your LTFTs before WOTing the shit outta your car, you will now be missing 15% of the necessary fuel to achieve a commanded AFR of say, 12.0:1. if THAT is the case, then you will run way leaner than 12.0:1 and probably end up in like the upper 12s or even 13s...that is pretty scary if you keep blasting it like that...=/ but, if you allow the LTFT to learn itself back up to 15% after an ECU reset, then it will still run perfectly normally and get you that 12.0:1 AFR that your ECU commanded.

long story short: it's nice to be able to tune your LTFTs all down to 0%, because that allows you to WOT without relearning fuel trims after an ECU reset, but even if your baseline LTFTs are rather high (10-15%), as long as you take the time to relearn the LTFTs after an ECU reset, you will have no problems running your car normally.