Gernby

I've finally gotten my fuel maps pretty well ironed out, and am considering reducing the min / max short term fuel trims. What are the pros and cons of doing this? Will it improve throttle response? Obviously, I would need to undo this if I make any changes to my setup, and would need to check my tune periodically to make sure everything is okay. However, I believe that limiting the STFT would not put any limits on the LTFT, so it would still be able to compensate for climate changes just fine.

scareyourpassenger

Are you trying to tune cruising a/f leaner. So far I don't see any reasoning why this would be beneficial.

Gernby

If you look at a datalog from normal stop and go driving, you'll see that the short term fuel trims are all over the place as the ECU tries to keep the AFR at 14.7. You can see that it is applying rather large fuel adjustments that are really unnecessary if the fuel map is "perfect". It seems to me that these incorrect / unnecesary STFT's will just reduce throttle response, and maybe even reduce efficiency some amount.

Gernby

I went ahead and limitted the STFT values to +/-5% for my commute home, and did some datalogging. I have to say that I really do think it improved throttle response significantly. I realize that most people will say just about anything improves throttle response due to the placebo effect, but I really do think this did. I even chirped the tires a couple times driving around my office parking lot with minor throttle twitches in 1st gear.

scareyourpassenger

Interesting, did it also limit the accel enrichment?

Gernby

What is accel enrichment? For the most part, it just prevented the STFT from going way negative. There were very few places where the STFT would go to +5%. In almost all cases, the STFT would max out at +/-5% for only about 1 second until the primary O2 would settle..

spectacle

There's no such thing as a perfect fuel map. If the map is tuned well though, limiting the STFT values won't do much since they self adjust to reach the target AF. Throttle response? I'd say negligible...no more than how good the fuel map is to begin with.

Gernby

The reason why I used the word "perfect" (in quotes) is because there could be a map where the map is so close to target that dynamic fuel trims aren't beneficial. In this case, any time the ECU tries to apply a dynamic fuel trim based on an O2 input, it's really just screwing up a good thing.

I can see in my datalogs that the ECU is applying excessively large fuel trims (due to the delay in the O2 sensor?) even when it shouldn't, and I believe this causes throttle response to suffer.

scareyourpassenger

It has to do when you accelerate. The ecu will add additional fuel to prepare for high load quickly.

Have you noticed in the logs it will go rich when accelerating?

Gernby

I'm beginning to think you've made up "accel enrichment" ...

I'm not aware of anything that the ECU does to prepare for accelleration or high load other than to apply LTFT and STFT (if in closed loop) to the fuel map. When my MAP is below 883 mBar (this is my open / closed loop threshold), the ECU dynamically alters STFT in an attempt to maintain 14.7:1 AF. Whenever I make throttle changes, there is always a short spike rich or lean with an accompanying spike in STFT (that I think is unnecessary for a well tuned fuel map). This doesn't happen above 883 mBar, since the ECU doesn't try to apply any sort of dynamic fuel trim in open loop. In open loop, I have my fuel maps tuned for a ~13.1 AF.