Is LTFT a value or a table?

[dstrout]

Is the Long Term Fuel Trim a single value that the ECU determines and applies to everything, or is it developing an LTFE for each RPM / load combination?

The root of the question is do I need to run in closed loop across a spectrum of load / RPM states (e.g. 6k rpm uphill against the wind towing a camper) or is running in the 2k - 4K rpm range on the way to work enough to get LTFT trained properly for the whole operating envelope ?

I ask because the car is pretty much dedicated to the track, so I’m more concerned abort having the trims correct in the 4K-7.5k rpm range, which are a bit tricker to train on the street.

[tomm.brz]

ltft is just the average in the long run of the Stft
stft is applied constantly all the time to the closed loop, while ltft could also be applied to the open loop

if you scale the maf really well you can disable ltft totally so that the open loop doesnt get influenced by that

you could also disable ltft in closed loop, modifying the closed loop afr target to 14.7 so that only stft works, that s what i do in my tunes and everybody is happy saying the closed loop is really smooth. You just need to scale really well the maf and be aware that the closed is really lean at 14.7 so be cautious with the timings

[dstrout]

I’m running E85 and spend most of my time in open loop (WFO, then brake!), so I am thinking I probably want ltft to apply to open loop. I’m just not sure how to most effectively train it.

[tomm.brz]

oh, yes. With ethanol blends you want ltft to work also in open loop. it trains constantly by itself btw, Maybe do some Light pulls with low throttle to 5k rpm,and log to check for knock and maf scaling

[Kodename47]

Originally Posted by tomm.brz

ltft is just the average in the long run of the Stft
stft is applied constantly all the time to the closed loop, while ltft could also be applied to the open loop

It's applied in MAF bands. These bands can be changed in the ROM. You can stop LTFT in open loop by adjusting the final band start point higher in the MAF range. Stock settings it's:
-1.6 g/s
1.6-5 g/s
5-12 g/s
12-20 g/s
20-60 g/s
60+ g/s

[tomm.brz]

what you mean with stop ltft? because to stop ltft working in open loop i just put the "long term max/min adjustment" tables to 0 without changing the bands

[steve99]

Unless you have a controlled source of e85 thats always a set ethanol percent or you have a flex fuel system you will need ltft in open loop to compensate for ethanol changes.


If your using ecutek you could enavle full time closed loop fueling though and achieve a good result with varing ethanol content as long as thte variance is only say 10% either side of a base range




USA E85 fuels can vary between 50 and 90% in some areas

[Kodename47]

Originally Posted by tomm.brz

what you mean with stop ltft? because to stop ltft working in open loop i just put the "long term max/min adjustment" tables to 0 without changing the bands

That doesn't always stop it, not sure why but I have seen LTFT active with that set to 0. There are a couple of ways to get LTFT negated or disabled. Raising the 60 g/s to around 80 g/s would have the desired effect. There would be no CL over 80 g/s so no STFT to set LTFT and thus 0 LTFT in WOT OL. It's how it was done on older Subarus by many tuners.

[tomm.brz]

Originally Posted by Kodename47

That doesn't always stop it, not sure why but I have seen LTFT active with that set to 0. There are a couple of ways to get LTFT negated or disabled. Raising the 60 g/s to around 80 g/s would have the desired effect. There would be no CL over 80 g/s so no STFT to set LTFT and thus 0 LTFT in WOT OL. It's how it was done on older Subarus by many tuners.

i dont know how you got ltft working with long term min max tables set to 0, i NEVER got it working in more than 290 maps that I tried in my car

that s also what Fensport in UK does, and even with stft and ltft working in closed loop with the bands not touched, i never saw ltft in open loop working..i saw more than 12 fensport cars tuned that way and 8 cars tuned by me.. never got ltft working in OL

[Kodename47]

Originally Posted by tomm.brz

i dont know how you got ltft working with long term min max tables set to 0, i NEVER got it working in more than 290 maps that I tried in my car...... and even with stft and ltft working in closed loop with the bands not touched

You just contradicted yourself there. You don't know how I got LTFT with it set to 0 but you've seen the LTFT active in CL? Is that not the same thing? I've seen similar, so I just disable LTFT completely if I don't want it. I have had seen cars with OL fuel trims after some time that have the "Mix/Max" set to 0.

Edit:
Just read you wrote this:

Originally Posted by tomm.brz

modifying the closed loop afr target to 14.7 so that only stft works You just need to scale really well the maf and be aware that the closed is really lean at 14.7 so be cautious with the timings

This is the only sure fire way of disabling LTFT completely, from my experience at least.

However the OP asked whether the LTFT is set as one value, the TL: DR is no it's not.

[tomm.brz]

i have not contradicted myself. i meant that those 2 tables "long term min max" work only in open loop. if you set them to 0, ltft will still work during closed loop but then on open loop will be forced to 0 because of the tables set to 0

of course is a bit of non sense having ltfr working in closed loop and not using it in open loop,so what i do is disable ltft both in open and in closed loop and scale well the maf
to disable ltft in closed loop the only way i found,thanks to steve and partially analysing fensport maps, is to force the closed loop afr adjustment to 14.7..a value of 14.69 would already enabling it again.. but anyway no need to mess with ltft bands

lately fensport has the same approach. they disable ltft both in closed and open loop, particularly in FI tunes. I have not access to ethanol so that s why i do this way,i only use petrol so i dont care about ltft, stft only is enough to have a good closed loop, and i enable also racerom v10 closed loop fuel control so i always have a spot on afr all the time

[tomm.brz]

Originally Posted by Kodename47

You just contradicted yourself there. You don't know how I got LTFT with it set to 0 but you've seen the LTFT active in CL? Is that not the same thing? I've seen similar, so I just disable LTFT completely if I don't want it. I have had seen cars with OL fuel trims after some time that have the "Mix/Max" set to 0.

i think it s impossible to have some ltft even after some time if the min max tables are set to 0, because as ecutek says, they force ltft in open to 0 if you set them to 0.
the only time i saw some ltft with those tables set to 0 is when i forgot to 0 the delay tables of the CL/OL transition, maybe that was your case too

[Kodename47]

Originally Posted by tomm.brz

i think it s impossible to have some ltft even after some time if the min max tables are set to 0.....the only time i saw some ltft with those tables set to 0 is when i forgot to 0 the delay tables

I'm not going to keep arguing with you. But yet again, you have just contradicted yourself.

The delay settings shouldn't matter, the fact they do suggests that the LTFT is still active as it's allowing the car to hold STFT into higher g/s. I know it says that those should stop the LTFT from activating but in my experience they don't. I could probably dig out a load of logs as well, seeing as those have been set to 0 on my car since my early tunes, but I don't fancy wading through thousands of files to try and find some examples.

The only sure fire way is to set the closed loop fueling targets to 14.7, but because I like to use CL to enrich the fueling under load I use the fuel trim bands to ensure that there is no trims set for WOT OL operation and so far, so good.

[Tor]

Just as another curiosity with LTFT. Since changing injector ratios to a uniform value above 2400 rpm, I only see the 20 g/s LTFT applied during OL. It's like it disregards the 60 g/s completely.

I have logs from cruising in CL, 5th gear, low rpm (exactly 20 g/s) and I see the LTFT changing. Later in the logs WOT and it has the exact trim throughout the full rev range that was set at 20 g/s.

I have multiple examples of this (including multiple new LTFTs set during CL and applied during WOT above 60 g/s in the same logs), so it makes me wonder if the descriptions of the of the brackets are correct. Maybe injector ratios play a part as well.

With the stock injector ratios, there is the classic split at 5000 rpm when port injection becomes active again. I've never seen this split with uniform injector ratios.