How important is it to target zero STFTs?

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Jeffman

Captain
Jan 7, 2017
1,340
In a recent post in another thread somebody commented on the importance of a custom tune targeting 0% trims. Certainly I understand that you don’t want to max out the trims one direction or the other thus leading to too rich or too lean, but is there any performance gain if trims are essentially zero?
 

SJ_1989

Sergeant
Aug 7, 2018
300
Illinoisssss
*insert response from someone who has zero tuning experience but reads the interwebs and is interested in learning in order to Make His Car Great Again*

Now that I've made that statement....I'm interested in knowing as well. But here's my take to get the conversation going. I'll fix this if someone points out errors in my reasoning. Hack away.

The few logs I've reviewed during a 4th gear pull from, in my opinion reputable tuners, show the STFT's decreasing nice and steady from ~30%ish at the start of the pull down to 0ish around ~4500 rpm. Then hovering around 0 until redline. During this time trims vary bank to bank by ~5-8%. This tells me that tuner was able to dial in the tune pretty well without relying on the closed loop control (STFT's) to adjust for "flaws" for lack of a better term in the tuning or hardware issues (i.e. leaky injector, vacuum leak, etc).
 
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Rob09msport

Major
Oct 28, 2017
1,850
Monroe CT
0 is target to allow for variables. But Logs wont be 0 cause variables like temp,gas,humidity,altitude. But a car that is tuned to 0 originally will have 78 percent swing 34 either way. So if you go to ethanol free pump you won't be in trouble etc
 
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noorj

10 Sec Club(N54)
Jul 12, 2017
128
Detroit, MI
I call fueling good once it’s within +\- 10% on these cars since the closed loop has so much authority. On O/L cars at wot I get it 5% on CL and +\-0.02lambda WOT.

There’s no performance benefit, just reliability benefit if something else goes wrong
 
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jyamona

Captain
Platinum Vendor
Nov 7, 2016
1,069
Philly
I would always try for as close to 0 as possible. I'm debating adding a toggle to temporarily turn off LTFT so that tuners can better dial in the scalars without the long term factor interfering.

Keeping it close to zero has a few benefits, especially post shift where the DME can see a large quick stft as an issue and put you in OL. Additionally, once you get kicked to OL for whatever reason, the scalars are still used, just no stft or ltft so the closer you were to 0% to begin with, the safer your OL AFR will be.
 

noorj

10 Sec Club(N54)
Jul 12, 2017
128
Detroit, MI
Not sure that's really needed. It's easy enough to get an LTFT map from a nice long log covering the full speed/load range and plop that into the scalars and bam, non-wot fuel is set.

I guess I'd consider going OL @ WOT a "if something goes wrong" scenario, but yeah being close to zero is good but it will always drift some with varying ambient conditions so 0% is sort of arbitrary since it only applies to the conditions it was calibrated for.

For reference, most OE's cal for standard conditions then 5 book-ended climate conditions. after that the passing criteria for releasing a production calibration +/-15% STFT for 85% of the time at any condition.
 
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jyamona

Captain
Platinum Vendor
Nov 7, 2016
1,069
Philly
The ambient conditions are mostly covered then by ltft. In these DME the ltft is stored in a multi dimensional neural network. I think it's 5 axis if I remember correctly? Rpm, load, ambient temp, coolant temp, and something else
 
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Jeffman

Captain
Jan 7, 2017
1,340
I would always try for as close to 0 as possible. I'm debating adding a toggle to temporarily turn off LTFT so that tuners can better dial in the scalars without the long term factor interfering.

Keeping it close to zero has a few benefits, especially post shift where the DME can see a large quick stft as an issue and put you in OL. Additionally, once you get kicked to OL for whatever reason, the scalars are still used, just no stft or ltft so the closer you were to 0% to begin with, the safer your OL AFR will be.
Thanks. Best answer.
 

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