Here the results:

0% offset change (stock) => Fuel Trims = +1.73% (FTC 6-14) and +1.70% (FTC 1-19)
+10% offset change => Fuel Trims = +0.14% (FTC 6-14) and -0.91% (FTC 1-19)
-10% offset change => Fuel Trims = +3.12% (FTC 6-14) and 3.39% (FTC 1-19)

The fuel trims values are a sum of LTFT and STFT (this to give a more precide result).
I averaged the counts and not the cells, so those numbers are a weighted average and give the real pictuer of the thing.

The change in the fuel trims is pretty logical.

With +10% offset the car feels stronger while accelerating ant the engine reacts quickly.

With -10% offset something is strange... it's not too bad but it's like an old car with a compression problem. A little asleep

Any comment? What else to try? The result with +10% is really interesting!

I'm sorry but I can't post the graphs. JR: is there a way to upload pictures directly from my PC?

In your case

After offset is changed and PCM has had relearn time then scan and adjust fuel flow to complete the changes related to offset values

I would need to see scanner data to say add or remove fuel as to the offset changes but assume -10 would make it richer and require less injector flow

To upload images simply use the options FILE MANAGER function listed below when entering a new post or full reply mode content window and it allows you to select images off your PC and embeds into the post.

Image cannot be larger then about 400K bytes

Here the results as a graph

I would do tuning tests with both -10 and +10 as to injector IFR table and see which one you like best.

OK let me try.
would you try with even bigger changes? like +/- 20%?

Do you see a reason why increasing the delay time (= advancing the fuel injection) the engine reacts better?
I can't express it in numbers, but it's one of those improvements that put a smile on your face when you test drive the car.

Better fuel vaporisation?

I think with stock injectors you should not do but a tweak of offset as you did and not as big a change as +/- 20 % as you could force the injectors to command on before or after the sparkplug fired

The delay, 2 parts, 1 is was the existing values not correct for the engine makeup and 2 did offset values for engine makeup require values different then what you had ( assumed stock values) not valid for your engine's needs.


Keep in mind if voltage level changes it then has effect on which offset row and its values are used.

Thus if for some reason voltage is low then so it the voltage to injectors thus different offset value row is being used and those values for delay might not suit the engine's needs.

I see.
The voltage is between 13.5 and 14 depending in the electrical load (lights, defogger, fan...).
I'll probably tweak the region between 12.5 and 15 V

My version of LS1Edit can read only 97-98 programs.
Could you take a look at the differences beween the years? Let's say 98 - 02?
If the cam grind plays a rule on how GM programmed those values each year should be different. If they are the same it would be interesting to understand why F-bodies LS1 have different values than Corvettes.

Stefano

The lower the voltage is at the battery means even lower voltage drop to the injectors and less the injector is spraying so it is important to assure good voltage level is maintained

If not then lower voltage also means the PCM is using the offset voltage values for that voltage row in the table.

Looking at a 1999 and 2002 Fbody you see there is offset value differences

Here the 1998 F-Body delay table, which is pretty different than the 99 and the 02

And here the cam specs of different F and Y bodies.
Notice how the 98 and the 99 F-body have the same cam specs but completely different injectors offset.

The 97-98 injectors flow is 28.5 Lbs-Hr @ 4 bar
The 99-00 injectors flow is 26.3 Lbs-Hr @ 4 bar

Is it possible that the injector design changed so much from 98 to 99 to request such a big change in the delay table?

Or did GM recognise an advantage firing the injectors a little sooner?