Tuning with fuel trims

[Dieselman]

Hello Ben,

Have you read through Swingtan's E38 VVE tuning tutorial ? It details tuning the VVE and MAF.

Attached below ....


Regards,
Taz

Yes I have done that and my VVE is ok apart from a few cells that I just can't get 100% spot on. I was just going to have a play with this and see if it is quicker to get the VVE close while being in the safety of Closed Loop (I have changed the cam)

My factory tune was actually actually putting in + trims through the whole rev range. I have a Tech Edge WB for doing my WOT work. I have talked to Swingtan and realize that the STFT will only work during closed loop and if LTFT's are not disabled it could make the PE cells either rich or lean.


Hope this makes sense!!!


Ben

[Taz]

OK ...understand what you are trying to achieve ...

Take a look at WeathermanShawn's CALC VET thread .... specifically the calc_pids.txt file posted ... obviously you can't use this technique with an E38, but some of the CALC PIDs may be of assistance to your current endeavours ... take a look at the CALC.LTFT, CALC.LTFTBEN, and CALC.SELBEN PIDs ... these examples may help you set up your own CALC PIDs for the E38 and your WBO.


Regards,
Taz

[joecar]

You will need to find out which E38 pids to replace some of those pids in that calc_pids.txt.

[Dieselman]

Thanks Guys


I have absolutely no idea how to make a CALC PID. If some one could help that would be great


Cheers

Ben

[joecar]

Ben, no problem, I can help you with calc pids.

[swingtan]

FYI, you can't really follow Shawns Calc VET tutorial for the E38 as there is at least one important PID not available. In the E38, we have no charge temp PID to correct the air density for the VVE. We know that it's there and is used bu the OS ( unless it's disabled by the tuner ), but we have no access to check what value is being applied. I did try and reverse engineer a PID, but with two unknowns, it got too hard and I gave up.

The closest you can do is to use GM.AIRPERSEC and then set up a BEN with SAE.MAF. It's a lot easier to get the MAF calibration spot on, so do that first, then use the BEN to make adjustments to the VVE. It's not a lot different to just using a WB BEN though, so it's really a case of what seems to work best for you.

Another tip if you do want to "tweak" using the trims, is to watch the O2 voltages as well, especially when the engine is cold and the trims are not yet active. It will only tell you if the mixtures are rich or lean, but can help to pin point issues. There are still only 2 reasons that I use the trims for any corrections....

1. The tune is running CL and I don't want to drop back to OL just to make a couple of minor changes.
2. The is no WB data available and I really feel some VVE / MAF adjustment is needed to fix specific issues.

I know many say you can't / shouldn't tune without a WB, which is true when looking at the whole tune. You can however make small measured changes using the trims.

Ben, if you want to go through an excercise for this, let me know and I'll do something up for the next tuning day.

Simon.

[Dieselman]

Ben, no problem, I can help you with calc pids.

Thanks Joecar that would be good

Ben, if you want to go through an excercise for this, let me know and I'll do something up for the next tuning day.

Simon.

That would be good Simon I wish I lived a bit closer to the city sometimes, it would make catching up a bit easier!!!

I don't know if I understand how the charge temp is calculated. Is it just calculating what the air temp entering the cylinder is???



Ben

[swingtan]

Basically, yes. The ECM uses IAT and ECT as start and end points for the charge temp calculations. Then using {B0179} and {B0180} it predicrs how much the air entering the cylinder will change from the IAT, toward the ECT. The idea is that the longer the air sits in the manifold, the close it will get to ECT. The ECM then uses this to sdjust the fueling for the motor, as warmer air is less dense and needs less fuel to maintain the same exhaust AFR.

Setting {B0179} to a flat value, will result in either a rich low air speed condition, or a lean high air speed condition. Sure, you can let the trims try and fix it, but that's not what we are about here. The only time I'd do this is if I wanted to actually see what the difference in fueling was like under similar conditions so I could set up the table correctly. You can probably understand then, that if you don't actually know the exact formula to use for calculating the charge temp, it's very difficult to try and correlate the MAF readings with GM.AIRPERSEC.

I just had a thought that the charge temp may be directly taken from the tables, but I don't think it is quite that simple.

Simon.

[Dieselman]

Thanks for the explanation Simon. I had a rough idea that was how it worked but you have cleared it up 100%

Maybe using STFT's would be ok to use when someone starts to do a VVE tune just to make sure they are not running too lean to start with ie if the trims are say +5 you could add 5% to the whole VVE table and go from there??


Cheers

Ben

[swingtan]

Yeah, you can. The basic idea is that, but in practice the trim percentage does not relate to a change in the VVE in a 1:1 format. It takes a bit of practice to judge the amounts.