First I would like to say thanks to all those that answered my question on how they obtain their LTFT's for tuning. I got answers ranging from very technical to someone monitoring a laptop while driving like a bat-outta-hell.

One answer said the TRUE value would be the sum of the short and long trim at the time of sampling. Hmmmm. Are the short trims measured in the SAME units as the long trims? That's my new question. If so, then I think a data log could have parameters in it like this:

1) Short Trim Bank 1
2) Long Trim Bank 1
3) Sum of #1 and #2 (Calculated Figure)
4) Short Trim Bank 2
5) Long Trim Bank 2
6) Sum of #4 and #5 (Calculated Figure)
7) Fuel Trim Cell
8) MAF Raw Data

Now average columns 3 and 6 for the LTrims. Any Thoughts on this???

John In Albuquerque

 

ChrisB, thanks for your answer. I am a little confused on a couple of things. First, I want to use the info to change the MAF table. You say I must log the MAF flow rate to do this. Could you explain?
Also, I monitor the fuel cells to make sure I don't use the WOT and deceleration ones in averaging my Ltrims. How do you average yours so that you don't use these states if you are not looking at the cell you are in? Thanks for any advice.

 

Re: maf flow rate - think about it this way.

We don't log it - we have MAF frequency and l-trim values.

Now what.

You can't apply trim values directly to frequency in any form or manner since

1) L-trims and maf frequency don't have any sort of direct relationship (like airflow and l-trims (fueling) do).

2) Frequency values are fixed - so you couldn't alter them if you wanted - you have to alter the airflow.

Now you could copy the MAF transfer function into excel, curve fit it, and calculate the airflow for each Hz you logged - but it seems much easier to me to just log the massflow as well - since that is what you are going to alter.

I am assuming you are going to do something similar to this
www.slowcar.net/shared/correctedmaf.xls

Re: fuel cells for decell and WOT. Yep, I guess you could do that. Honestly when I am logging to tune I just don't engine brake and don't go WOT (pop it into neutral when slowing down). But good point, that could be a possible use.

Chris

 

Chris, please don't interpret my questions as being argumentative; rather, I am just try ing to learn how this system should be working. Correct me where I am wrong in my logic here. First, the MAF produces a signal of what it THINKS the airflow rate is and sends that to the PCM. A different flow rate produces a signal of a different frequency (???). The PCM uses this signal and the MAF table (which we can change with LS1-Edit) to come up with a value used in determining STFT's, which ultimately lead to the LTFT values. I thought airflow depended on the load and throttle opening and RPM, and we could not control that with programming. Help me where I am wrong here. Thanks

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Vettepartz:
<strong>Chris, please don't interpret my questions as being argumentative; rather, I am just try ing to learn how this system should be working.
</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif"></strong>

No, I wasn't interpreting it like that at all - I was just trying to explain the logic a little behind it! Sorry if it came across as argumentative on my part!

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">
Correct me where I am wrong in my logic here. First, the MAF produces a signal of what it THINKS the airflow rate is and sends that to the PCM. A different flow rate produces a signal of a different frequency (???).
</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">

Yep - the MAF has a wire in it that it maintains at a constant temp by adjusting the current flowing through it. As more air comes through the wire will loose heat faster, so more current must be applied to compensate.

Based on the amount of current flowing through the wire at any given point the MAF outputs a frequency to the PCM.

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">
The PCM uses this signal and the MAF table (which we can change with LS1-Edit) to come up with a value used in determining STFT's, which ultimately lead to the LTFT values.
</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">

Almost, but you missed a few steps; this might be the problem.

The PCM takes this signal and based on a calibration that was lab derived for that particular sensor setup (originally anyway) it translates the frequency input to an air massflow value (g/sec, lb/min, etc.). The PCM then decides what the target a/f ratio is and inputs the amount of fuel it thinks is required to achieve that a/f ratio based on the incoming air amount.

*If* you are at part throttle (and the target value is thus 14.7:1) the computer looks at the feedback from the O2 sensors which tell it how close it actually is to the value it is trying to achieve. If the MAF is perfectly calibrated, as well as the injector flow rates, etc. then theoretically the l-trims/s-trims should be zero. But everything is not perfect, so the O2's are actually what "originate" the l-trim/s-trim correction values.

When you use the fuel trim values to tune the MAF transfer function you are taking the position that inaccuracies in the results (as measures by fuel trims) are a result of inaccurate MAF calibration - so you will use the feedback of the O2's (through fuel trims) to correct the MAF calibration.

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif"><strong> I thought airflow depended on the load and throttle opening and RPM, and we could not control that with programming. Help me where I am wrong here. Thanks</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">We can not alter actual airflow, but we can change what the computer thinks the actual massflow value is for a given MAF input frequency.

Chris

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Vettepartz:
<strong>One answer said the TRUE value would be the sum of the short and long trim at the time of sampling.</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif"></strong>

That would be the true fuel trim correction. If it's the "true" value or not depends on what you are applying value to.

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif"> Hmmmm. Are the short trims measured in the SAME units as the long trims? That's my new question.</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">

Nope, the s-trims are somewhat of an iterator for the l-trims.

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif"><strong> If so, then I think a data log could have parameters in it like this:

1) Short Trim Bank 1
2) Long Trim Bank 1
3) Sum of #1 and #2 (Calculated Figure)
4) Short Trim Bank 2
5) Long Trim Bank 2
6) Sum of #4 and #5 (Calculated Figure)
7) Fuel Trim Cell
8) MAF Raw Data

Now average columns 3 and 6 for the LTrims. Any Thoughts on this???
</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Well 3 and 6 would be a direct sum, though if anyone knows the exact methodology that the s-trims use to incriment the l-trims then you could still achieve the same effect.

A couple of points - I think I replied to your original question on some forum or another - I still would submit that what you do with the l-trims values will depend entirely on what your goal is. Whatever scales the table in the pcm you are going to modify is what you should log.

For instance I see you are logging raw maf frequency - so I surmise you want to scale the MAF transfer function? You will also need to log maf flowrate (you could calculate it I guess but it's much easier to log it).

Also with regards to the fuel trim cell - I see lots of people logging this/talking about spreadsheets to array your l-trims by fuel trim cell. I never honestly understood what the point of this was. Yes, you can get a nice representation of what the pcm stores - but what good does it do you? You can't really use it to address any tuning issues, so it seems like nto really too usefull of a parameter to log. Unless you are going to alter the fuel trim cell boundaries - but then I have never had a reason/need to do that.

Chris