LTFT vs VE Table

[NewV]

So I have a log w/out a wideband and I'm trying to interpret what I'm looking at. My understanding is that based on the VE table, MAF, MAP, STFT and LTFT along with the narrowband O2 sensors the system is basically trying to maintain 14.7 AFR at all times.

Based on that, I have setup a new map table in Scan that has the same rows and columns as the VE table from Tune tool (RPM and SAE.MAP inHg). I then set the data as SAE.LONGFT1 which is a %.

From this if my LTFT % is 8.19% at 2400 RPM and 7.4 inHg MAP, the VE table that corresponds is 64.7 can I assume the VE table for this cell is 8.19% lower than what is needed to maintain 14.7 AFR?

As such if I decrease that VE cell amount by 8.19% I could expect the LTFT for this cell to drop to 0%?

[Rhino79]

Correct except if you have positive fuel trims you need to increase the VE by the 8.19% because the pcm is adding 8.19% currently. If you increase that area by 8.19%, ideally your trims would be around 0. Your target is -5 to 0 on fuel trims.

[SSpdDmon]

Yeah, Rhino's right about increasing the VE for positive trims and decreasing for negative trims. However to make things a little clearer, you need to multiply the 8.19% to the affected cell. If you add in 8.19%, it will be way too much. Think of it as 5% + 50% in a cell = 55%. But, 5% * 50% in a cell = 52.5%. So, adding the 5% would have been 2x as much as you needed in this example.

On top of the average LTFTs, it would also be wise to monitor the average of the STFTs on the same kind of map. Then, you can add the average LTFT and average STFT for a given cell to get a closer correction. In other words, the cell you mentioned said 8.19%. If the average of the STFTs was -2.5% for that cell, then you'd be increasing the cell too much. Make sense?

If you know anything about Excel, it's not that hard to make something that will do the math for you. If you want to see what I put together, I can shoot you an email with my spreadsheet. Just need an address...

Jeff

[NewV]

If you know anything about Excel, it's not that hard to make something that will do the math for you. If you want to see what I put together, I can shoot you an email with my spreadsheet. Just need an address...

Jeff

Jeff - Please send your spreadsheet if you don't mind I would like to see it.

svassh@msn.com

Thanks

Scott

[Rhino79]

Yeah, Rhino's right about increasing the VE for positive trims and decreasing for negative trims. However to make things a little clearer, you need to multiply the 8.19% to the affected cell. If you add in 8.19%, it will be way too much. Think of it as 5% + 50% in a cell = 55%. But, 5% * 50% in a cell = 52.5%. So, adding the 5% would have been 2x as much as you needed in this example.

Correct, I was vague in my first post. I don't know much about excel, but my wife, which is majoring in accounting, sure in heck can whip around excel pretty good.:rockon:

[Doc]

AutoVE tuning is the way to go. Follow the tutorial. You need to be in Open Loop Speed Density for starters. Too many cooks (modifiers) in the kitchen (your tune) will have you chasing your tail forever. Download and read, and print out the tutorials while you are waiting for your wideband to show up.

[SSpdDmon]

Also, my statements were assuming your MAF was disabled and you were in speed density mode.

BTW - Scott, you've got mail. :)