Magic Chicken

iTrader: (1)

I'm new to tuning and everything I've read pretty much says I should get my LTFT's in line (slightly negative) before anything else. I was having a discussion with a buddy and he is under the assumption that if the computer compensates automatically for a rich or lean condition there is absolutely no point in tuning the LTFT's. Is there a real purpose or reason for tuning them to slightly negative like everyone claims they should be? Thanks in advance for any light you can shed on this issue.

Another_User

Except at WOT. The computer doesn't compensate properly at WOT.

bomax

iTrader: (3)

You will need to get them negative. The computer tries to compensate, but nothing is perfect.

jimmyblue

iTrader: (11)

Trims compensate things to steady-state "ideal".

But grossly large trims are indicating something
not-ideal with the air/fuel metering, a "cover-up".
Sometimes the cover-up is worse than the crime
(like when fat positive LTFTs corrupt your WOT
tune).

Then there is the transient performance issue,
bad open-loop mixture control can expose more
tip-in leanness / pinging, etc. if you are over-
lean in the base setup and "compensated"
later ("the check is in the mail").

Best to have it tuned tight with just enough
margin for drift & environmentals, which means
just so slightly negative (in the closed-loop
cells; "right" in higher-MAP, power-enrichment
regions is another ball game).

Once you are certain that the base air & fuel
models are proper, you have more consistent
base for tuning performance. Build yer house
upon the rock, and all that.