LTFT's, do they really matter?
 

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.

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

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

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.