Altitude Compensation

[Darrell Norquay]

At 11:24 AM 9/11/96 -0700, RJH wrote:

>>From what I read and hear, the Air Flow sensor used in production
>EFI's at best compensate for temp and density changes in the 
>intake air mass.   Then the oxygen sensor is used to fine tune
>the mixture to a rich power setting - I.E. no uncombined oxygen

>Does anyone do any altitude compensation?  Because, as altitude

Most production systems use barometric compensation in one form or another.
Some systems take a baro reading from the MAP sensor after the ignition key
is turned on, but before the engine starts, and store this as a reference.
This can also be updated at WOT, since manifold pressure is essentially =
baro pressure at this point (with some flow related pressure drop).  Some
systems have a separate baro sensor in addition to MAP.  Some MAPs are not
absolute sensors at all, but differential sensors, referenced on one side to
atmospheric, so as the baro changes, the MAP reference point changes as
well.  Some compensation is possible with the fuel pressure regulator, since
it is usually referenced to manifold pressure and thus atmospheric
indirectly.  And, some systems have none at all.  Remember that most carbs
have no baro comp, and they worked for the last 100 years or so...   As you
mentioned, the O2 sensor can compensate somewhat at steady state conditions.  

regards
dn
dnorquay at awinc.com