Altitude compensation

[Fred Miranda]

>> Isn't it true that for a speed density efi system with an _absolute_ MAP 
>> sensor, then altitude compensation is not needed?  For example, if you 
>> changed altitude, the barometric pressure changes, which changes the 
>> pressure in the manifold, which the MAP sensor directly measures. 
>>  Temperature (air density) compensation is still needed.
>
>This is correct to a certain extent.  There is still the backpressure
>on the exhaust which makes a small difference.  I have never come across
>an aftermarket efi which takes this into account.

Autronic makes a box that has provisions for ex backpressure comp, they also
seem to make barometric pressure estimations while running, off the same sensor
measuring engine vacuum.

As for the affect of altitude on a speed density system with no alt comp,
take the case
of an engine with little manifold vacuum, say it idles at 70-80kpa absolute
at sea level.
It's operating range of vacuum is so compressed that a change in elevation
of 1000ft
can through it's calibration all out of wack. It will go lean as you as you
go up the
mountain. Any system that only looks at barometric pressure at startup is
useless in
the real world. The motor could be gone well before you reach the top of the
pass.

Fred