<VV> RE: MAP sensor - utlimately for a Corvair

[Craig Nicol]

Chuck asked:
<snip>Manifold Air Pressure sensors used on more modern cars sense vacuum in
the manifold as I understand it. Some are mounted on a firewall with a
vacuum line to the manifold. 
Questions. What voltage is supplied and what voltage range vs. vacuum range
do they sense? What is the reference? Absolute or ambient? If ambient
pressure, how does the sensor get it? Is there a port on the sensor?
Thanks Chuck S. BBRT <unsnip>

Craig Replies:
The common GM sensor senses ambient (baro) at Key-on and this value
(voltage) is stored in the ECM as "BARO".  There is no separate port for
sensing barometric pressure. Sensor variations after engine start (caused by
the engine operating) become the MAP (Manifold Air Pressure) value.  Note
that this value is always a pressure - the concept of manifold vacuum is not
typically used or relevant anymore.  The weakness of this sensor setup is
that BARO remains fixed in the ECM until the next key cycle, so if you start
in LA and drive to the mountains, the LA value is still the supposed BARO.  

Newer ECMs have on-board (literally) baro sensors that track the actual BARO
value.  GM MAP sensors are available in 1-bar (for NA engines) and 3-bar
(for turbocharged engines).  They both run off of a regulated 5v signal
that's sourced and monitored by the ECM.

Craig (Now you are talin' my bread and butter) Nicol