<VV> MAP sensor - utlimately for a Corvair

[BBRT]

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