MAP vs MDP

[wmcgonegal at rr.etc.ncr.doe.ca]

>Most MAP sensors are really differential pressure sensors with one port open 
>to atmosphere.  This automatically compensates for any differences due to 
>altitude or Barometric Pressure, and essentially measures the difference 
>between atmospheric and manifold pressure.  Some systems (Cadillac springs to 
>mind) have a separate BP sensor as well as the MAP, and compensate that way.  
>Some systems use an absolute pressure sensor (referenced to a hard vacuum) 
>and simply take a reading before the engine starts to determine current BP, 
>and remove the BP as an offset.  This obviously doesn't work too good if you 
>drive up a mountain without stopping the engine once in a while to "catch up".

>dn

Why would they call a Manifold Absolute Pressure sensor a MAP sensor 
and not a MDP (Manifold Differential Pressure) sensor if it measures 
differential pressure?  All that I have ever read or heard about MAP 
sensors is that they are absolute.  There may be a few designs that 
use only a differential sensor, I don't know.  With a MAP sensor 
(truly absolute that is), you can measure the atmospheric pressure 
before the vehicle is started and compensate accordingly once the 
vehicle is running.  I do not see how a differential sensor could 
compensate for altitude or barometric pressure unless you had a second 
absolute sensor.  With only a differential sensor you have no idea of 
what the barometric pressure is.  Without this information, how do you 
correct for air density?  A differential sensor would work fine if you 
always drove at the same altitude and your barometric pressure hardly 
ever changed.  Please enlighten me further.

Will McGonegal