MAP vs MDP

[Darrell Norquay]

At 10:13 AM 6/7/96 EST, Will M. wrote:

>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?  

Probably a matter of semantics.  (Could also be interpreted as MAnifold 
Pressure sensor) Maybe the first one made was an absolute sensor and the
name stuck.  An absolute sensor is just a differential sensor with one side
referenced to 0 PSIA. In any case, either sensor (directly or indirectly)
measures the pressure in the manifold.  One just has a different reference 
point than the other.

>There may be a few designs that use only a differential sensor, I don't know.  

I should have said Some instead of Most.  When it comes right down to it, the
differences in density due to barometric pressure extremes are fairly small
and may be ignored with little error. (around +/-3%) The effects of altitude
are a different story, however, around -10% per 1000M.  Closed Loop systems
may be  able to take care of this condition.  Let's face it, when it comes
to auto mfgrs. cheep is better; if it wasn't for emissions laws they would
not try to calculate AFR that accurately.  Remember, carburetors have no
altitude or barometric pressure compensation, and they were good enough for
70 or 80 years...  (The air on top of mountains is too damn pure anyway, it
could stand 
a little pollution :)

>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.

You read my comment about that one...

>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.  

You are right, since the well known PV=nRT equation is based on P being 
absolute, with this system you would need both sensors (Manifold Pressure 
sensor and BP sensor) to be absolutely correct.

regards
dn