Map vented to Atmosphere ???

[Phil Lamovie]

Hi Laura & Neil,

> Surely engine vacuum is relative to the ambient atmospheric 
> pressure. And as the MAP sensor is vented to atmosphere it will 
> have a fair degree of self correction for altitude.

Lets set the record absolutely. Absolute means what it says.

It is a barometer in every sense. It has an integral nitrogen
bubble as its reference and it's output is set to read  101.3 kPa
as the P of STP. It is not a differential sensor. 

Thus it is ALWAYS reading the manifold pressure relative
to ABSOLUTE pressure.

Air flow meters are major profit centres for auto manufacturers.
Some times they make more profit on the AFM replacement than the
original car sale.

Engine vacuum is controlled by the butterfly and thus is relative to
it's position and the load on the engine.

Phil 

Injec Racing