Posted for "George Braly" <gwbraly@gami.com>:

 Skip writes:
 
 “MAP (displayed in inches of Hg) is not actually “absolute” since it is
ambient pressure (AP) plus gauge sensed pressure (GP).   “
 
 
 Ah…  Skip,  actually,   manifold pressure as the term has been commonly used
for aircraft engine operation for the last 80 years or so, is truly an
“absolute” pressure measurement, referenced to a perfect vacuum.  It has
nothing to do with  “gage  sensed pressure (GP) “
 
 Any other notion adopted by VM,  or anybody else,  is inconsistent with  the
normal and common engineering usage of  “manifold pressure”  as used by
engineers with respect to aircraft piston engines.  It would also introduce an
enormous source of error, uncertainty, and confusion in all engine test data.
 
 Ram air effects are fully accounted for by an absolute manifold pressure
sensor that is properly plumbed into the induction system.
 
 Any other “adjustment” for some other source of  air pressure is simply an
erroneous concept.
 
 Many systems have a vent system incorporated into the pressure fuel sensor
designed to pick up ambient pressure, or a designated reference pressure,  but
that has nothing to do with manifold pressure, even if the fuel pressure
 mechanism happens to be located inside the same 3” round instrument as the
manifold pressure sensing system, which is the common way of doing that.
 
 In a very well designed  normally aspirated engine installation,  adding and
  installing a ram air system and bypassing the induction air filter may raise
the cruise MP by up to 1.5 to 2” of MP at normal cruise speeds, compared to
not having such a system.
 
 On a turbocharged engine,  it can improve the critical altitude by several
thousand feet, as one enjoys a multiplier on that 1.5 to 2 inches,  the
magnitude of the multiplier being the pressure ratio across the compressor
under the applicable cruise conditions.
 
 Regards,  George