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Scott:

I figure 174 knots IAS should provide maximum ram pressure boost of a bit
under 1.5 inches of Hg which is within the experimental error of your data.  
It shows that you have a good diffuser getting virtually all of what is
available.  Some bugs that always pollute the data ointment:

1) NASA tests in the 80's measured profiles of total pressure behind a
propeller at various radii.  These showed that one can get up to 20% more ram
pressure than the free stream ram pressure if you chose you distance from the
centerline correctly.  It depends on flight conditions, cowl configuration,
etc.  So there is a bit more pressure to be had behind the prop at the right
position.

2) Keep in mind that the manifold pressure gage is attempting to read an
average of a wildly pulsating instantaneous pressure within the intake
manifold.  Depending on the details of the inlet from the intake manifold to
the tube running to the manifold pressure gage (and resulting differences in
inlet and outlet flow coefficients at the fitting location) the pressure read
by the instrument can be above or below the average.  Usually it is a bit
below because inlet coefficients are invariably lower than discharge
coefficients (arising as the flow oscillates in and out of the tube going to
the instrument).  It all goes to show that making accurate measurements is
not as easy as might first seem.

I would say your inlet system is working every bit as well as could be
reasonably expected.  Declare victory and go flying!

Fred Moreno
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