[Diy_efi] Re: +AFs-Diy+AF8-efi+AF0- MAP vs actual vacuum

[David Allen]

  It's easy to figure this out.  Vacuum is how much LESS pressure you have than atmospheric pressure.  We can take for granted that atmospheric pressure is about 101.3 KPa or 14.7 PSI or 29.9 In Hg.  This varies with altitude and weather but not by much.  

20 KPa would be about 24+ACI-Hg Vac
14+ACI-Hg vac would be about 53.8KPa

To convert from vacuum in Inches of Mercury to KPa, use the following formula:
(29.9 - +AFs-In Hg Vac+AF0-) +ACo- 3.386388  +AD0-KPa     

To go from KPa to Inches of Mercury Vacuum:
(101.3 - +AFs-KPa+AF0-) +ACo- 0.2952999 +AD0- In Hg Vacuum

To go from PSI Boost (pressure GREATER than atmospheric) to KPa:
(14.7 +- +AFs-boost PSI+AF0-)  +ACo- 6.894757 +AD0- KPa

To go ftom KPa to PSI Boost:
(+AFs-KPa+AF0- - 101.3) +ACo- 0.1450377 +AD0- PSI Boost

(someone good with math feel free to check over this+ACE-)

  What you have to remember is that KPa is absolute pressure.  The MAP sensor contains a load cell which measures pressure relative to it's own internal standard.  The ambient pressure around the exterior of the sensor makes no difference in the reading it takes from the pressure inside the fitting connected to the manifold. It is indipendent of atmospheric pressure.
  A mechanical pressure gauge, be it a boost/ vac gauge or a PSI gauge measures +ACI-gauge+ACI- pressure, or +ACI-relative+ACI- pressure.  The reading reflects a difference between the pressure in the pressure on the gauge inlet fitting, and the ambient pressure around the gauge.
  A mechanical gauge is affected by the ambient pressure surrounding the gauge.  Assuming the absolute pressure on the gauge inlet fitting remains constant, a low ambient pressure will increase the reading while a high ambient pressure will lower the reading.
  An electronic gauge with an absulute pressure transducer is immune to this.  The transducer works like a MAP sensor (which is actually a transducer).  My manifold gauge face is marked off in Inches Hg and PSI Boost, but it reads absolute.  With the transducer sitting open on the table, the gauge will read near +ACI-0+ACI-  Let a storm approach, and it will sit a little below zero because it is measuring absolute pressure. Even though pressure IN and AROUND the transducer dropped equally, it still measures the change because it does not depend on a difference in pressure inside and outside the transducer.
  A mechanical gauge sitting open will not reflect changes in atmospheric pressure because it is comparing pressure on the gauge inlet fitting against ambient atmospheric pressure.  Both changed equally so there is no difference to display.
  They both have strengths and weaknesses.  My absolute pressure gauge is marked showing vac and PSI which is a little misleading.  But it has a large 4+ACI- backlighted face and the transducer cable is easier to route in the car than a vacuum hose.  Plus it was free. :-)
  Later,
David



+AD4- Generally speaking... how much vacuum does it take to hit 20 on the MAP.  The reason I ask is because my beast will idle at 800 with 14 inches of vacuum.  So I'm trying to determine values for my spark table...