Gentleman (I know some who are) and Ladies,

 

There has been a gargantuan struggle to get one’s arms around the ram air affect on MAP – Manifold Absolute Pressure – a struggle accompanied by missteps, pratfalls, traps, blind alleys and just plain bad measuring equipment of all sorts.

 

The discussion will focus on Vision Microsystem engine monitors used for normally aspirated piston avgas-sucking engines.  I believe the sensors used by VM 1000 and EPI 800 systems are identical.  Other systems may operate differently.

 

MAP (displayed in inches of Hg) is not actually “absolute” since it is ambient pressure (AP) plus gauge sensed pressure (GP).   That is indicated MAP = AP + GP.  At wide open throttle (WOT), GP can be positive from pressure created via a ram air induction system and limited by KIAS.  GP can be negative (vacuum) at reduced throttle (reducing air in the induction system).  Ram air may overcome the slight closing of the throttle to where the MAP = AP.

 

Ambient Pressure (AP):

1) Before engine start:  Theoretically, a properly calibrated sensor would be expected to read the barometric setting before engine start.  However, the AP part of the MAP sensor is not required to match the TSO requirements of even a non-sensitive altimeter.  There also may be display constraints when the engine is not running.  In my case, the MAP indication on my EPI 800 generally reads 29.2” over a broad range of barometric settings.  Since my airport is 710 MSL, this reading (710 ft = 29.16) is consistent, all things being equal. 

 

2) The VM MAP sensor is usually mounted thru the firewall with the AP port open to the cabin.  This means that the ambient pressure varies with speed, venting and altitude.  That is, the cockpit low pressure is less at lower speeds, less when vents are opened.  Generally, in my airplane at cruise speeds, the inside pressure is .25” Hg less that “static” ambient pressure, depending on altitude.  This gives rise to the question: Should I connect the AP port to the same static air used by the altimeter?

 

Side Note:  If an electronic ignition is used and one of the controlling parameters is MAP, the ambient pressure is obtained from the position of the sensor, generally located in the “brain” box that is often placed under the cowling and is sensing the cooling air output pressure.  This is generally slightly above “static” ambient pressure.  I misplaced the measurement I once made of this but I remember it was maybe about .25” Hg greater.  At cruise, this is a true difference of about .5” in what the MAP indicator is showing and what the electronic ignition is using for timing adjustments.

 

 3) True ambient (static) pressure at altitude is an elusive number.  This pressure is dependent on air density which is sensitive to absolute altitude, temperature and humidity.  Note that our altimeters are built to make assumptions about temperatures at altitude and the MAP sensor is not.  Additionally, the true pressure is dependent on what has gone on at lower altitudes with respect to temperature, moisture, etc.  The higher the altitude the more difficult it is to even estimate the ambient pressure, let alone back calculate it from the altitude and OAT not adjusted for compressibility due to air speed.  Luckily, most of us use sensitive altimeters built to TSO standards and set to the local barometer setting so that we read altitudes the same way even though we may be far off from the true altitude.  Maybe this helps to understand one of the reasons why GPS altitude determination and vertical guidance is not trivial. Also, I better understand why the Flight Levels above 18000 MSL require the 29.92 Kollsman setting.

 

Remember:  Look out below when going from hi to lo or hot to cold.

 

Gauge Pressure (GP):

GP is obtained from a tap into the induction system, usually from a primer port on one of the cylinder heads ahead of the intake valve (at least that’s where it generally is on an injected Lycoming engine). Any reduction in the throttle denies air to the engine thus creating a vacuum  in the induction system.  The sensor is protected against pressure pulses (valve operation) in the induction system by a “snubber” valve. 

 

I went back to my parts box and checked an older one from a prior VM MAP sensor.  As Lorn reported, it is a thin porous metallic disk contained in the large brass fitting on the firewall side of the sensor.  It seems like it won’t pass air when one tries to blow thru it.  At least 4 people I have talked to have had problems with the snubber valve causing problems in MAP readings, especially after throttle change.  Remember that its placement creates a chamber from which the pressure is sensed.  There can be at least two problems because of fouling:

 

1)       Inability for the chamber to return completely to ambient pressure – This might yield a low MAP reading before engine start?

 

2)       Fouled in a way that makes it very slow to sense vacuum or incorrectly requires a greater vacuum to reduce the pressure in the chamber.

 

 

How can it get fouled?  There is no movement of air thru the sensing system.  Is the sensor located lower than the tap into the induction system? If so, can fuel, oil or water condensations run down the line to the snubber?  This certainly happens in those canard airplanes that park with the nose on the ground.  Hmmmmmm……  What about the sensor in the electronic ignition?

 

Ram air effect:

Precisely what is the ram air effect and how much does it improve the MAP?  The airspeed system is a perfect example of a closed system which measures the differential pressure between ambient (static) and ram in the pitot which is then converted to the airspeed display.  The KIAS takes into account the less dense air at altitude where both the engine and wing have to deal with fewer molecules.  The rise in the air pressure difference due to ram in a closed system is thus:

 

KIAS  Inches of Hg

100      .5

150    1.1

200    2.0

250    3.1

 

These numbers could be considered a maximum assuming that there is a special inlet and no drop due to a filter.  If filtered induction air were taken from a well sealed engine cooling air plenum, the filter and ram effect may cancel each other out at say, 160 KIAS (a guess from pressure measurements taken from my upper cowl area).  Of course, none of this takes into account that the air is just passing thru.

 

Side note:  Consider that ram air can raise the induction air pressure higher than that utilized by the fuel injectors (obtained, for example, from the upper cooling plenum) to help atomize the fuel.  This may suppress the best atomization and shrouded injectors utilizing the higher ram air, available before the throttle body, may improve atomization.  Such a system is similar to the way super/turbo charged engines operate.

 

Well, what the heck, with unfiltered ram air I know my induction system is performing better, at a lower altitude than my wing is seeing. Something for nothing (except cash).  Now I want to know precisely how much better so that I can have bragging rights over somebody else’s system.  However, as can be seen from the above discussion, comparable numbers are hard to come by.

 

Wait!  There is a way.  Join me in the next Air Venture cross country race and we shall see how well your ginned up system works.  Yep, the gauntlet has been tossed at your feet.

 

Disclaimer: These data and opinions are provided free and, as such, are worth what you are paying for them.