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MAP Sensor reading

  1. Apr 12, 2015 #1
    Hi guys,

    I have read a lot about car's MAP sensor and it's function but there are some areas that are not completely understood yet!

    Let me put it this way:

    My car's MAP sensor reads 337 mbar (millibar) @ idle (800 RPM).
    When i open the throttle, the reading DROPS (for example at 2500 RPM it shows around 260 mbar)

    With the engine OFF and ignition ON it reads exactly the barometric (atmospheric) pressure of our city which is about 870 mbar.

    My car is a Peugeot.

    The problem is that i saw in a video that MAP sensor reading of a Ford was the exact opposite of my car! When he opened the throttle the reading went HIGHER not LOWER!

    (both cars have naturally aspirated engines)

    I think that my car's MAP shows manifold VACUUM and that Ford's MAP shows manifold PRESSURE not vacuum.

    I know that highest vacuum is created in high rpm and closed throttle (like going downhill)

    In this situation my car shows highest numbers and that Ford shows lowest numbers.

    But someone told me that my car isn't showing vacuum, it shows Absolute Pressure. Someone else said something about Relative Pressure.

    My interpretation of my own car is this:

    @ idle the MAP reads 337 mbar. It means that @ idle my manifold pressure is 337 mbar lower than atmospheric pressure so my REAL manifold pressure is 533 mbar (870 - 337 = 533).

    So my MAP does NOT directly show my manifold pressure but it shows how much my manifold pressure is lower than ambient pressure.

    Am i correct?

    Please shed some more light into these concepts.
    Last edited: Apr 12, 2015
  2. jcsd
  3. Apr 12, 2015 #2


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    Without knowing what kind of Ford car was shown in the video, it's impossible to say why the MAP readings are different. In a car with a turbo, after pressing the accelerator, the turbo will kick in and start to force air into the intake manifold. In a naturally aspirated engine (no turbo), idle will generate a certain amount of vacuum in the intake, and wide open throttle should produce the most vacuum.

    MAP stands for Manifold Absolute Pressure, so I would tend to think that a reading of 337 mbar is the absolute pressure in the manifold. A perfect vacuum is 0 mbar and atmospheric pressure is 1013.25 mbar. Note: 1000 mbar = 100 kPa.

  4. Apr 13, 2015 #3

    Doug Huffman

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    Gold Member

    I recommend also this article
    https://en.wikipedia.org/wiki/Pressure_measurement to understand the differences among pressure and vacuum and gauge, relative and absolute measurements.
  5. Apr 13, 2015 #4

    Randy Beikmann

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    Gold Member

    karabiner98k, I've run into the same questions, and had to figure it out. The MAP sensor does indeed read out "manifold absolute pressure." I'll try to answer your questions/observations in order:
    The reason your MAP readout dropped when you increased throttle in neutral is that the engine speed increased. Manifold pressure doesn't just depend on throttle, but also on engine speed (pressure drop is roughly proportional to density times the square of the flow velocity, at a given throttle opening). When the engine runs faster, more air flows through the throttle opening, and a larger pressure drop results - your manifold pressure drops. At idle, a small throttle opening can raise the MAP to nearly atmospheric pressure.
    When you had your engine stopped, there was no flow, and no pressure drop across the throttle (even if it's "closed"), so the MAP equaled atmospheric pressure.
    In the video you saw of the Ford, the engine speed may have stayed the same (or nearly so), so when the throttle opened more, the pressure dropped decreased, andMAP went up.
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