MAP Sensors: What They Are And What They Do.

Remember science class when the teacher shouted “Pay attention! You might need this someday!” ?  Well, as much as I don’t want to admit it, that teacher was right.  Lets touch upon the basics and see if it rings a bell?

The Basics:

  • First off – MAP is an acronym for Manifold Absolute Pressure.  MAP sensors measure the air pressure in your intake manifold which helps the engine’s computer determine air / fuel ratios.  MAP sensors are set to “zero” from the factory. So with the car off, and the key on, the MAP sensor will read “zero” at sea level.
  • On earth, we have 14.7 Pounds per Square Inch (PSI) on us at all times at sea level.
  • “Bar” is a measurement of pressure.  1 Bar = 1 atmospheric pressure, which is 14.7 PSI.
  • The absence of pressure is measured in “Inches Of Mercury” (in. hg). (Finally we get to use the periodic table of elements in real life!)
  • -1 Bar = -29.4 in. hg
  • 1 Bar = 29.4 in. hg
  • Naturally Aspirated = without a turbocharger or supercharger. Also known as “N/A”.
  • Forced Induction = with a turbo or supercharger
  • In forced induction applications “Boost” is automotive slang for PSI
  • Stoichiometric Air Fuel Ratio = The ratio of the exact amount of air it takes to burn a fuel completely.
  • Stoichiometric for Gasoline Engines = 14.7 : 1 (14.7 parts air to 1 part fuel).

Now some of that good stuff:

With a N/A engine running, the MAP sensor may see readings anywhere from -29.4 in. hg to 0 PSI depending on how hard you smash the pedal. The more you hit the throttle, the closers to 0 psi the MAP sensor will read because there is less vacuum in the intake manifold.  On an engine with forced induction, the MAP sensor will also measure boost (finally above zero!).

When MAP sensor data is combined with an air temperature sensor and a known engine speed, the ECU (engine’s computer) can accurately calculate the air flow rate of the engine, which then means it can calculate fuel.  It does this with fuel maps that are programmed into the ECU.  The fuel map guides the engine to its happy stoichiometric place.  Easy enough right?

So what is so great about a 3 or 3.3  Bar MAP sensor? Why do people use them on cars that they don’t belong on?

Some cars that come stock with superchargers or turbochargers have 3 or 3.3 bar MAP sensors from the factory.  So horsepower addicts like to take their own project car, stuff more boost into it than it was ever intended to have (More Powahhh!), and run a tunable computer system to handle the changes.  Since a 1 Bar sensor can only read up to 14.7 psi (which is really zero here on earth), a 1 Bar sensor can’t handle any forced induction applications.  EEEK! Any type of forced induction puts pressure (above zero) into the intake manifold, and therefore horsepower seekers need a MAP sensor that can accurately read those pressures.  This is where the 3 or 3.3 Bar comes into play.  A 3 Bar sensor can read up to 44.1 PSI (Subtract the 14.7psi of atmosphere, and it can actually can read up to 29.4 PSI.)  So, if a person were to put a 1 Bar sensor where a 3 Bar goes, the ECU would freak out when the boost arrives, and wouldn’t know what to do with the air/fuel ratio because the numbers on the fuel maps don’t add up anymore.  The moral is that the 3 and 3.3 bar sensors are perfect for this type of thing because of their simple 3 wire connector, reliability, and accuracy.   Oh and the price is great too!

Hopefully that makes sense.  Some of the pressure type stuff can get a little confusing, but I did my best explaining it.  If anybody has any other input, I’d love to hear it.  Leave me some comments!

Full Disclosure: I lied.

  • Atmospheric Pressure = 1.01352932 bar……. Not 1 Bar.  I did it for simplification.

Jeremy Nutt

Hi, I'm Jeremy.

One thought to “MAP Sensors: What They Are And What They Do.”

  1. Jeremy Hi! 1989 GMC Safari V6
    I am really impressed with your Sensor article. However, even though I have been a mechanic for over forty years I am failing to comprehend the meaning of all the intricate functions.
    About atmospheric pressure yes I have learned at mechanics school many, many years ago. The Toriceli experiment with his square glass tube filled with mercury putting his thumb at the bottom of the tube, submerging it in a basin of water and then releasing it by pulling his thumb away he measured the mercury and figured that the atmospheric pressure is equal to 1033grams per sq. centimeter. Of course, the 14.7 is calculation is by sq. in.
    However, this is not why I am writing to you. Since I retired I am going to Europe every year. I have an older Van there for my needs and the last four or five years I have the “Service engine soon” light appears sporadically on my dash. Since my experience is on old and older cars my knowledge stops around the mid seventies.
    By purchasing a very good one OBD2&1 Scan Tool I thought it might help to diagnose some troubles and correct them. Wrong, after I went in the Internet and got the Engine diagram and location of the sensor which “is” on the upper part on the Passenger side, I opened the hood I took the back cover off no sensor. The sensor looks exactly like the one you have at the beginning of your article the upper one. The Van is a 1989 GMC Safari V6, I am sure that there is a sensor but shape and location must be different. Last year I had to replace a Muffler, could the sensor have been the reason?
    Jeremy, if you have any spare time from your busy schedule would it be possible for you to write to me and explain where to find the sensor? But please your explanation let it be in common Language.

    Thank you in advance
    Nick M. Nomikos

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