map sensor

[supldys]

whats the difference between a barometric pressure and a manifold diff. pressure?

im trying to go from a 2bar map to a 3bar map sensor, figured it'd be easy with GM but its being a pain in the ass.

thanks guys

-Jono

[Renner]

The barometric pressure (BP) sensor allows the ECU to compensate for changes in the altitude by measuring the actual pressure of the atmosphere, it should have nothing to do with measuring the absolute pressure inside your manifold. Although some companies do combine the MAP sensor, and BP sensor into one unit because they work similar mechanically, even though they measure different things.

MAP sensor means manifold absolute pressure sensor, which you probably already knew. Absolute pressure is different then relative pressure; the manifold difference pressure you are referring to is probably a relative pressure device. A MAP sensor combined with a BP sensor is what gives you a relative pressure. MAP-BP=Relative pressure, which would most likely be the manifold difference pressure, it all just depends on what terminology they decided to use.

 

Atmospheric pressure near the surface of the earth is just that, 1 atmosphere (1 atm), which equals about 1.013 bar, and also equals about 14.69 psi. As you go higher up in altitude this pressure decreases obviously, which is where the BP sensor comes in.

 

So when your tire gauge reads 30 psi , this is a relative reading, so you really have 44.69 psi absolute, and when your aftermarket boost gauge reads 15 psi, this is a relative pressure reading, this is why it settles to zero when the car is off, you are at zero psi relative to the atmosphere. But at 15 psi relative you are really running about 29.69 psi absolute.

 

So, if you have a 3 bar Manifold Absolute Pressure sensor, that is basically the same as a 2 bar manifold relative pressure sensor in terms of measuring the pressure in your manifold. Now, whether or not the output values of the sensor to the ECU will be the same, is totally dependent on how the sensor is set up. If you have any more details, then I might be able to help a bit more.