sample local atmospheric pressure at boot time, before the engine is started, and then subtract this value from the sensor readings while logging. This way, the mechanical boost gauges(which always true themselves to zero) should be in sync with the readings from the MAP sensor.

http://www.freescale.com/files/sensors/doc/data_sheet/MPX4250A.pdf

http://www.plxdevices.com/sensormodules.html

For a 5 Bar MAP sensor use a scalar of 1250 and offset of -625
Motorola 2.5 bar:  Scalar 500  Offset 100
This works for the AEM and Motec 5 bar MAP sensors (0.5V = zero absolute pressure, 4.5V = 5 bar absolute pressure).

be warned that GM actually makes 3 different 3 bars and each uses a different scaler and offset.

http://www.made-in-china.com/showroom/yuanxiao/product-detailbojndqLVEuWA/China-Pressure-Transducer-HG40-.html


Here is how to calculate the scalar and offset.

1. Find the minimum and maximum voltage and absolute pressures.
2. Convert the pressures to mbar.
3. Subtract the minimum and maximum voltages to find the range, then do the same for the pressures.
4. Divide the pressure range by the voltage range to calculate the scalar.
5. Multiply the minimum voltage by the scalar to calculate the offset. Note that a positive voltage gives a negative offset.

So:

1. For the AEM 3.5 bar data sheet:
0.5V @ 0 kPa (absolute)
4.5V @ 345 kPa (absolute)

2. 345 kPa = 3450 mbar.

3. Voltage range = 4.0V, pressure range = 3450 mbar.

4. 3450 / 4.0 = 862.5 = scalar

5. 862.5 x 0.5 x -1 = -431.25 = offset

Plug in the minimum and maximum voltages to check your work:
0.5V x 862.5 - 431.25 = 0 mbar @ 0.5v
4.5V x 862.5 - 431.25 = 3450 mbar @ 0.5v

If you have trouble then draw a graph and exterpolate the lines until they cross the pressure axis.


Picture: GM 3 port