So here I will try to explain how I got the equation.

Many of the engine sensors output to the PCM a varying voltage that represents the value of the measured parameter. In this case we are looking at the Intake Manifold Absolute Pressure.

So this variable voltage comes from the sensor to the PCM and at some point the PCM converts this analog voltage to a digital value, in this case expressed as a single hexadecimal byte in the range of 0x00 to 0xFF. This corresponds to a decimal value in the range of 0 to 255 representing the number of bits in the hexadecimal. It is this decimal value that Torque Pro represents with the token "A" in equations. There are other parameters such as engine RPM that report with a two byte value for higher precision and in these cases Torque Pro (as well as other apps) assign "A" to the first byte and "B" to the second byte.

So the first step is to convert the decimal value of "A" back to the original voltage output from the sensor. There will be a very mimimal amount of degredation but too little to be of concern. The voltage came to the PCM in the nominal range of 0 to 5 and got cast to the range of 0 to 255 bits where 0 volts = 0 bits and 5 volts = 255 bits. So 255/5 = 51 bits per volt. Thus A/51 = volts.

So now I need to convert this voltage back to the kPa value that the sensor saw when it ouput the varying voltage to the PCM. In this case I searched online for "2 bar map" and came up with a few suppliers and datasheets. The datasheets supplied some kPa vs output voltage calibration data. Here is one from pe-ltd.com...

View attachment 101136
Circled above are the output vs kPa values from this particular sensor. I could have used these values, they would have worked, but I searched further for some values atribbuted to a GM sensor itself. I found a sensor at Bmotorsports.com which at the time I thought was a genuine Delphi / GM sensor but upon further review just now I see is also an aftermarket sensor. They say the China made Delphi sensor has quality control issues. At any rate I used the values I found there, seen below...

View attachment 101137
So here we see a range of values and corresponding voltages. We have a kPa of 200 down to a kPa of 20 for a difference of 180 kPa. We have corresponding voltages from 4.896 (@200 kPa) down to 0.306 (@20 kPa) for a difference of 4.59 volts. So there we see a change of 180 kPa for a change of 4.59 volts. 180/4.59 yields 39.2156862745 kPa per volt. This is our scale,,,, but we are not through yet.

If we were to use what we have thus far we would have A/51*39.2156862745. Lets try that for one of the values stated in the sensor spec... we will use the 0.306 volts = 20 kPa.

0.306/51*39.2156862745=12. We are low by 8 kPa. Lets try the upper spec for the datasheet, 4.896 volts = 200 kPa...

4.896/51*39.2156862745=192. Again we are low by 8 kPa. There is an 8 kPa offset (linear) in the values.

So adding the offset to the equation we have.....

(A/51*39.2156862745)+8

The above is clearly not precisely what I posted a few messages ago, but is closely equivalent. Earlier I moved the offset to the other side of the multiplication and somehow came up with a very slightly different value in doing so, no idea how but the difference at 200 kPa amounts to about 4 hundreths of one psi so I figure close enough!!

When I was about 9 or 10 years old, at school, a sister (attended parochial school) looked over my shoulder while I was doing some work with a compass or the like and stated "Whatever you do in life, you are going to do it with precision".

Naturally this equation could be simplified and sometimes I do that but I find leaving them in this form can make deciphering them later much easier.

One further note: the above equation yields the MAP value as kPa as well as an absolute value. As one desired it could be converted to psi by multiplying the result by 0.145037738 (round as desired). Alternatively you could go back to the sensor spec, convert the 20 and 200 kPa to psi and refigure the equation

for psi instead of kPa. And you could also offset for atmospheric pressure if desired to display gauge pressure as opposed to absolute pressure.

So this is my methodology if that is the proper word for working out some equations. Often there is no datasheet to work from so I have to collect sample data points from the vehicle and that introduces a good bit of error from a lack of precision in the readings of varying data as well as an inability to view the extremes of the measured parameter.