Originally Posted by Mike Rainbird

The MAP sensor always reads absolute pressure (which is atmospheric pressure plus boost pressure). As atmospheric pressure is 1 bar (zero positive pressure).

So a 3 bar MAP sensor measures atmospheric + 2 bar to give your 3 bar figure. Therefore the max it can read is 2 bar of boost (well actually it's a smidge over, but that will complicate things). Obviously a 2 bar MAP sensor only measures atmospheric + 1 bar boost, this is why a stage 1 engine should not run too far above 1 bar (again it can read a bit more than that, the figure is round down) when using the standard MAP sensor. Accordingly, the scaling is always done in absolute pressure, so you just subtract 1 to get the boost pressure. So when anyone refers to the 2.87 bar top line reading, this is what the MAP sensor is reading, and to get the actual boost pressure, subtracting 1 gives you 1.87 bar of boost.

Hope this clarifies?