MAP sensors are absolute sensors so they start at zero pressure, or a perfect vacuum, just like a gauge calibrated in pounds per square inch absolute. They have a reference cell which is usually vacuum, and the electronics read the pressure above this reference in this case. I posted something that was wrong when this came up in the alky forum a while ago, so I want to get it right here. By definition 1 pascal = 1 newton / meter^2 and 1 Pa = 0.00001 bar, so 1 bar = 100000 N/m^2. Plug in the conversions between N, m, pound force, and square inches, and 1 bar = 14.5 psia. It's sheer coincidence that this is almost 14.7 psia which is one atmosphere. [In that other thread I said 1 bar = 1 kilogram/m^2, sigh - sorry about that, guys.] If the reference cell in a MAP sensor is at one standard atmosphere, it could read pressures both above and below this, depending on the electronics. Apparently the Motorola sensor doesn't read below this, only above, which means it can go to higher pressure with a lower range strain gauge, but of course it can't read vacuum.