one bar sensors cover from about 15" of vacuum to 0, normal for standard engines. Two bar sensors cover from 15" of vacuum to 15 lbs of pressure, typical for a supercharged/turbo'd vehicle.

http://www.injector.com/sensors.php

follow that link to examples of 1 bar, 2 bar and 3 bar. 1 bar isn't designed for boosted applications. 2 bar is.