its a replacement for your current map sensor (on a cossie, 2bar)

what a map sensor does is read "manifold abslute pressure" which put simply... boost

a 2 bar sensor will red from 0 to 2 bar absolte (-1 to +1bar boost) a 3bar will read 3bar absolute (-1 to +2bar boost)

therefore if youre running a car with more than 1bar of boost, it stands to reason that you'd want a 3bar map sensor :P

incidentally, both sensors give identical outputs (but these outputs mean different things) therefore it is vital that you let the ecu know if youve changed map sensors... you do this by fitting a different chip with tht info lready on it (or get the car livemapped)

hth