If I remember correctly, there are advantages to the different scales. You can also measure pressure in Pascals (I don't think that I spelled it right

) Anyway, I will ask my wife about it, as she deals with the different pressure scales on a daily basis.
Just a FYI: Pressure at sea level is a good reference for 1 bar or 14.7 PSI, but remember it is not "equal" to it, because atmospheric pressure is not stable, and therefore not a constant 14.7
Weather or not there is any difference in the scales when simply measuring a relative pressure change in the intake manifold of a turbo application is another story though, I doubt that it makes much difference, but is just easier for the person who looks at them to comprehend what information is trying to be relayed. Think of it as just degrees Farenheit or Celcius. They both accomplish the same thing, people are just more comfortable with one or the other. I personally do not like the Bar scale as it is to big. Its like measuring the length of an item with a rule that is 1 meter long instead of a rule with centimeters marked on it.
Greg