OK, have to admit I'm a little confused. It was my understanding that 1 bar (1.013 Atm, or 14.7 psi) is when the engine is neither under vacuum nor getting "boost". Some say they get max boost around 1 bar and others around 2 bar. I recently got my 951 and I am pretty sure it hits 1 bar under moderate acceleration. Under slightly higher acceleration (keeping nicely up with the traffic when the light tuns green) it goes considerably over 1 bar (say 1.2+) and though I haven't driven it hard, it seems willing to climb to 2 bar easliy. I know if I would let it, it would build much more boost. I would like to verify all this but the car is put away for the winter. What boost is everyone else getting? What is "normal"?

Reactor2

 

The dash gauge in our Porsches displays pressure such that a reading of "1 " means atmospheric pressure. That is, exactly the pressure that is all around us. The pressure of the air that we breathe. A reading of greater than 1 means that boost is actually building. I.e, if your in-dash gauge shows "2", then your are 1 bar above atmospheric pressure. If the reading is less than 1, then there is actually a vacuum in your intake (during deceleration, or more generally during cruising and light-load conditions).

The confusion comes in when you compare this to aftermarket boost gauges. They generally are calibrated such that "0" conforms to atmospheric pressure. Thus, "2" on your in-dash gauge would be the same as "1bar" (or 14.7psi) on an aftermarket gauge.

BTW, 951s typically build 0.7-0.8bar of boost (dash: 1.7-1.8) in stock form.

HTH,

Gary.

 

Thanks Gary

 

just a matter of absolute pressure and calibrated pressure... like Gary said... the Porsche gauge favors absolute pressure where as most american aftermarket guages calibrate atmospheric pressure to zero for easier reading (anything negative= vac, anything positive= boost)... on the Porsche it's anything below 1 is vac, anthing above is boost... just preference, thats all

 

FSAEracer03 is correct in the difference between ABSOLUTE pressure relative to a perfect vacuum, vs. GAUGE pressure, which is relative to atmospheric pressure. Which, BTW, is 14.5psi, not 14.7 (1-bar does not equal 1-atm).

Typically with boost, using a BAR measurement typically refers to absolute pressure above vacuum, whereas using PSI usually means gauge pressure above atmospheric pressure.