So what happens when the BOV opens. The pressure on the intake manifold side starts immediately venting to the atmosphere reducing the pressure inside the manifold – this is of course what the BOV is designed to do. The compressor wheel has been spinning at 90,000 rpm with 1.5 boost pressure ratio. Now the boost pressure ratio may be 1.2 or less in a very short duration of time (milliseconds). This immediately reduces the backpressure on the compressor wheel caused by the manifold pressure being less than it before the venting. . The engine has not yet reacted to this change, as it is still combusting the boost density air it had ingested into the combustion chamber before the BOV opened. Neither has the exhaust gas flow been affected - yet.