All depends on the relative pressure of the crankcase vs the point at the exhaust where the EVAC is connected. One thing to keep in mind.... as the exhaust cools while moving down the tubes, its pressure goes down. So it is not hard to see if you go far enough back the average absolute pressure gets down to, or close to, atmospheric.... the velocity of the exhaust at that point keeps it moving on down the line....if things are pretty open all the way to the back.