Join Date: Jan 2007
Location: Sacramento, CA USA 96814
Actually, the problem would not be allowing a tall enough AIR column on top of the water column, before the tube turns down. Normal atmospheric pressure is about 30 inches of water column, so in a perfect vacuum you would need at least 30 inches of airspace between the water level and the top of the air column.
Note that manifold pressure is actually lowest (highest vacuum) at idle, and it is highest at wide open throttle, at low RPM.