By calculation it is possible to know the flow rate of a part at a different test pressure than that used during measurement. This eliminates the need to control the test pressure (fixed standard pressure) in test benches with low power air source such as a vacuum cleaner.

A powerful racing or road engine does not see a fixed standard pressure drop. In this case, we must face the reality shown in the figure opposite. This low pressure is caused by the negative pressure wave that reaches the exhaust valve and acts as an extraction of combustion residues. It does so with enough amplitude to pass through the chamber and act on the intake manifold. This vacuum generated by the exhaust starts the intake flow in the cylinder well before the piston even starts to descend into the bore.

To simulate as closely as possible what happens in a running engine, intake flow tests must be performed at a high test pressure at low valve lift and a lower test pressure at high valve lift. This is exactly the situation that occurs with a floating pressure air source test stand. An unregulated source, such as a vacuum cleaner, draws a large vacuum when the inlet valve is closed and a progressively smaller vacuum when the valve is opened.