What if the engine throttle is fully open?

When the throttle opening increases, the vacuum at the intake manifold decreases and the pressure increases.

This should be explained by the knowledge of fluid mechanics, that is, "total pressure = dynamic pressure+static pressure", and total pressure represents all the energy when air flows in the pipeline. Dynamic pressure is the pressure that reflects the phenomenon of air flow. It only works in the forward direction of the air and is always positive. There are positive and negative static pressure and positive and negative total pressure. The total pressure in the suction pipeline is negative and the total pressure in the compressor pipeline is positive. The static pressure of still air is the total pressure of air.

Therefore, the vacuum degree will increase only when the throttle opening continues to decrease.