Here are two videos showing what to expect from a ported Vacuum source at the throttle plate (normally used to control vacuum advance on a distributor), and then manifold vacuum taken from the intake plenum. Notice the ported vacuum source has essentially no vacuum until the throttle is opened, whereas the manifold vacuum reduced directly in relation to increasing engine RPM. My cheap vacuum gauge zeros at about 5 inches by the way.
![](https://s2.save4k.org/pic/WtMLHuN90Zo/mqdefault.jpg)