vacuum psi ???

Well to start if you are not running a blower or turbo then you are not talking about psi, you are talking about inches of vacuum (hg). Vacuum is negative pressure and in a stock engine works like this. At idle your vacuum shoud be in the area of 15-20 inches. When you stomp on the throttle vacuum goes away and will head toward 0. As the engine speed goes up vacuum goes along with it. Normal cruise is 15 or so. The higher you can get the vacuum meter to read the better fuel mileage you will get.

Well as high as it will go is the max for your engine. If you watch the gauge you will notice very little gas will cause the gauge to go towards 0. In a carbed engine you can get it close to 20 inches but I am not sute the max but it should be between 15 and 20.