Originally Posted by 70GS455
There are some fpr that do lower fuel pressure with vacuum. One example is the Buick GN; once base fuel pressure is set with vacuum line off, re-attaching the line with the engine idling under vacuum results in a drop in fuel pressure. It then rises with decreasing vacuum and (obviously) increasing boost.
the older cars do that, since they didnt have such precise control. The aftermarket fpr's such as aeromotive might drop 2-4psi but its nothing dramatic.