[jonnyk]

I know 12:1 is common. I sold the FMU out of my Paxton kit not long ago.

I'll spare you the research. In return style fuel systems, the way to increase fuel pressure in the lines and rails is to pinch off the return line. This causes the fuel pump to work against the restriction and builds pressure in the lines and rails.

So it simply has a diaphragm that controls the amount of flow going through the FMU, and responds to a positive pressure from a vacuum feed.