10-28-2006, 08:08 PM
Join Date: May 2005
Location: Chantilly, VA
Total Cats: 1,356
The SFMU has five separate adjustments:
1.) The main spring adjusting screw
2.) The calibration ring & disc
3.) The vacuum screw
4.) The optional bleed screw
5.) The boost screw
What do these adjustments do?
1.) The main adjusting screw sets the basic static fuel pressure level of the unit. By adjusting the main screw any initial pressure can be set.
2.) The calibration ring and disc are used to set the slope of the fuel pressure increase with increasing boost pressure. Four different diameter rings and discs are included with the SFMU.
3.) The vacuum screw acts like a vacuum regulator. If it is set to give, for example, 5 in hg gauge, the upper chamber of the SFMU will only see 5 in hg even though the engine manifold pressure may drop to 18 in hg at idle. The result of this is the fuel pressure remains constant from idle vacuum up to 5 in hg then the pressure will start to increase.
4.) The optional bleed screw does two things. First, it will lower the slope of the curve set by the calibration ring. So, it can be used to "fine tune" the pressure vs. boost curve. Second, it acts as a damper to "soften" the transitions in the curve.
5.) The boost screw acts in a similar manner as the vacuum screw in that it performs like a pressure regulator. It can be set to limit the maximum fuel pressure. If the screw is set all the way in it will not regulate and the curve will continue to rise as boost increases. If the screw is set to relieve at, say, 10 psig then once that manifold pressure is reached any manifold pressure above that will not increase the fuel pressure. More here.