igo4bmx

I understand its necessary but why?
does the increase in air pressure have to go through the fpr so it functions properly? what would happen if you didn't have it hooked up to a vacuum source?

drewspeed

The BOV needs a vacuum source to open. Once it detects vacuum it will open to release the charge pipe pressure.

asterix_2ksi

When the throttle plate closes the engine quickly consumes the remaining air in the intake manifold and creates a vacuum where the pistons are sucking... but there's simply nothing there. At some point the vacuum gets high enough to suck air pass the throttle plate's edges preventing the engine from stalling and at the same time the idle valve opens as controled by the ecu to help maintain a steady idle rpm. All along though there's positive pressure being built up from the compressor. The bypass valve / blow off vavle is used to releave this pressure. There's a diaphram working on the piston of the BOV to make it open and close. This diaphram pushes the pistonout to open the valve when there's vacuum... but there's boost in the intake pipe. So the vacuum line is connected to the intake manifold where there is vacuum when the throttle plate closes. Once the BOV opens the positive pressure is released and the compressor can relax as the force of air pushing back on it goes away. With out this release the compressor would begin to burp as the pressure would be too great and begin trying to escape back through intake piping... forcing the compressor to quickly slow down while working against the shaft that spins it (either a shaft contected to a turbine or a shaft contected to a pulley). These forces can wear the shaft bearings along with damaging the impellar fins caused by quick speed changes.

The title bar says BOV, but your post says FPR. The FPR works in the same way as far as the diaphram goes, but the diaphram regulates the fuel return orfice allowing fuel pressure to build when the orfice is smaller and to release pressure when the orfice is larger. The purpose of an FPR is to maintain positive pressure so that the fuel will flow through the injectors and into the cylinders rather than back to the fuel tank. The purpose of having the FPR vary the fuel pressure is to maintain a dynamic pressure that will match manifold pressure. Consider this, one-pound = one-pound So if you have a pound of intake pressure (positive) pushing on the nozzle of the injector that's going to affect the amount of pressure needed to push the same amount of fuel through the injector. So as positive air pressure increases on the nozzle side of the injector so does the fuel pressure on the feed side of the injector. This also holds true for vacuum where less fuel pressure is needed as vacuum increases.

Now I've posted this not because I know what I'm talking about, but because I hope someone who does will correct me if I'm wrong. I sure hope I'm right though...