here's a picture of an untouched fuel pressure regulator. On the right, is a fuel regulator I tore apart. Basically, the spring as well as the air pressure, through that nipple, applies pressure to a diaphragm and metal seal which, when open, bleeds off excess fuel pressure to the tank. Stock, we run a 4 bar fuel pressure regulator.



Here's how the regulator does it....

Basically, #1 seals to #2. When the fuel pressure on #5, going into the screen, goes up beyond 4 bar + manifold pressure, the excess fuel is dumped through #4, back to the gas tank. Say, under boost, the fuel injectors are seeing that boost pressure, so, that same pressure is applied to the nipple on the hat of the regulator, so, as the injectors are fighting the boost pressure of the manifold to spray fuel, the fuel pressure is also increased by that same amount so that the pressures cancel out and you maintain the 4 bar of fuel pressure across the injectors. It's really important that the manifold pressure, whether it be vacuum or boost, be applied to the fuel regulator, that way the fuel pressure across the injectors is constant. Say, that the pressure regulator lost its manifold pressure by a broken hose or something and you're trying to push 1.5 bar of boost through your motor, your fuel pressure will only be 2.5 bar because the regulator is going to try and hold 4 bar with only the spring now and not the additional boost, which would be applied to that red rubber diaphragm in the picture. What will happen is, as the injector opens, the 4 bar of fuel pressure can't be maintained because now the manifold pressure is fighting the fuel pressure/fuel pump and the regulator can only help up to 4 bar minus the manifold pressure.

Yes, I'm up on a Friday night talking about fuel pressure. What a romantic night. :/

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'01.5 S4, Silver, GT28 6spd, Sedan
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'92 G60 Corrado, 5spd with built 20V 1.8T, GT2871R/0.60/0.86, full drive by wire

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mike AT vastperformance DOT com