FPR - quick question

[jimberg]

Pressure goes down when engine vacuum increases, so yes, it should drop when the engine is at idle.

Think of it this way, you have the cylinder trying to pull fuel into it and you have the fuel rails try to push fuel into the cylinders. The injector is the opening between the two. You want to maintain a constant relative pressure between the two since the injector is rated to let through a certain amount of fuel at a certain fuel pressure (39 psi). Since the cylinder is pulling hardest at idle, say at 10 psi, the fuel rail pressure needs to drop to 30 psi to maintain a constant relative pressure of 40 psi.

Vacuum drops at WOT to nearly 0 so you need to push the fuel at 40psi. The pressure you set the FPR to maintain with the vacuum line disconnected is the pressure that the engine will try to maintain all throughout the RPM range.

By the way, you should probably drop your fuel pressure to 38 psi.

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351W 89 Mustang GT Convertible