Originally Posted by XLR8NSS

If you look at the fuel pressure regulator on the driver side fuel rail there is a little vacuum line attached to it that comes from a port on the top of the intake manifold. That is the manifold pressure reference. Our fuel systems raise fuel pressure as manifold vacuum decreases or gets closer to outside pressure(0psi on your boost guage).

So at idle or cruise(low load) the vacuum is high in the manifold which causes the regulator to open the fuel return up which causes the pressure to be low. When you get on the gas the manifold goes from having a vacuum to having the same pressure as outside. If you were naturally aspirated this would show up on a boost guage as 0psi. Now when the pressure in the manifold is high(0psi) it causes the fuel pressure regulator to close the return down some which raises the fuel pressure.

The fuel pressure regulator on our trucks is very similiar in function to a FMU but, a FMU raises pressure at much more than a 1:1 rate like our regulators.

When I had my mechanical fuel pressure guage hooked up and the truck idling the pressure was 50psi. Then I just pulled that little vacuum line off the regulator for a couple seconds which causes the regulator to see regular outside pressure(0psi on a boost guage) instead of the manifold vacuum. When the reg. sees outside pressure the fuel pressure rises to 59psi.

Your fuel pressure shouldn't be staying at 50psi. Could be a bad regulator or clogged vacuum line??

Originally Posted by DanaliHD

with my whipple at 10lbs boost, it was suggested to me to leave that vacuum line unplugged and to cap off both ends. Good idea, bad idea? Fuel pressure sits at 60ish all the time when I do that.

Originally Posted by DanaliHD

with it unplugged, the fuel pressure is constant all the time. always 60ish. I guess I'm confused.