I was doing a little bit of testing on my fuel pressure and I can't find a definate answer on this.

at idle the pump primes and holds 40psi, then when you pop off the vac line to simulate WOT it raises to 47-48psi. I figured no problem. I had the car on a lift and threw it in gear and noticed with no load on the tires and just revving it up at part throttle fuel pressure drops to about 30-35 psi. Is that normal? would having no load on the tires cause it to be off? Unfortunately the line on my pressure guage isn't long enough to reach the windshield so I can't actually drive the car and watch the guage.

 

When was the last time you changed your fuel filter?

 

Intake manifold pressure adjusts your fuel pressure. When your intake is under higher vacuum, like on deceleration, your vacuum can drop to 35 psi.

 

The regulators function is actually to maintain a set pressure differential between the manifold vacuum and the fuel system. If your gauge was referenced to engine vacuum in theory the pressure would never change, but they are open to the atmospheres pressure. If it was not setup like this low throttle high vacuum would suck extra fuel from the injectors.

This is why boosted cars need regulators capable of pressure rise under boost other wise 30lbs injectors at 43.5 psi would only act as 22.1lbs once there was 20psi in the manifold.

 

From the Helms manual:

"When the engine is idling, manifold pressure is low (high vacuum) and is applied to the pressure regulator diaphragm. Vacuum will offset spring pressure and result in lower fuel pressure."