I want my fuel pressure to be 55 - 56psi. I set the pressure (according to my elect. fuel pressure gauge) to that with the vaccuum line off. When I hook the line up the press. drops to 50psi. Which is actually the correct reading? Which pressure are the injectors seeing. Also, from doing searches, the stock fuel pressure 43-45psi is measured with the vacuum line off right? So I have in fact raised the fuel pressure by 10-12psi right? Just need reassurance.

 

The purpose of the regulator is to adjust the pressure according to load. It lowers the pressure at idle, since higher pressure is not needed, then. When the engine is at higher loads, less vacuum is available and the regulator sees this, because of the vacuum line that goes to it. Then, you get your higher pressure that you adjusted to when you had the vacuum line off. Both your readings are "correct".

 

Thanks for chiming in shoebox. I was hoping you or injuneer would have something to say. Make sense.

 

Fred can probably give you a better explanation, but that is the gist of it. You pull the vacuum line off to simulate WOT, so you can adjust the max pressure you want during that crucial time.

 

You explained it fine. I didnt put 2 and 2 together. I didnt realize pulling the vacuum line simulates WOT, but I did know vacuum drops @ WOT. Like I said, I didnt connect the 2. Thanks for opening my eyes.

 

Its a matter of raising or lowering the rail pressure to reflect load, but only because "load" affects intake manifold pressure.

Stock injectors are flow rated at 43.5psi differential pressure. Differential pressure is the difference between the fuel pressure coming into the injector (rail pressure) and the pressure at the point into which the injector is firing... the pressure at the tip of the nozzle. That pressure = the negative pressure (vacuum) in the intake manifold.

At idle, intake manifold vacuum is about 20"Hg (stock cam), or about minus 10psi. Since the injector needs a DIFFERENTIAL pressure of 43.5psi, the rail pressure must drop to 33.5psi..... then the differential pressure across the injector is:

(+33.5psi) - (-10psi) = +33.5psi + 10psi = 43.5psi differential

(remember... minus times a minus = a plus)

When you go WOT, almost all the intake manifold vacuum disappears. In effect, vacuum = 0, so intake manifold pressure = 0psi. The rail pressure has to rise to 43.5psi, to maintain DIFFERENTIAL pressure:

(+43.5psi) - (0psi) = 43.5psi differential

This is called "vacuum compensation". Your fuel pressures are fine, if you really wanted to raise them 10-12psi.

 

You should teach for a living, if you don't already.

 

I figured 55-56psi would be good for my new setup (380-390rwhp) since I am still using stock 24lb/hr injectors. Do you think this is too much? Any recommendations for now until I can buy 30lb/hr's? I dont want to max out the stock injectors.