i could imagine FPR on TII works this way :

- vac or 0 pressure = lower pressure in fuel rail (FPR is more open, letting fuel return to tank)

- positive pressure = higher pressure in fuel rail (FPR is closing to raise pressure)


but how it works in NA setup where engine is not making (?) positive pressure ?

 

N/A one just maintains a constant pressure.

turbo one increase fuel pressure with boost.

 

Not arguing with the above answer.
Works the same as the Tubo fpr. At a vacuum the N/T reads just like the Turboii, approx 28 psi and the pressure in the fuel rail rises as pressure to the fpr diaphram rises, but only to approx 38psi in the fuel rail for a N/T because there is no pressure rise above atmospheric pressure, but in a turbo, depending on how much boost you have, can raise the rail pressure to approx 50psi.

I put a fuel pressure gauge on the fuel rail of a N/T and then put a MItty vac on the fpr vac hose. I raised the pressure to the fpr with the MittyVac, and the rail pressure rose just like it would on a Turbo car....to approx 50psi. That seems to be the max figure the fpr will raise fuel pressure for either a turbo or a N/T.

N/T fpr works just like the Turbo fpr except the N/T is limited due to no boost pressure.

 

Not trying to argue with what said

The differential between fuel pressure and manifold pressure is constant.

in other words, the fuel pressure follows manifold pressure (positive or negative) by some (positive) constant

Hugues -

 

***Not trying to argue with what said***

Your right. Well stated. Thank you.

 

Nope, Hailers and hugues are right.

Both NA and Turbo FPR's work in exactly the same manner. Basically they keep the fuel pressure a constant amount above the manifold pressure at all times. For FC's this is ~38 psi, so both engines will idle with ~30psi fuel pressure (@ 16inHg vac) which rises to ~38psi at zero manifold gauge pressure (atmospheric), which is WOT for an NA. A Turbo's fuel pressure will keep on rising as manifold pressure keeps rising (i.e. boost), and should be ~48psi at 10psi boost. These are approximate figures of course.

This is what's known as a constant-rate FPR, because the pressure difference remains constant. With an aftermarket rising-rate FPR, that difference also increases as manifold pressure increases.

 

oh I didn't know they moved between vac and 0...

o well. can't be right all the time