On electronic injected cars, Fuel pressure does change with manifold pressure.

Fuel pressure changes in exact proportion to boost/vacuum.
(This applies to a normal fuel pressure regulator )
(FSE power boost valves raise the pressure more than boost.... crap IMO)

For example, on a cossie.... (and most electronic injected systems)

At 0 pressure (atmosphere) the fuel pressure should be around 3.5 bar / 52 ish psi.
This is called the base fuel pressure.

So on boost of 1 bar/ 15 ish psi, the fuel pressure should be 4.5 bar / 67 ish psi

Therfore also, under vacuum, the fuel pressure will go down.

The idea is that a constant pressure differential is applied across the injector
so the flow rate/spray pattern can be predicted/fixed.


As for mechanical injection (KE JET) S1/S1 escort,
usually the fuel pressure is constant and the flow rate is adjusted
by the airflow meter flap.