INJECTORS AND FUEL PRESS.

most injectors are flow rated at a specific pressure. alot of them (mustangs) are are 39 psi.

you set the base FP with the regulator with the vac line OFF. this simulates the atmospheric pressure at WOT.

whatever you set your base fuel pressure to is what you should see at WOT ( atmospheric pressure in manifold, only applies to N/A engines)

with the vac line attached. during idle and normal driving, the throttle plate is going to restrict airflow. the engine is going to want to move more air than the closed/slightly open TB will move. so the pressure in the manifold decreases.

the purpose of the FPR is to adjust fuel pressure to compenstate for the changing manfold pressures. the idea is to keep the same pressure differential across the whole operating range.... if you had it stuck at one pressure, while the pressure in the manifold change constantly while you drive. in a sense your injector flowrate would be constantly changing. making any sort of tuning impossible.

having locked out FP at 39 psi during idle would run rich, the pressure difference is so much more, more fuel will exit the injector for a given PW.

the exact opposite applies for boost. you want to make sure your FP increases under boost. you want 1 psi of FP for every one psi of increases manifold pressure. this is also to compensate for the changing pressure difference. except this time. if no FPR active. your injectors become smaller. since your manifold pressure would increase and it less fuel will leave the injector for a given PW. in a sense the extra manifold pressure is pushing back more on the fuel trying to come out of the injector.