How do these work?
Is there more than one setting, meaning can you tune fuel at idle and at full throttle without the other suffering?
The fuel pressure regulator is a calibrated pressure release device. There's a spring inside which holds a valve closed. Fuel pressure pushes the valve open. When fuel pressure is enough to open the valve, fuel flows out of the rail and back to the tank. This is "base" pressure with key on, engine off.
The spring trying to close the valve is sealed in a chamber. The vacuum fitting on the regulator is also attached to that chamber. When the engine is running vacuum tries to squeeze the spring, which helps the fuel force the valve open from the other side. So fuel pressure is lower when there's vacuum at the regulator, like at idle.
Generally a stock replacement or adjustable FPR affects fuel everywhere. wide open throttle, idle, and mid range. RRFPR's are used to alter fuel more when there's no vacuum or when there's boost. There are usually different settings for the amount of pressure increase desired.
HTH
-->Slow
I have never heard of an rrfpr, what is it?
