How have you been searching for the optimal fuel pressure? I take it that you have at least installed 155lph fuel pump or better. If you're still running a stock fuel pump an adjustable FPR can't really help you.
We've gone over this a lot of times, but it's probably good to bring up every now and then.
Your air/fuel mixture is controlled by the computer via sensor input and injector pulse length. The MAF, ACT and ECT sensors help the computer measure how much air is going into your cylinders. The computer then does some calculating and determines how long to open the injectors based on how much air it measured going into the cylinders. The computer has preprogrammed air/fuel ratios that it must keep. Changing fuel pressure will only change the amount of fuel that is delivered for a given injector pulse length. The computer is programmed to expect that the fuel pressure is at a specific value like 39psi (I don't know what it really expects, but I'm sure it's around what the injectors are rated at). If you raise your pressure to, let's say, 50psi you'll increase the amount of fuel delivered during a given pulse length by about 13% compared to same pulse length at 39psi. You've esentially richened the air/fuel mixture.
This is where adaptive strategy becomes a factor. The computer will monitor feedback from the oxygen sensors and determine that the amount of fuel it provided for the measured amount of air caused a richer condition than the computer is programmed to keep. The computer will then gradually decrease the pulse width compensation factor until the oxygen sensors report the correct air/fuel mixture that the computer is trying to maintain. The opposite is true if you try to lower fuel pressure and lean out the air/fuel mixture.
What I described is closed loop control. When in open loop control the computer will just look up pulse lengths from a table and then apply the compensation factor determined during closed loop control against them to determine how much fuel to deliver. This is the control mode used during WOT. Your oxygen sensors are only checked to be sure that you aren't running dangerously lean in this mode.
I'm pretty sure the reason the computer uses open loop mode during WOT is that the 12.8-13.1:1 air/fuel ratio that it is trying to maintain is outside of the range of the narrow band oxygen sensors that are used.
So, the question now should be, "Why get an adjustable FPR if it doesn't really do anything to change the air/fuel mixture?". The answer is that sometimes the injectors used don't have enough capacity to supply the amount of fuel necessary to match the amount of air coming into the engine. You esentially have an engine capable of more power than your fuel system can handle. This would be evident on a dyno graph where your horsepower line just flattens out after peaking. If this happens, you can raise the fuel pressure to essentially increase the capacity of your injectors to the point that they can deliver enough fuel.
Injectors also have a minimum amount of time that they can be open. The higher the injector rating and fuel pressure, the less control the computer will have at idle. If emissions is an issue, choose an injector that has a capacity nearest to your horsepower potential without going over. If you need 28# injectors, get 24# injectors and raise the fuel pressure to make 28#. If you need 23#, stick with the 19#ers.
There is some potential for adjustment but not a lot and I'm not sure to what degree. The oxygen sensors put out a voltage ranging from .100 V to .900 V. Voltages less than .400 V being considered lean and voltages over .600 V being considered rich. Since you have that window of .200 V, it may be possible to stay on the lean side if you start off lean or stay on the rich side if you start off rich. Once you're in that window whether on the rich side or lean side you may be able to make small adjustments up or down in fuel pressure as long as you stay in that window. Maybe 1 or 2 psi at a time.
Oh, to figure out what the capacity of your fuel injectors is at a given fuel pressure, take your fuel pressure and divide it by 39. Take the square root of that number and multiply it by your rated injector capacity.
Well, there's what I know about fuel pressure in a nutshell. I have my pressure set at 60psi right now.
I hope this helps with your adjustments.
------------------
351W 89 Mustang GT Convertible