FPR is a set it and forget it proposition, so adjustable IMO only makes it easier to set initially. There are enough variables in the FPR that simply changing the spring can't be a one-size fits all proposition. It may be close, or raise pressure higher if the spring is higher tension, but if it was possible to say X spring equals Y pressure, every truck would have exactly the same pressure off the assembly line, there wouldn't be a range of acceptable pressure.
I don't see fuel pump making a difference, X PSI is X PSI. I *suppose* if somehow the pump was able to overwhelm the return line in terms of volume then you might see higher pressure, but I suspect that is highly unlikely. Impossible with the stock TBI pump.
And Vic is being generic talking about FPR. It is true TO A POINT, which also assumes you don't have an open loop idle like some of the trucks run. The ECM can only compensate a minimum and maximum amount for too much or too little fuel during closed loop operation. Once you exceed the systems capability (programming capability, not physical limits edit: and there are physical limts of the injectors, which is why too big is as bad as too small) to add or subtract fuel, you will run into problems. It either runs lean or rich. If you are near the limit of the ECM's ability to add fuel under normal conditions, then add a trailer, you are going to run lean. Conversely, if you are rich under normal conditions, then add a trailer, fuel demand may be enough to get it back into a range the ECM can control. This is a cumulative effect. Swapping cams, intake, heads, etc., anything that increases the efficiency of the engine, is going to eat into the "margin" that is designed within the ECM's programming.
As a for-instance. Stock 454 TBI setup on a 454 with headers, mild "RV" cam, and Edelbrock dual plane intake resulted in fueling roughly 20% below what was necessary. The engine was running lean because there wasn't enough fuel being delivered. The only fix in that case is increasing fuel pressure, then correcting the numbers within the ECM programming to deal with the "artificially" increased flow rate of the injectors (which affects open loop operation), and the changed engine efficiency.
Not trying to make this more difficult than it is, but these things are complete systems. If you monkey with one aspect, it changes a host of others. Which may or may not result in differences you can tell, like better or worse idle, more or less power, more or less fuel economy, etc. Is it *likely* you will destroy an engine because you are running an AFPR? Of course not. But it IS going to change how it runs, and isn't how it runs why we like EFI?
Mess with fuel pressure if it's needed. Not because it's possible.
