You do not use an adjustable fuel pressure regulator to change your air/fuel ratio. That can only be done with a chip. You can, however, use fuel pressure to compensate for injectors that don't have enough capacity for the amount of horse power your engine is capable of producing.
Changing fuel pressure can cause measurable, temporary changes in the A/F ratio and performance of your engine which ta dyno will pick up, but the computer will compensate for the addition or removal of fuel by using the readings coming from the O2 sensors.
There are exceptions to this, of course. Anyone who has seen an a/f gauge in action knows that the thing constantly bounces back and forth between rich and lean. I think this is because of the timer resolution used to control how long the injectors are open. The computer just makes sure that the bounce occurs between a predefined rich and lean level so that it averages out to be the stoichiometric a/f ratio of 14.7:1.
Think of this range as two lines on a table that are 1 foot apart. One line is the maximum richness allowed and the other line is the maximum leanness allowed. The actual a/f ratio can be represented by a pendulum. It swings back and forth 8 inches and is positioned between the two lines on the table. Optimally, as far as having a perfect, stochiometric a/f ratio, the pendulum would only go within 2 inches of either line. As you increase fuel pressure, the pendulum would come closer and closer to the max richness line. As long as it doesn't cross that line, the computer will not do anything to correct it. If, however, it does cross the line, the computer will compensate by removing some time from how long the injectors open and that will move the pendulum back between the lines. It does this over time but will eventually negate any adjustment that you made.
What I just described is how closed-loop mode would work. Any adjustments that the computer makes are stored in the KAM and factored into the calculations used during open-loop mode.
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