Any dyno tips.

[jimberg]

fiveohpatral, it doesn't change the fuel pressure at all. That is mechanical and will remain fairly constant. The computer compensates by adjusting the injector pulse length based on feedback by the O2 sensors. This is all done during closed-loop operation of your computer. Here's how it normally works:

You computer uses the MAF, ACT (air charge temperature), ECT (engine coolant temperature), and BAP (barometric air pressure) sensors to measure how much air is going into the cylinders. The computer calculates the length of injector pulses based on that information. The computer has a predetermined air/fuel ratio that it must maintain for fuel economy and it uses the O2 sensors to measure whether or not that air/fuel ratio is being achieved. If the A/F ratio is too lean, it compensates by lengthening the injector pulse. If it's too rich, it will shorten the injector pulse. It will average that adjustment over time and store it as a compensation value. Let's say your fuel pressure was exactly where the computer expected it to be. The compensation value would be 1.00. Now you adjust the fuel pressure to deliver 10% more fuel than the computer expected. Eventually, the computer would set your compensation value to .90.

When you're at WOT or your engine is cold, you run in open-loop operation. This means that the computer will not use O2 sensor feedback and base injector pulse length only on information by the MAF, ACT, ECT and BAP. These are the times that the computer wants to run rich, but it's too rich for the O2 sensors to measure so the computer just guesses based on lookup tables. The computer measures air, looks up how long to open the injectors, applies the compensation value that was stored during closed-loop operation, and then opens the injectors accordingly.

Adjusting fuel pressure isn't completely useless, but it's not something you tweak on a dyno and hope to maintain. If you want to run on the rich side, you can bump your pressure up and eventually it will come down to the rich side of normal. If you want it to run on the lean side of normal, you can drop it some and it will move up to the lean side of normal. What I mean by the lean or rich side of normal is that normal is a window of voltages that the computer will look for from the O2 sensors. It's a range that as soon as the computer compensates the air/fuel ratio to be within, the computer will stop compensating.

I hope this helps.