I adjusted my AEM FPR to 36psi at idle, and did so with the car running, on a warmed up 600rpm idle. I went out on a test drive to make sure it stayed. It would shoot up .5psi if I turned the a/c on, and a psi or two after some driving, but would always settle back down to 36psi several seconds later. Well, later that day I went out on an extended drive, doing many WOT runs. After one run, I checked my gauge and it read 33psi. Several seconds later (cruising with car put in neutral) it shifted back up to 36psi. Now, this is scaring me, becuase what if I do back to back runs, before the FPR "decides" to adjust back up to the set psi?? Why on earth is it doing this? The lock nut is on, everything is "fine".

 

The fpr is going to adjust fuel pressure based on manifold vacuum. It should give the lowest fuel pressure at the engine's highest vacuum (i.e. at idle), and maximum fuel pressure when manifold vacuum goes to zero (during acceleration but not boosting). When you are boosting the fmu takes over and drives the fuel pressure even higher based on boost pressure.

When you turn on the AC, the IAC allows a higher rpm with a subsequently lower manifold vacuum, so it is not surprising that the fuel pressure went up a bit. So it sounds like your fpr is behaving normally.
The fact that the fuel pressure was at 33 psi and then went up to 36 psi when you put it in neutral just means the manifold vacuum changed when you put the car in neutral.

 

So Steve, heres another instance. Today on the highway I had it in 4th gear around 3500-4000rpm, gunning it and letting off at various points. I also d/shifted into 3rd and floored it alot. After doing it several times, I looked at my gauge and it read 33psi! (while in gear) after putting the car into neutral it shot up some, but i'm still scared. Does this mean that my base fuel pressure, when I floor it, will start from that 33psi? Becuase I know I will probably detonate if so!

 

If you're flooring it, there will be positive manifold pressure so shouldn't the FMU be taking over??

 

This is what I *know* to be true. The FPR lowers base fuel pressure a set amount throughout the entire rpm band. The FMU may be taking over, but its taking over with X less psi, that was adjusted down with the FPR.

 

Happens to me all the time.

AT 18in-hg I get about 34psi of fuel pressure. On a high speed type highway run, if I let off the gas and leave the car in gear, my vaccum will drop to under 20in-hg...almost 22in-hg. Because of that the fuel pressure dips below that "base idle" pressure.

I've seen it hit high 20's before.

 

True, but since the manifold pressure will be at zero or above (when boosting), the fpr will always produce the same base pressure, since it doesn't react to positive pressure. In other words, your fpr may make a varying lower fuel pressure depending on manifold vacuum, but the upper fuel pressure (which should be around 43 psi) will always be the same. So you can tune based on a non-varying base pressure.

Well, this is clear as mud, I'm sure. But what I'm trying to say is that I don't think your fuel pressure wandering around a bit during non-boosted operation really matters.