It has been some time since I've messed with my MS3 tunes. Car has been sitting a lot since the move last year. I am trying to get back at it. I was digging through some setting trying to knock the cobwebs out and noticed this Fuel Pump Control section. At the moment the only Vac line I have goes to the MS3 onboard MAP. I still have the stock fuel system, Walbro 255 pump, and FM dual feed rail (modified to single feed). To my knowledge I have no Vac Reference and I don't know if the pump has some atmosphere opening on its end. My setup has been this way since I got the base map I presume. Unless a firmware update lost its mind. That was a fun week.

Also as a side note I am going back to school for Mech Engineering and lead of the FSAE car. I am the only one on the team with any tuning experience. One of the older cars I'm trying to fix tunes on has a fuel pressure regulator open to air reference. I would have though to close it up so fuel pressure doesn't change with altitude. It is a 1:1 rising rate regulator. I was always under the impression that was for when you don't have a standalone. And if it is boosted then use the ECU to compensate pulse width not a higher fuel pressure. I think I am now wrong in those assumptions.

 

I'm not sure what your question is.

FPR is not a sealed can. They are open to atmosphere (so called constant pressure) like on you 1999, and apparently your FSAE car, or connected to the intake manifold. Either works. Intake referenced gives a little better control, especially at idle (low manifold pressure).

If it is 1:1, I would not call it "rising rate". It is 1:1. Rising rate would be like 2:1, such that fuel pressure goes up faster than manifold. Those are the older technology that allowed smaller injectors to still work, as you were thinking.

Don't worry about the Vac Referenced. If your table was tuned that way, it will make no difference at all whether it is set there. Just leave it.

Actually IIRC, I switched mine and saw no change in fueling. I'm not sure where MS actually does the fuel calculation mentioned in the help box. My conjecture is in the MPG calculation, not in the fueling calculation. In fact, if you look at all of the fueling calculations, there is no term for such compensation.