what fuel pressure should i use?

[CRAIG___]

hey i have a 95 gst with a 50 trim turbo a big front mount intercooler walbro 255 fuel pump type s bov i just installed a fuel rail with adjustable fuel pressure regulator still has stock injectors what should i set my fuel pressure on?? and was gonna install rc 750cc injectors what fuel pressure should i use after that install?

 

[1992awdlaser]

Stock fuel pressure of 43.5 with the vacuum line disconnected. Why would you use a different fuel pressure?

 

[Splitpi]

Stock fuel pressure of 43.5 with the vacuum line disconnected. Why would you use a different fuel pressure?

I agree that is the pressure to use in the scenario above. But the reason to use lower or higher fuel pressure should be obvious with the equation:
New Flow Rate = SQRT(New Pressure / Old Pressure) * Old Flow Rate

i.e. Stock = 43.5 psi, new pressure is 39 psi on 750 cc's min

New Flow Rate = SQRT(39/43.5) * 750 = 710 cc

or Stock = 43.5 psi, new pressure is 45 psi on 750 cc's min

New Flow Rate = SQRT(45/43.5) * 750 = 763 cc

The reason for increasing the pressure is to get more flow from the injectors without increasing the Duty Cycle.

Fuel pump has a flow rate that is related to output pressure and voltage. Meaning, our fuel pressure matches boost 1:1 so if I am running 20 psi on stock fuel pressure, the fuel lines are pressurized to 43.5 + 20 = 63.5 psi. This ensures that the fuel is always entering at 43.5 psi over intake pressure. Now as the pressure increases the fuel pumps have harder and harder times to keep the ltr/hr flow rate at that pressure. Thus, at higher pressures the pumps cannot fully flow enough to feed the injectors. The solution is to decrease the base fuel pressure. This allows the pump to operate in a better power band. But ofcourse with the equation above, the injectors flow less.... so you must use larger injectors to compensate.

There are benefits for doing it both ways. But in either case you need to know what you are doing and have a method to log and tune the ECU to account for the different fuel flow rates. Hope that clarified the "Why". But I agree with your assessment for what the OP should use.

 

[1992awdlaser]

The question was for his set up not as a question of what advantages or disadvantages there are. I know what lowering and raising fuel pressure will do for lack of a fuel system that works together. Good explanation for somebody that needs to read up though.

 

[Mr Peepers]

I think the real question is if that's all that's done to the car.

Tuning?

"stock injectors what should i set my fuel pressure on?? and was gonna install rc 750cc injectors what fuel pressure should i use after that install?" - this brings up red flags.

You can't just throw 750's in there without something to tune/compensate for them. DSMlink, SAFC, DSMchip, etc.