Im trying to figure out the idle fuel pressure. Im going to be running stock injectors, SFMU, 6psi boost. So far through my calculating ive figured out my max fuel pressure will have to be ~101psi.

Ive figured that a good static fuel pressure should be 50psi. How can I figure out what idle pressure to set the SFMU to? Is there a general pressure that should be used when running that setup? Thanks.

NeoVB
2005 Subaru Impreza 2.5RS
1997 Mitsubishi Eclipse GS TURBO
1995 Mitsubishi Galant ES DOHC TURBO
1991 Eagle Talon TSI AWD

The static pressure is easy. You set it to whatever the car can idle at. Start with 45psi and then work your way down. I would expect you to be able to set it around 35psi. Once you know your static and idle(vacuum), you then choose the correct disk based on what maximum pressure you are trying to achieve.

05 Mazda RX-8
06 Lotus Elise

I just noticed you said stock injectors. Leave your static at stock. No reason to change.

05 Mazda RX-8
06 Lotus Elise

I calculated the proper rising rate for my setup would be 8.5. What is the stock static pressure and idle pressure settings?

NeoVB
2005 Subaru Impreza 2.5RS
1997 Mitsubishi Eclipse GS TURBO
1995 Mitsubishi Galant ES DOHC TURBO
1991 Eagle Talon TSI AWD

static for stock injectors is about 50 psi. good number to tune by. idle will be around 35-40, if i remember correctly.

2016 WRX STi Limited (current)
2001 A4 1.8tqms (donated)
1991 GVR4 #1933 (sold)
old.dsmregistry.com/detail.php?carid=218 (dismantled)

static fp should be 60 psi and idle @ 47-50 on stock injectors.

96 Eclipse RS - sold
92 Eclipse GSX

Just as a suggestion,

I wouldn't try to get exact mathmatical numbers for fuel pressures. I would only use them to guide you in the right direction as in what disk you would need to run in the vortech SFMU. It is as simple as a straighline formula.

For a Stage I system, you should be seeing 80 to 90 psi of fuel and the disk should be the 2.72 disk which has a ratio of 5:1.

Note: The optional faster rise rate spring may change the numbers slightly.

y = mx + b
y = WOT fuel pressure
M = disk ratio
x = max boost
b = idle pressure

y = 5 * 6 + 50 (for 50 psi idle pressure)
y = 80 psi

y = 5 * 6 + 55 (for 55 psi idle pressure, just about perfect)
y = 85 psi



I am running a stage II (8 psi) turbo kit and I started out around 50 psi at idle running stock injectors with the 3.62 disk (7:1). When I tuned the car according to the A/F guage and EGT guage, I ended up to be slightly lean. I then bumped up my idle (because idle will effect your maximum pressure at WOT shown above) to 55 psi. At that idle, the car runs perfect.

Keep in mind that each car will need slightly different fuel settings based on where you live and the weather. Cold air produces more power and your fuel may need to be increased more where as in hot weather less power is produced and you may be running rich.


Hope this helps,
Kurt

I'm finding this conversation interesting, as I plan to turbo my car in the near future. Would someone explain in a little detail what exactly we're talking about here. I understand that the purpose of the SFMU is to raise the fuel pressure by a certain ratio, depending on the level of boost. What are these discs that you use in a SFMU? What is static fuel pressure? How do you change your idle fuel pressure?


1996 Eclipse RS / 5-Speed / More at www.dalnetcars.com
Mark Galyan - Greenwood, Indiana

The reason a turbo car needs a rising rate fuel pressure regulator is the simple fact that we are adding more air to the combustion cycle. When you add more air (more boost), the more fuel you need. This is the job of the SFMU. The SFMU knows through a vac/boost line that is run from the back of the intake manifold to the SFMU how much air is being forced into the engine. When it detects more air (boost), it closes a bleed valve inside the regulator and prevents fuel from returning to the tank. This builds fuel pressure. When pressure rises in your fuel lines, more fuel is forced through your injectors and fed to the engine.


The best choice for a rising rate fuel pressure regulator is the Vortech SFMU. It comes with four calibration disks (a 7:1 - 3.72" disk, 5:1 - 2.62" disk, 3.25:1 -2.47" disk, and 2.5:1 - 2.22" disk), 3 rings (largest disk does not use a ring), and two springs. With the combination of different springs and disks, it allows you to fine tune how much pressure builds up in your fuel lines.

Static fuel pressure is fuel pressure at atmospheric conditions. It is not really important. (The signal line to the vortech is not connected)

Idle pressure is fuel pressure under engine vacuum (signal line connected). This is very important as idle can affect your WOT fuel pressures (max boost pressures). As an example, if you set your static fuel pressure and then connect the vacuum line, your fuel pressure will drop. (Static will always be higher then idle). When you set fuel pressures, you are only concerned about idle (static can be ignored).

The vortech SFMU has an allen key screw on top of the unit that you turn in to decrease idle fuel pressure and turn out to increase idle fuel pressure. You are simply closing or opening the bleed valve.

Hope this helps,
Kurt

Thanks

1996 Eclipse RS / 5-Speed / More at www.dalnetcars.com
Mark Galyan - Greenwood, Indiana