What Fuel pressure should I run?

95 King GT · 10-25-2001, 01:19 AM

I just hooked up my BBK adjustable fuel pressure regulator. what would be good PSI right now I have it at 38 PSI.

MODS: headers,h-pipe,2.5 cat-back,U/D Pulleys,pro-m 75mm,cold air induction w/KN,custom chip, 3:73 gears,65mm TB,subframes,FPR, shift kit.

Future mods (I already bought them) Trick Flow street Intake, 1.6 RR, E-cam,( and when the money comes back stage 2 ported winsdor JR. heads)

mustang17 · 10-25-2001, 11:18 AM

38 psi at WOT? I think 32 is stock, correct? You'll be fine around the stock setting. You can try +/- a few psi in both directions, but stay close to stock. Your current set up doesn't need more.

When you do the rest of the mods, go to the dyno. I have heard that some people lower their FP, even with more mods! You can do some guess and check to gain a bit more power.

------------------
1995 GT Black- Edelbrock 6037's,Intake,Edelbrock cam,70mm TB,75mm pro-m,Taylor wires, pulleys,Equal length headers,K&N filter
Flowmaster,3.73's, Eibach pro rate springs
lower control arms Centerforce clutch
New motor in the works!

89 Cobra LX · 10-25-2001, 01:29 PM

You should definitely be around 32psi. Just to give you an idea, when I went to the dyno we ended up setting mine at 34psi for the best results(still with 19lbers). Check out my sig and you'll see I have considerably more mods.

------------------
Eric - 89 Mustang LX coupe
302 - Cobra Intake - GT40p heads - TFS stage 1 cam - FRPP 1.6 roller rockers - Naturally Aspirated
3:73 gears - KYB shocks and struts - Eibach springs - HPM Mega-bite Jr lower control arms - HPM upper control arms

exgmguy · 10-25-2001, 03:42 PM

38-40 psi is the stock and recommended setting. If you drop it to 32 the engine will lean out for a short time. Then the computer will simply widen the pulse width of the injectors so the proper air/fuel ratio is met.

So basically set it at 38 psi with the engine running and the vacuum hose disconnected. Removing the hose simulates wide open throttle.

------------------
1988 Mustang GT
11.8 @ 123 (7/14/01)
11.5 @ 120 (9/14/01)
1992 BadAzz Wrangler
1993 Explorer

See my car at: www.mustangworks.com/cgi-bin/moi-display.cgi?179

95 King GT · 10-25-2001, 04:02 PM

thanks for all the help. BBK said that 39 PSI is stock, I set it at 40 PSI at WOT. It feels damn good.

John · 10-25-2001, 05:22 PM

I was under the impression that 43 is stock w/ vac. disconnected. Pls. correct me if I'm wrong.

------------------
89 Notch: ASP Pullies,
K&N Air,B&M shift kit, C4, JW 2800 torque conv., B&M Zgate shifter, Trans. cooler,FMS 3 core rad., FMS 4:10's, 1.5" cowl, cold air kit, hooker shorties(jet hot coated), O/R H-Pipe, ASP solid mounts, Energy Sus. Trans. Mnt. BBK 70mm TB, MSD 6al, Dual elec. fans, 5.0 Firepower Kit, Scorpion 1.6 RR's, Flowmasters, 73 mm MAF, cobra intake(ported), 1" phenolic spacer,ported stock heads and crower #512 cam.

Unit 5302 · 10-25-2001, 06:37 PM

Stock is 39psi with the vacuum line disconnected.

jimberg · 10-26-2001, 08:20 AM

Unit is correct, of course, but I think this is a good time to bring up the reason for getting an adjustable fuel pressure regulator. exgmguy is also correct that the computer will simply extend the injector pulse width when it sees that the air/fuel ratio is too lean via the oxygen sensors.

Applying vacuum to the FPR will cause the fuel pressure to go down so that there is a constant relative pressure maintained between the fuel rails and the cylinder. If, for example, the intake stroke has 8psi of pull, you want the fuel rail to be pushing at 31psi for a total 39 psi. Replace 39 psi with whatever you set the pressure at with the vacuum line disconnected.

If you set your fuel pressure too low, you can create two problems. One is that the pressure is so low that the injector will trickle instead of spraying the fuel. Think of it like a bottle of Windex. If you pump hard, it has a nice spray that covers your window. If you pull the pump slowly, the fluid will just slowly stream out onto your window. The other problem is that you will be cutting off the amount of fuel available at WOT. Yes, this can lean out your air/fuel ratio at peak horsepower, but it will be abrupt and relatively worthless when it comes to gaining any horsepower because it won't allow for a change that is constant and repeatable. I could go into a lot more detail about why this is so, but for now I will just leave it at something that you just shouldn't do.

The best reason to have an adjustable FPR is that your engine pulls in more air than your injectors can provide fuel, but it's not enough air to require going to the next highest capacity set of fuel injectors. Let's say that you have 24# injectors but you need 28# injectors to meet the fuel requirements for your engine. Just use the following formula:

N = New Fuel Pressure
T = Target Injector Capacity
S = Injector Capacity at 39 psi.

N = ( T / S )^2 * 39

Plugging in the numbers for our example, we get.

53.08 = ( 28 / 24 )^2 * 39

You can set it to 54psi just to be safe.

If you search on Fuel Pressure you'll find many messages where we go into this in a lot more detail, but I figured I'd enter a little refresher.

------------------
351W 89 Mustang GT Convertible

10-25-2001, 01:19 AM	#1
95 King GT Registered Member Join Date: Jun 2001 Location: coweta OK USA Posts: 93	What Fuel pressure should I run? I just hooked up my BBK adjustable fuel pressure regulator. what would be good PSI right now I have it at 38 PSI. MODS: headers,h-pipe,2.5 cat-back,U/D Pulleys,pro-m 75mm,cold air induction w/KN,custom chip, 3:73 gears,65mm TB,subframes,FPR, shift kit. Future mods (I already bought them) Trick Flow street Intake, 1.6 RR, E-cam,( and when the money comes back stage 2 ported winsdor JR. heads)

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10-25-2001, 11:18 AM	#2
mustang17 Registered Member Join Date: Sep 2000 Location: Concord, MA, USA Posts: 264	38 psi at WOT? I think 32 is stock, correct? You'll be fine around the stock setting. You can try +/- a few psi in both directions, but stay close to stock. Your current set up doesn't need more. When you do the rest of the mods, go to the dyno. I have heard that some people lower their FP, even with more mods! You can do some guess and check to gain a bit more power. ------------------ 1995 GT Black- Edelbrock 6037's,Intake,Edelbrock cam,70mm TB,75mm pro-m,Taylor wires, pulleys,Equal length headers,K&N filter Flowmaster,3.73's, Eibach pro rate springs lower control arms Centerforce clutch New motor in the works!

10-25-2001, 01:29 PM	#3
89 Cobra LX Don Corlione Join Date: Jun 2001 Location: Omaha NE USA Posts: 491	You should definitely be around 32psi. Just to give you an idea, when I went to the dyno we ended up setting mine at 34psi for the best results(still with 19lbers). Check out my sig and you'll see I have considerably more mods. ------------------ Eric - 89 Mustang LX coupe 302 - Cobra Intake - GT40p heads - TFS stage 1 cam - FRPP 1.6 roller rockers - Naturally Aspirated 3:73 gears - KYB shocks and struts - Eibach springs - HPM Mega-bite Jr lower control arms - HPM upper control arms

10-25-2001, 03:42 PM	#4
exgmguy Registered Member Join Date: Feb 1999 Location: Utica, Michigan Posts: 2,631	38-40 psi is the stock and recommended setting. If you drop it to 32 the engine will lean out for a short time. Then the computer will simply widen the pulse width of the injectors so the proper air/fuel ratio is met. So basically set it at 38 psi with the engine running and the vacuum hose disconnected. Removing the hose simulates wide open throttle. ------------------ 1988 Mustang GT 11.8 @ 123 (7/14/01) 11.5 @ 120 (9/14/01) 1992 BadAzz Wrangler 1993 Explorer See my car at: www.mustangworks.com/cgi-bin/moi-display.cgi?179

10-25-2001, 04:02 PM	#5
95 King GT Registered Member Join Date: Jun 2001 Location: coweta OK USA Posts: 93	thanks for all the help. BBK said that 39 PSI is stock, I set it at 40 PSI at WOT. It feels damn good.

10-25-2001, 05:22 PM	#6
John Registered Member Join Date: Dec 1998 Location: Ft.Myers, FL. Posts: 150	I was under the impression that 43 is stock w/ vac. disconnected. Pls. correct me if I'm wrong. ------------------ 89 Notch: ASP Pullies, K&N Air,B&M shift kit, C4, JW 2800 torque conv., B&M Zgate shifter, Trans. cooler,FMS 3 core rad., FMS 4:10's, 1.5" cowl, cold air kit, hooker shorties(jet hot coated), O/R H-Pipe, ASP solid mounts, Energy Sus. Trans. Mnt. BBK 70mm TB, MSD 6al, Dual elec. fans, 5.0 Firepower Kit, Scorpion 1.6 RR's, Flowmasters, 73 mm MAF, cobra intake(ported), 1" phenolic spacer,ported stock heads and crower #512 cam.

10-25-2001, 06:37 PM	#7
Unit 5302 Registered Member Join Date: May 1999 Posts: 5,246	Stock is 39psi with the vacuum line disconnected.

10-26-2001, 08:20 AM	#8
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Unit is correct, of course, but I think this is a good time to bring up the reason for getting an adjustable fuel pressure regulator. exgmguy is also correct that the computer will simply extend the injector pulse width when it sees that the air/fuel ratio is too lean via the oxygen sensors. Applying vacuum to the FPR will cause the fuel pressure to go down so that there is a constant relative pressure maintained between the fuel rails and the cylinder. If, for example, the intake stroke has 8psi of pull, you want the fuel rail to be pushing at 31psi for a total 39 psi. Replace 39 psi with whatever you set the pressure at with the vacuum line disconnected. If you set your fuel pressure too low, you can create two problems. One is that the pressure is so low that the injector will trickle instead of spraying the fuel. Think of it like a bottle of Windex. If you pump hard, it has a nice spray that covers your window. If you pull the pump slowly, the fluid will just slowly stream out onto your window. The other problem is that you will be cutting off the amount of fuel available at WOT. Yes, this can lean out your air/fuel ratio at peak horsepower, but it will be abrupt and relatively worthless when it comes to gaining any horsepower because it won't allow for a change that is constant and repeatable. I could go into a lot more detail about why this is so, but for now I will just leave it at something that you just shouldn't do. The best reason to have an adjustable FPR is that your engine pulls in more air than your injectors can provide fuel, but it's not enough air to require going to the next highest capacity set of fuel injectors. Let's say that you have 24# injectors but you need 28# injectors to meet the fuel requirements for your engine. Just use the following formula: N = New Fuel Pressure T = Target Injector Capacity S = Injector Capacity at 39 psi. N = ( T / S )^2 * 39 Plugging in the numbers for our example, we get. 53.08 = ( 28 / 24 )^2 * 39 You can set it to 54psi just to be safe. If you search on Fuel Pressure you'll find many messages where we go into this in a lot more detail, but I figured I'd enter a little refresher. ------------------ 351W 89 Mustang GT Convertible