Is 47 For Fuel Pressure to high

pete1 · 05-29-2001, 06:42 PM

I just got done checking my fuel pressure and it is set at 47pds with the vacuum off and am wondering if this is to high.set up is below. Thanks

Stock Heads, 24 LB Injectors, E303 Camshaft,
Edelbrock Performer Efi Intake, H-pipe, 2 Chamber Flowmasters,75 Bullet Mass Air Meter, 65TB, Mac Cold Air Induction, Mac Chrome Headers, AOD TCI Street Fighter Transmission, Tci 3000 stall converter, Lentech Valve Body, 4.10 gears, Timing Set At 14, TPS Set at .98, 190 Walbro Fuel Pump, Fuel Pressure set at 41, Msd 6AL, Dual Biondo Line Locks, 15x3.5 Weld Wheel On Front, 15x10 Welds On Rear with 26x9.5x15 Hoosier Quick time Pro Tires.

jimberg · 05-29-2001, 06:53 PM

Yeah, that's way too high. Set your pressure to 39.

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

pete1 · 05-29-2001, 07:13 PM

is that 39 with the vacuum off or on

------------------
1991 Mustang Gt,TCI AOD Streetfighter Transmission,Mac Chrome Headers,Mac air induction,3.73 gears,H-pipe,2 Chamber Flowmaster Mufflers,75 bullet mass air,65m Throttle Body, NSCA S/ET 7050,Best Time 13.47 1/4, 8.47 1/8
P&V Racing

red82gt · 05-29-2001, 07:23 PM

With those injectors you've got enough fuel for ~400 N/A horsepower!
You'd be better off with the 19 lb'ers. Since you've got the Bullet mass air meter I would knok the fuel pressure down to about 30 psi, even that will support ~320hp.

Here's my logic:
24lb inj. X8 inj.=192lbs
N/A cars need ~.5lbs of fuel/hp so: 192lbs/(0.5lbs/hp)=384 hp @40 psi fuel pressure.

to figure out the flow at 47 psi. take the square root of the new fuel pressure and divide it by the square root of the stock fuel pressure.
(47^1/2)/(40^1/2)=1.08
take this number and multiply it by the max number of hp at the stock pressure so
1.08X384hp=416hp! Your injectors won't be able to flow at capacity all the time so this much fuel pressure will still support 400hp.

So at 30psi: (30^1/2)/(40^1/2)=0.87
0.87X384hp=332hp.

In short, set it to 30 psi, anything more and you'll be wasting fuel!

------------------
GT-40 heads (ported, polished, + milled), B303 cam, 1.7rr's, JE pistons, Offy intake, Carter AFB 625cfm carb, Flowtech 1 5/8" shortys, Flowtech X-pipe, MAC Flowpath exhaust, MAC pulleys, 373's, subframes, Eibachs+Tokicos, B&M ripper, FMS Clutch, Zoom Quadrant+cable, 17" CSA Ultra rims, 235/45ZR17 Yokohama AVS S4's, MSD 6A ignition+coil, FMS 9mm wires, Carbed, Naturally Aspirated, and Nasty!

jimberg · 05-29-2001, 07:59 PM

It's 39psi with the vacuum line off and the engine off.

Find your test connector on the driver side up by the fire wall. It's got one flat side and another side that is kind of bezelled. There are 4 connectors in a row on the flat side. The fuel pump test lead is leftmost one if the flat side is on the top. Ground that lead with your key in the on position and your fuel pump will run continuously. Check your pressure that way.

Don't set it for 30psi. Set it to 39psi which is what the 24# rating is determined by. You won't be wasting fuel. The only time you need to bump it up is when you are having fuel starvation problems at WOT.

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

red82gt · 05-29-2001, 08:52 PM

Yes, the 24lb/hr rating is at 39 psi but he has a long way to go before he needs the flow of 24lbs /hr. He's maybe running about 265-270 hp, that is a long way from 384!

I tell ya what pete1, go to the track run at 39psi and then at 30psi and I'll bet you'll have a difference.

------------------
GT-40 heads (ported, polished, + milled), B303 cam, 1.7rr's, JE pistons, Offy intake, Carter AFB 625cfm carb, Flowtech 1 5/8" shortys, Flowtech X-pipe, MAC Flowpath exhaust, MAC pulleys, 373's, subframes, Eibachs+Tokicos, B&M ripper, FMS Clutch, Zoom Quadrant+cable, 17" CSA Ultra rims, 235/45ZR17 Yokohama AVS S4's, MSD 6A ignition+coil, FMS 9mm wires, Carbed, Naturally Aspirated, and Nasty!

jimberg · 05-29-2001, 09:35 PM

Fuel pressure has nothing to do with the air/fuel ratio. These are computer controlled vehicles. The computer will only deliver as much fuel as needed.

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

withamc · 05-30-2001, 12:44 AM

"Fuel pressure has nothing to do with the air/fuel ratio. These are computer controlled vehicles. The computer will only deliver as much fuel as needed."
Speaking from experience, I can tell you that is not true. I had my car on the dyno - 24# injectors and the other stuff in my sig - at 39 PSI (vacuum off) I was putting out 243 HP at the wheels and running rich at WOT. Dropped the fuel pressure to 33 PSI and the car had the proper A/F ratio and put 277 HP to the wheels. At WOT the computer ignores the O2 sensors. That's why we use adjustable regulators.
It doesn't look like pete1 is going to be putting out the kind of HP I am, so yes he should try dropping his fuel pressure to 30 PSI.

------------------
86 GT, CCM long block, GT40P heads, Crane 2031, 1.7 RR's, Cobra intake, 65mm TB, 73mm Vortech, 24# injectors, MAC GT40P headers, 2-1/2" BBK H-pipe (w/cats), Flowmasters, 3.55s, Groundpounder subs and lower control arms, 277 RWHP.
93 GT - Moroso cold air, 3.55 gears, ?? shifter, Black Magic fan, soon to get the engine and other goodies from the 86

jimberg · 05-30-2001, 03:08 AM

withamc, read the other fuel pressure thread to see my response to what you just said. Just for a little clarification hear, however, I will say my statement was referring to air/fuel ratio throughout the rpm range and not just at max HP. Your fix for running rich only effect air/fuel ratio at peak power.

How do you know that pete1 is running rich at WOT? Maybe his O2 sensors are working properly and yours aren't. His MAF meter may be calibrated differenlty. How do you know that his car doesn't run better rich at WOT? It varies from engine to engine.

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

Mach 1 · 05-30-2001, 10:04 AM

Jimberg- if your setting your fuel pressure with the engine off, whats the difference if the vacuum line is connected or not?

------------------
1993 GT/AOD
'93 Mustang GT

jimberg · 05-30-2001, 10:58 AM

Probably not much.

The important thing is that the engine is off.

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

red82gt · 05-30-2001, 02:17 PM

No disrespect, jimberg, Adaptive strategy is fine and all but how does your car know its making peak power in open loop? The oxygen sensors are not used during open loop therefore your computer would not be able to precisely find the right a/f ratio, it only controls the pulse width and the computer has no way of knowing if it is supplying a rich mixture (especially with bigger injectors), the only way to lean it out is to drop fuel pressure.
I've seen cases where the same car ran the same et/mph time after time, month after month, only to pick up .2sec/1.7mph by dropping the fuel pressure from stock to 32psi, only to go back to where it was before after going back to the stock fuel pressure. It works!

I'm not chiming in on this one anymore, I know I'm right, and its been proven too many times in practice for me to believe what was interpreted from a book.

jimberg · 05-30-2001, 03:39 PM

I've explained this at length in the other thread. The computer uses base calibrations determined during closed loop operation to calculate injector pulse widths for open loop operation. And, it's possible that the particular car you're talking about was running a little too rich at peak hp and the adjustment helped.

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

05-29-2001, 06:42 PM	#1
pete1 Registered Member Join Date: Sep 1999 Location: Radcliff, Ky U.S.A Posts: 202	Is 47 For Fuel Pressure to high I just got done checking my fuel pressure and it is set at 47pds with the vacuum off and am wondering if this is to high.set up is below. Thanks Stock Heads, 24 LB Injectors, E303 Camshaft, Edelbrock Performer Efi Intake, H-pipe, 2 Chamber Flowmasters,75 Bullet Mass Air Meter, 65TB, Mac Cold Air Induction, Mac Chrome Headers, AOD TCI Street Fighter Transmission, Tci 3000 stall converter, Lentech Valve Body, 4.10 gears, Timing Set At 14, TPS Set at .98, 190 Walbro Fuel Pump, Fuel Pressure set at 41, Msd 6AL, Dual Biondo Line Locks, 15x3.5 Weld Wheel On Front, 15x10 Welds On Rear with 26x9.5x15 Hoosier Quick time Pro Tires.

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05-29-2001, 06:53 PM	#2
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Yeah, that's way too high. Set your pressure to 39. ------------------ 351W 89 Mustang GT Convertible

05-29-2001, 07:13 PM	#3
pete1 Registered Member Join Date: Sep 1999 Location: Radcliff, Ky U.S.A Posts: 202	is that 39 with the vacuum off or on ------------------ 1991 Mustang Gt,TCI AOD Streetfighter Transmission,Mac Chrome Headers,Mac air induction,3.73 gears,H-pipe,2 Chamber Flowmaster Mufflers,75 bullet mass air,65m Throttle Body, NSCA S/ET 7050,Best Time 13.47 1/4, 8.47 1/8 P&V Racing

05-29-2001, 07:23 PM	#4
red82gt Sober voice of Reason Join Date: Nov 1998 Location: Kelowna, B.C., Canada Posts: 1,514	With those injectors you've got enough fuel for ~400 N/A horsepower! You'd be better off with the 19 lb'ers. Since you've got the Bullet mass air meter I would knok the fuel pressure down to about 30 psi, even that will support ~320hp. Here's my logic: 24lb inj. X8 inj.=192lbs N/A cars need ~.5lbs of fuel/hp so: 192lbs/(0.5lbs/hp)=384 hp @40 psi fuel pressure. to figure out the flow at 47 psi. take the square root of the new fuel pressure and divide it by the square root of the stock fuel pressure. (47^1/2)/(40^1/2)=1.08 take this number and multiply it by the max number of hp at the stock pressure so 1.08X384hp=416hp! Your injectors won't be able to flow at capacity all the time so this much fuel pressure will still support 400hp. So at 30psi: (30^1/2)/(40^1/2)=0.87 0.87X384hp=332hp. In short, set it to 30 psi, anything more and you'll be wasting fuel! ------------------ GT-40 heads (ported, polished, + milled), B303 cam, 1.7rr's, JE pistons, Offy intake, Carter AFB 625cfm carb, Flowtech 1 5/8" shortys, Flowtech X-pipe, MAC Flowpath exhaust, MAC pulleys, 373's, subframes, Eibachs+Tokicos, B&M ripper, FMS Clutch, Zoom Quadrant+cable, 17" CSA Ultra rims, 235/45ZR17 Yokohama AVS S4's, MSD 6A ignition+coil, FMS 9mm wires, Carbed, Naturally Aspirated, and Nasty!

05-29-2001, 07:59 PM	#5
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	It's 39psi with the vacuum line off and the engine off. Find your test connector on the driver side up by the fire wall. It's got one flat side and another side that is kind of bezelled. There are 4 connectors in a row on the flat side. The fuel pump test lead is leftmost one if the flat side is on the top. Ground that lead with your key in the on position and your fuel pump will run continuously. Check your pressure that way. Don't set it for 30psi. Set it to 39psi which is what the 24# rating is determined by. You won't be wasting fuel. The only time you need to bump it up is when you are having fuel starvation problems at WOT. ------------------ 351W 89 Mustang GT Convertible

05-29-2001, 08:52 PM	#6
red82gt Sober voice of Reason Join Date: Nov 1998 Location: Kelowna, B.C., Canada Posts: 1,514	Yes, the 24lb/hr rating is at 39 psi but he has a long way to go before he needs the flow of 24lbs /hr. He's maybe running about 265-270 hp, that is a long way from 384! I tell ya what pete1, go to the track run at 39psi and then at 30psi and I'll bet you'll have a difference. ------------------ GT-40 heads (ported, polished, + milled), B303 cam, 1.7rr's, JE pistons, Offy intake, Carter AFB 625cfm carb, Flowtech 1 5/8" shortys, Flowtech X-pipe, MAC Flowpath exhaust, MAC pulleys, 373's, subframes, Eibachs+Tokicos, B&M ripper, FMS Clutch, Zoom Quadrant+cable, 17" CSA Ultra rims, 235/45ZR17 Yokohama AVS S4's, MSD 6A ignition+coil, FMS 9mm wires, Carbed, Naturally Aspirated, and Nasty!

05-29-2001, 09:35 PM	#7
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Fuel pressure has nothing to do with the air/fuel ratio. These are computer controlled vehicles. The computer will only deliver as much fuel as needed. ------------------ 351W 89 Mustang GT Convertible

05-30-2001, 12:44 AM	#8
withamc Registered Member Join Date: Aug 2000 Location: Thousand Oaks, CA Posts: 396	"Fuel pressure has nothing to do with the air/fuel ratio. These are computer controlled vehicles. The computer will only deliver as much fuel as needed." Speaking from experience, I can tell you that is not true. I had my car on the dyno - 24# injectors and the other stuff in my sig - at 39 PSI (vacuum off) I was putting out 243 HP at the wheels and running rich at WOT. Dropped the fuel pressure to 33 PSI and the car had the proper A/F ratio and put 277 HP to the wheels. At WOT the computer ignores the O2 sensors. That's why we use adjustable regulators. It doesn't look like pete1 is going to be putting out the kind of HP I am, so yes he should try dropping his fuel pressure to 30 PSI. ------------------ 86 GT, CCM long block, GT40P heads, Crane 2031, 1.7 RR's, Cobra intake, 65mm TB, 73mm Vortech, 24# injectors, MAC GT40P headers, 2-1/2" BBK H-pipe (w/cats), Flowmasters, 3.55s, Groundpounder subs and lower control arms, 277 RWHP. 93 GT - Moroso cold air, 3.55 gears, ?? shifter, Black Magic fan, soon to get the engine and other goodies from the 86

05-30-2001, 03:08 AM	#9
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	withamc, read the other fuel pressure thread to see my response to what you just said. Just for a little clarification hear, however, I will say my statement was referring to air/fuel ratio throughout the rpm range and not just at max HP. Your fix for running rich only effect air/fuel ratio at peak power. How do you know that pete1 is running rich at WOT? Maybe his O2 sensors are working properly and yours aren't. His MAF meter may be calibrated differenlty. How do you know that his car doesn't run better rich at WOT? It varies from engine to engine. ------------------ 351W 89 Mustang GT Convertible

05-30-2001, 10:04 AM	#10
Mach 1 Registered Member Join Date: May 1999 Location: Georgia Posts: 3,866	Jimberg- if your setting your fuel pressure with the engine off, whats the difference if the vacuum line is connected or not? ------------------ 1993 GT/AOD '93 Mustang GT

05-30-2001, 10:58 AM	#11
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Probably not much. The important thing is that the engine is off. ------------------ 351W 89 Mustang GT Convertible

05-30-2001, 02:17 PM	#12
red82gt Sober voice of Reason Join Date: Nov 1998 Location: Kelowna, B.C., Canada Posts: 1,514	No disrespect, jimberg, Adaptive strategy is fine and all but how does your car know its making peak power in open loop? The oxygen sensors are not used during open loop therefore your computer would not be able to precisely find the right a/f ratio, it only controls the pulse width and the computer has no way of knowing if it is supplying a rich mixture (especially with bigger injectors), the only way to lean it out is to drop fuel pressure. I've seen cases where the same car ran the same et/mph time after time, month after month, only to pick up .2sec/1.7mph by dropping the fuel pressure from stock to 32psi, only to go back to where it was before after going back to the stock fuel pressure. It works! I'm not chiming in on this one anymore, I know I'm right, and its been proven too many times in practice for me to believe what was interpreted from a book.

05-30-2001, 03:39 PM	#13
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	I've explained this at length in the other thread. The computer uses base calibrations determined during closed loop operation to calculate injector pulse widths for open loop operation. And, it's possible that the particular car you're talking about was running a little too rich at peak hp and the adjustment helped. ------------------ 351W 89 Mustang GT Convertible