What fuel Press. Does everyone run..?

PrplMonster · 08-18-2001, 11:45 AM

I have a 347 with Performer heads/matching intake. Pretty radical cam

70mmTB/EGR Spacer, 24lb injectors with a Matching PRO-M meter(not quite sure what size it is) curious as to what you all run for fuel pressure, i have been searching for the optimal, but not much luck... any suggestions..?

.dave

jimberg · 08-18-2001, 01:30 PM

How have you been searching for the optimal fuel pressure? I take it that you have at least installed 155lph fuel pump or better. If you're still running a stock fuel pump an adjustable FPR can't really help you.

We've gone over this a lot of times, but it's probably good to bring up every now and then.

Your air/fuel mixture is controlled by the computer via sensor input and injector pulse length. The MAF, ACT and ECT sensors help the computer measure how much air is going into your cylinders. The computer then does some calculating and determines how long to open the injectors based on how much air it measured going into the cylinders. The computer has preprogrammed air/fuel ratios that it must keep. Changing fuel pressure will only change the amount of fuel that is delivered for a given injector pulse length. The computer is programmed to expect that the fuel pressure is at a specific value like 39psi (I don't know what it really expects, but I'm sure it's around what the injectors are rated at). If you raise your pressure to, let's say, 50psi you'll increase the amount of fuel delivered during a given pulse length by about 13% compared to same pulse length at 39psi. You've esentially richened the air/fuel mixture.

This is where adaptive strategy becomes a factor. The computer will monitor feedback from the oxygen sensors and determine that the amount of fuel it provided for the measured amount of air caused a richer condition than the computer is programmed to keep. The computer will then gradually decrease the pulse width compensation factor until the oxygen sensors report the correct air/fuel mixture that the computer is trying to maintain. The opposite is true if you try to lower fuel pressure and lean out the air/fuel mixture.

What I described is closed loop control. When in open loop control the computer will just look up pulse lengths from a table and then apply the compensation factor determined during closed loop control against them to determine how much fuel to deliver. This is the control mode used during WOT. Your oxygen sensors are only checked to be sure that you aren't running dangerously lean in this mode.

I'm pretty sure the reason the computer uses open loop mode during WOT is that the 12.8-13.1:1 air/fuel ratio that it is trying to maintain is outside of the range of the narrow band oxygen sensors that are used.

So, the question now should be, "Why get an adjustable FPR if it doesn't really do anything to change the air/fuel mixture?". The answer is that sometimes the injectors used don't have enough capacity to supply the amount of fuel necessary to match the amount of air coming into the engine. You esentially have an engine capable of more power than your fuel system can handle. This would be evident on a dyno graph where your horsepower line just flattens out after peaking. If this happens, you can raise the fuel pressure to essentially increase the capacity of your injectors to the point that they can deliver enough fuel.

Injectors also have a minimum amount of time that they can be open. The higher the injector rating and fuel pressure, the less control the computer will have at idle. If emissions is an issue, choose an injector that has a capacity nearest to your horsepower potential without going over. If you need 28# injectors, get 24# injectors and raise the fuel pressure to make 28#. If you need 23#, stick with the 19#ers.

There is some potential for adjustment but not a lot and I'm not sure to what degree. The oxygen sensors put out a voltage ranging from .100 V to .900 V. Voltages less than .400 V being considered lean and voltages over .600 V being considered rich. Since you have that window of .200 V, it may be possible to stay on the lean side if you start off lean or stay on the rich side if you start off rich. Once you're in that window whether on the rich side or lean side you may be able to make small adjustments up or down in fuel pressure as long as you stay in that window. Maybe 1 or 2 psi at a time.

Oh, to figure out what the capacity of your fuel injectors is at a given fuel pressure, take your fuel pressure and divide it by 39. Take the square root of that number and multiply it by your rated injector capacity.

Well, there's what I know about fuel pressure in a nutshell. I have my pressure set at 60psi right now.

I hope this helps with your adjustments.

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

PrplMonster · 08-18-2001, 03:14 PM

well jim, i understand all of that, so i guess i will rephrase the question. I am running 24#'ers at 42Psi should i be up pn fuel pressure..? I have heard from a few different people that with the bigger injector, the more press you need to make the injector work to it's potential.. is this true..?

.dave

jimberg · 08-18-2001, 05:13 PM

I suppose it's possible for a really big injector to be rated at a higher fuel pressure, but the 24# injectors we usually use are rated at 39psi. Anything near or over that will give you the 24# capacity or more.

Running it on a dyno with a wide band oxygen sensor will probably be the only sure way to confirm that you are getting enough fuel at your peak power.

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

PrplMonster · 08-18-2001, 05:27 PM

Well, i have her at 60psi now..

It seems as if she is running better, the hesitation under 2k settled a little more. At 60psi i really don't have much black smoke, can't even notice it while she's running. At 65psi my eyes started to burn just being near it, i'd say it's rich.. any thoughts..?

.dave

PrplMonster · 08-18-2001, 05:30 PM

duh, sorry i aslo have a 190lph intank.. brand new..

.dave

jimberg · 08-18-2001, 07:25 PM

The richness will go away in a while. Just go for a drive for about 100 miles.

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

PrplMonster · 08-18-2001, 10:06 PM

Well i went out an beat her up tonight!! The buck/hesitation only happens when you have I push ever so slightly on the gas pedal. At 60psi she runs like a raped ape, like right under 55 is when the buck/hesitation starts to come back(worse than it is at 60) At 65 i start to get a rough idle.. I am running 14*. I checked my MAF and everything looked clean(took it apart) Also, my car takes a minute to return back to idle. I have sprayed Carb cleaner on just about everything and no increase in RPM. any suggestions..? One thing i did notice, is however much i did change fuel press. this afternoon the plugs seemed to be OK.. weird. both o2 sensors are brand new.

Are those Fuel/Air ratio gauges any good..? I heard they only work good for WOT.

.dave

jimberg · 08-19-2001, 07:36 AM

Make sure that your TPS is set well below 1 V. Set it to something like .95-.97. If you have it at .99 and you tap on the accelerator the computer will think you're at part throttle when you're still at closed throttle and open the EGR valve and thermactor causing your hesitation. The easiest way to confirm this theory is to disconnect the vacuum line from the EGR and plug it with a golf tee or something. If you still get the hesitation, it is still possible that your EGR valve is stuck open a little, though.

Any changes you make to fuel pressure will require some time for the computer to adapt. How long did you wait between each fuel pressure you tried?

The air/fuel gauges bounce back and forth since that's how the computer works. You're correct that they probably aren't that much help until you're at WOT. Even then it will only indicate whether you are rich or lean but not by how much. The only way to do that would be to get a gauge that works with a wide band o2 sensor. Knowing if you go lean, though, would be some good info.

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

PrplMonster · 08-19-2001, 08:21 AM

well jim, i put about 200 miles on the old gurl last night, ALL of my emissions are disconnected, i just have the EGR plugged in with no vacuum. I know the MAF is working i mean hell, she'd barely run without it connected.. lost... any help..?

.dave

PrplMonster · 08-19-2001, 10:39 AM

TPS is .965, I am not complaining about performance, i mean hell, i am going to need new tires soon.. >

It's just annoying to not be able to drive slow at a low RPM

PrplMonster · 08-19-2001, 11:12 AM

I just hooked up a vacuum gauge, it's reading about 10in/hg. I have checked for leaks about 3 times now, no air leak(s) any idea's..?

.dave

PrplMonster · 08-19-2001, 11:23 AM

Hey guys, For shits and giggles I used the jumper wire to see if i could get any codes. and the check engine light won't even come on(with or without the jumper) and the actual bulb is good. Could my computer just be in limp mode..?

.dave

PKRWUD · 08-19-2001, 11:28 AM

You don't have a vacuum leak. if you did, your vacuum would be below 9". how many miles are on your engine? Who installed the cam? What kind of timing chain set was used, and how many miles are on it? I think your valve timing may be retarded. When is the last time you did a compression check? What were the numbers wet and dry?

Take care
~Chris

------------------
Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

PrplMonster · 08-19-2001, 11:34 AM

dry-150/145 i haven't done a wet test yet. Everything has like 1k miles except the vavle train(lifters are new) and heads/intake.

Should i atleast get a flash from the check engine light..?

The timing is dead nuts on, I had no problems timing it. Great power, just a slight surge.

The spark plug as of 5 minutes ago is light with a hint of dark tan(at 62psi) it's definately darker on one side of the porcelin than the other, but no major buildup.

The car is an 89lx with an A9L. i know the computer is original. But i mean shouldn't it atleast give some sort of code..?

.dave

08-18-2001, 11:45 AM	#1
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	What fuel Press. Does everyone run..? I have a 347 with Performer heads/matching intake. Pretty radical cam 70mmTB/EGR Spacer, 24lb injectors with a Matching PRO-M meter(not quite sure what size it is) curious as to what you all run for fuel pressure, i have been searching for the optimal, but not much luck... any suggestions..? .dave

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08-18-2001, 01:30 PM	#2
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	How have you been searching for the optimal fuel pressure? I take it that you have at least installed 155lph fuel pump or better. If you're still running a stock fuel pump an adjustable FPR can't really help you. We've gone over this a lot of times, but it's probably good to bring up every now and then. Your air/fuel mixture is controlled by the computer via sensor input and injector pulse length. The MAF, ACT and ECT sensors help the computer measure how much air is going into your cylinders. The computer then does some calculating and determines how long to open the injectors based on how much air it measured going into the cylinders. The computer has preprogrammed air/fuel ratios that it must keep. Changing fuel pressure will only change the amount of fuel that is delivered for a given injector pulse length. The computer is programmed to expect that the fuel pressure is at a specific value like 39psi (I don't know what it really expects, but I'm sure it's around what the injectors are rated at). If you raise your pressure to, let's say, 50psi you'll increase the amount of fuel delivered during a given pulse length by about 13% compared to same pulse length at 39psi. You've esentially richened the air/fuel mixture. This is where adaptive strategy becomes a factor. The computer will monitor feedback from the oxygen sensors and determine that the amount of fuel it provided for the measured amount of air caused a richer condition than the computer is programmed to keep. The computer will then gradually decrease the pulse width compensation factor until the oxygen sensors report the correct air/fuel mixture that the computer is trying to maintain. The opposite is true if you try to lower fuel pressure and lean out the air/fuel mixture. What I described is closed loop control. When in open loop control the computer will just look up pulse lengths from a table and then apply the compensation factor determined during closed loop control against them to determine how much fuel to deliver. This is the control mode used during WOT. Your oxygen sensors are only checked to be sure that you aren't running dangerously lean in this mode. I'm pretty sure the reason the computer uses open loop mode during WOT is that the 12.8-13.1:1 air/fuel ratio that it is trying to maintain is outside of the range of the narrow band oxygen sensors that are used. So, the question now should be, "Why get an adjustable FPR if it doesn't really do anything to change the air/fuel mixture?". The answer is that sometimes the injectors used don't have enough capacity to supply the amount of fuel necessary to match the amount of air coming into the engine. You esentially have an engine capable of more power than your fuel system can handle. This would be evident on a dyno graph where your horsepower line just flattens out after peaking. If this happens, you can raise the fuel pressure to essentially increase the capacity of your injectors to the point that they can deliver enough fuel. Injectors also have a minimum amount of time that they can be open. The higher the injector rating and fuel pressure, the less control the computer will have at idle. If emissions is an issue, choose an injector that has a capacity nearest to your horsepower potential without going over. If you need 28# injectors, get 24# injectors and raise the fuel pressure to make 28#. If you need 23#, stick with the 19#ers. There is some potential for adjustment but not a lot and I'm not sure to what degree. The oxygen sensors put out a voltage ranging from .100 V to .900 V. Voltages less than .400 V being considered lean and voltages over .600 V being considered rich. Since you have that window of .200 V, it may be possible to stay on the lean side if you start off lean or stay on the rich side if you start off rich. Once you're in that window whether on the rich side or lean side you may be able to make small adjustments up or down in fuel pressure as long as you stay in that window. Maybe 1 or 2 psi at a time. Oh, to figure out what the capacity of your fuel injectors is at a given fuel pressure, take your fuel pressure and divide it by 39. Take the square root of that number and multiply it by your rated injector capacity. Well, there's what I know about fuel pressure in a nutshell. I have my pressure set at 60psi right now. I hope this helps with your adjustments. ------------------ 351W 89 Mustang GT Convertible

08-18-2001, 03:14 PM	#3
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	well jim, i understand all of that, so i guess i will rephrase the question. I am running 24#'ers at 42Psi should i be up pn fuel pressure..? I have heard from a few different people that with the bigger injector, the more press you need to make the injector work to it's potential.. is this true..? .dave

08-18-2001, 05:13 PM	#4
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	I suppose it's possible for a really big injector to be rated at a higher fuel pressure, but the 24# injectors we usually use are rated at 39psi. Anything near or over that will give you the 24# capacity or more. Running it on a dyno with a wide band oxygen sensor will probably be the only sure way to confirm that you are getting enough fuel at your peak power. ------------------ 351W 89 Mustang GT Convertible

08-18-2001, 05:27 PM	#5
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	Well, i have her at 60psi now.. It seems as if she is running better, the hesitation under 2k settled a little more. At 60psi i really don't have much black smoke, can't even notice it while she's running. At 65psi my eyes started to burn just being near it, i'd say it's rich.. any thoughts..? .dave

08-18-2001, 05:30 PM	#6
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	duh, sorry i aslo have a 190lph intank.. brand new.. .dave

08-18-2001, 07:25 PM	#7
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	The richness will go away in a while. Just go for a drive for about 100 miles. ------------------ 351W 89 Mustang GT Convertible

08-18-2001, 10:06 PM	#8
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	Well i went out an beat her up tonight!! The buck/hesitation only happens when you have I push ever so slightly on the gas pedal. At 60psi she runs like a raped ape, like right under 55 is when the buck/hesitation starts to come back(worse than it is at 60) At 65 i start to get a rough idle.. I am running 14*. I checked my MAF and everything looked clean(took it apart) Also, my car takes a minute to return back to idle. I have sprayed Carb cleaner on just about everything and no increase in RPM. any suggestions..? One thing i did notice, is however much i did change fuel press. this afternoon the plugs seemed to be OK.. weird. both o2 sensors are brand new. Are those Fuel/Air ratio gauges any good..? I heard they only work good for WOT. .dave

08-19-2001, 07:36 AM	#9
jimberg Registered Member Join Date: Oct 1998 Location: Rogers, MN Posts: 2,089	Make sure that your TPS is set well below 1 V. Set it to something like .95-.97. If you have it at .99 and you tap on the accelerator the computer will think you're at part throttle when you're still at closed throttle and open the EGR valve and thermactor causing your hesitation. The easiest way to confirm this theory is to disconnect the vacuum line from the EGR and plug it with a golf tee or something. If you still get the hesitation, it is still possible that your EGR valve is stuck open a little, though. Any changes you make to fuel pressure will require some time for the computer to adapt. How long did you wait between each fuel pressure you tried? The air/fuel gauges bounce back and forth since that's how the computer works. You're correct that they probably aren't that much help until you're at WOT. Even then it will only indicate whether you are rich or lean but not by how much. The only way to do that would be to get a gauge that works with a wide band o2 sensor. Knowing if you go lean, though, would be some good info. ------------------ 351W 89 Mustang GT Convertible

08-19-2001, 08:21 AM	#10
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	well jim, i put about 200 miles on the old gurl last night, ALL of my emissions are disconnected, i just have the EGR plugged in with no vacuum. I know the MAF is working i mean hell, she'd barely run without it connected.. lost... any help..? .dave

08-19-2001, 10:39 AM	#11
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	TPS is .965, I am not complaining about performance, i mean hell, i am going to need new tires soon.. > It's just annoying to not be able to drive slow at a low RPM

08-19-2001, 11:12 AM	#12
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	I just hooked up a vacuum gauge, it's reading about 10in/hg. I have checked for leaks about 3 times now, no air leak(s) any idea's..? .dave

08-19-2001, 11:23 AM	#13
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	Hey guys, For shits and giggles I used the jumper wire to see if i could get any codes. and the check engine light won't even come on(with or without the jumper) and the actual bulb is good. Could my computer just be in limp mode..? .dave

08-19-2001, 11:28 AM	#14
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	You don't have a vacuum leak. if you did, your vacuum would be below 9". how many miles are on your engine? Who installed the cam? What kind of timing chain set was used, and how many miles are on it? I think your valve timing may be retarded. When is the last time you did a compression check? What were the numbers wet and dry? Take care ~Chris ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

08-19-2001, 11:34 AM	#15
PrplMonster Registered Member Join Date: Mar 1999 Posts: 88	dry-150/145 i haven't done a wet test yet. Everything has like 1k miles except the vavle train(lifters are new) and heads/intake. Should i atleast get a flash from the check engine light..? The timing is dead nuts on, I had no problems timing it. Great power, just a slight surge. The spark plug as of 5 minutes ago is light with a hint of dark tan(at 62psi) it's definately darker on one side of the porcelin than the other, but no major buildup. The car is an 89lx with an A9L. i know the computer is original. But i mean shouldn't it atleast give some sort of code..? .dave