Fuel Pressure - Tuning Forum

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Fuel Pressure
Thursday, September 08, 2011 2:22 PM
I know I am gonna get the do it right the first time stuff and all that, but I am still researching things and would like to know about stuff just for my own education.

GM says stock fuel pressure on a L61 is between 50 and 60. Right now with the fuel pressure at 50, my IDC with the 36's on the GM flash is 80ish%. Theoretically if I raise the fuel pressure to the 60 PSI high limit with my AFPR, wont IDC go down, allowing me to to get more fuel from the 36's, but with a lower duty cycle?




Re: Fuel Pressure
Thursday, September 08, 2011 4:57 PM
Yes. Just running a few quick calculations you would drop down to somewhere around 70%.
Re: Fuel Pressure
Thursday, September 08, 2011 5:14 PM
I have always known the L61 fuel pressure to be 55-62. 55 idle, and 62 WOT.

I do believe you should drop some IDC by going up.

What is your fuel pressure with the vaccum hose off the AFPR? That should simulate WOT, but does not account for it under boost (if you have a 1:1 ratio)



FU Tuning



Re: Fuel Pressure
Thursday, September 08, 2011 5:27 PM
According to this thread it is 50-60. So thats what I went by.

I forget how i set it. I think I set it to 50 with the hose connected. But I know its at 50. Ill have to check for sure when I get home. my AFPR is 1:1. So I should remove the hose, and set it to 62 and call it a day?



Re: Fuel Pressure
Thursday, September 08, 2011 5:30 PM
I would.

Regardless of what pressure you set it to, do so with hose off. So 50psi with hose on will be higher with hose off.

When they say 50-60 or 55-62 the low is idle hose on, and the high is WOT (hose off).



FU Tuning



Re: Fuel Pressure
Saturday, September 10, 2011 9:22 AM
Kind of on the same topic, how many of you think our pressure drops pretty hard on the stock pump when we start using much bigger injectors, say 42-60 lb/hr. Sometimes the duty cycles just don't make any sense. Such as maxing out 60 lb/hr injectors well under the horsepower level you would think you'd get out of them. That, and i've seen lower duty cycles on very similar set ups with a racetronix pump for example.

Also, If your intake is pressurized, doesn't that pressure combat the pressure behind the injector when it opens? So if you don't have a boost referenced FMU, your ~60psi fuel pressure at WOT may be down to 45-50psi depending on the intake manifold psig.



Re: Fuel Pressure
Saturday, September 10, 2011 11:18 AM
oldskool wrote:

Kind of on the same topic, how many of you think our pressure drops pretty hard on the stock pump when we start using much bigger injectors, say 42-60 lb/hr. Sometimes the duty cycles just don't make any sense. Such as maxing out 60 lb/hr injectors well under the horsepower level you would think you'd get out of them. That, and i've seen lower duty cycles on very similar set ups with a racetronix pump for example.

Also, If your intake is pressurized, doesn't that pressure combat the pressure behind the injector when it opens? So if you don't have a boost referenced FMU, your ~60psi fuel pressure at WOT may be down to 45-50psi depending on the intake manifold psig.


I do think some pressure drop might happen on stock pump. I also wonder how much of a difference the wiring kit on a stock pump would make.

I would like to know what the pressure does on a stock FPR on boost.




FU Tuning



Re: Fuel Pressure
Saturday, September 10, 2011 11:18 AM
That's correct. If you don't have a boost referenced regulator the pressure across the injector gets lower once you get into positive pressure.
Re: Fuel Pressure
Saturday, September 10, 2011 3:28 PM
Wagonwes wrote:

That's correct. If you don't have a boost referenced regulator the pressure across the injector gets lower once you get into positive pressure.


I know that is what it is suppose to be, but have a question on stocker.

At idle full vaccum. Say 20inches. The diaphram is being sucked/pulled upwards? Correct? This would be the lowest psi of fuel. At WOT on a n/a car you would have 0psi/inches diaphram is sitting normal?? Correct? This should give the highest/high? Add boost you are now pushing down on the diaphram?? Correct?

I remember years back someone (I thought SpeedRacer) saying even a stock FPR will add fuel psi under boost.



FU Tuning



Re: Fuel Pressure
Saturday, September 10, 2011 4:00 PM
oldskool wrote:

Kind of on the same topic, how many of you think our pressure drops pretty hard on the stock pump when we start using much bigger injectors, say 42-60 lb/hr. Sometimes the duty cycles just don't make any sense. Such as maxing out 60 lb/hr injectors well under the horsepower level you would think you'd get out of them. That, and i've seen lower duty cycles on very similar set ups with a racetronix pump for example.


I was having that issue on the stock regulator for some reason, not on the aftermarket one. Though I still havent gotten my fuel pressure gauge to work right. I'm still working on it.

oldskoolAlso, If your intake is pressurized, doesn't that pressure combat the pressure behind the injector when it opens? So if you don't have a boost referenced FMU, your ~60psi fuel pressure at WOT may be down to 45-50psi depending on the intake manifold psig.[/quote wrote:



Thats the whole point of a fuel pressure regulator so that it maintains the same pressure differential no matter what the intake manifold pressure is.


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Sponsored by: Kronos Performance

WPI Class of '12 Mechanical Engineering
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Re: Fuel Pressure
Saturday, September 10, 2011 4:46 PM
Um, the whole point of a fuel pressure regulator is to set the fuel pressure. Unless the stock unit adds more pressure with postive manifold pressure, you will experience some drop with a boosted system. Hence the question if the stock unit adds pressure with boost.




Re: Fuel Pressure
Thursday, November 17, 2011 8:20 PM
Would it be safe to bump the fuel pressure up to 60 at idle (w/vacuum) to get 42 lb/hr ratings outta the injectors? this would allow the injectors to deliver more fuel per pulse and should help me lower my IDC. Am I thinking about this right? Is 60 PSI at the rail safe? That would take my WOT PSI to about 70ish + 1PSI for every pound of boost?



Re: Fuel Pressure
Thursday, November 17, 2011 10:04 PM
IRRC most injectors should not be run above 80 psi and the flow rate gained by increasing pressure is not linear. And you would bump up the fuel pressure with the car running but the vac line to the fpr not connected so it would run at less than 60 at idle.


1994 Saturn SL2 Home Coming Edition: backup car
2002 Chevy Cavalier LS Sport Coupe: In a Junk Yard
1995 Mazda Miata R-package Class=STR
Sponsored by: Kronos Performance

WPI Class of '12 Mechanical Engineering
WPI SAE Risk and Sustainability Management Officer
Re: Fuel Pressure
Thursday, November 17, 2011 10:09 PM
I suppose I will jump in and answer everyone's questions here haha. Yes, if you increase the fuel pressure at the injectors the IDC usually drops down under closed loop conditions. Now it's been several years since I have played with the GM reflash so I can't remember what the open loop power enrichment AFR was set at (Not that it really matters for this discussion). I remember it being rather rich but my point is that the IDC will typically go to those set parameters (PE AFR) to achieve that AFR with much less IDC change in reference to fuel pressure. Essentially in open loop (WOT) the IDC is typically less affected than it would be in closed loop. Did I lose you yet haha? The GM 36lb injectors are actually rather impressive when it comes to handling extreme fuel pressures (trust me I have put them to the test). They do not clip even well beyond their max duty cycle limits however, this is not to say that the spray pattern doesn't begin to deteriorate with extreme pressures and max duty cycle. You can get those GM 36s to perform pretty close to 42lb norms with fuel pressure manipulation.

In regard to the normal fuel pressures it will range from 50psi to 60psi (idle) depending on several factors but primarily the pump itself.

The stock GM FPR has a 10 psi sweep in vaccum from absolute to atmospheric however, have you guys ever noticed where the stock vacuum line runs on the factory intake system? This is where it gets somewhat comical and I haven't entirely determined if this was a design hiccup or what but the "vacuum" reference is pre-throttle body on the J-bodies. In other words there is NO vacuum reference to the stock FPR even though there is a dummy vacuum line connected. One might ask, well if that's the case perhaps the stock air box is just so restrictive that perhaps it still sees vacuum. Well this is not the case, the stock FPR NEVER sees vacuum on the factory fuel system. The FPR is simply in place to maintain a static pressure at the rail at all times at atmospheric fuel pressure conditions (approx. 60psi) regardless whether you are at WOT or idle. One might now say that can't be! There must be vacuum at that reference under load then, again the answer is NO there isn't. The FPR on a stock J-body fuel system literally does nothing but maintain a static pressure regardless of vacuum reference haha!

Now, for those interested in what the stock FPR does under atmospheric plus conditions (boost). The stock FPR is actually a 1:1 FPR that will increase fuel pressure 1 psi for every 1 psi of boost. I have tested the stock FPR up to 20 psi which actually ends up dead-heading the stock fuel pump. For those not familiar with the term "deadhead" in regard to pressure and fuel pumps, this means the pump reached it maximum pressure limits.

For those who were curious about bigger injectors and the affects on the fuel pump. This is a volume issue when it comes to bigger issues and stock pumps. The stock pump has a limit as to how much volume of fluid it can pump which in turn directly affects rail pressures as well.

Chris the rail and most every other fuel system component I have personally tested over and over to well over 120 psi (not that you would ever need to go that high nor do I condone it but it answers your question as to whether the system can handle the increase). Putting that system at the pressures you want is not an issue.

Hope this helps.



ASE Master Certified Automotive Technician

Re: Fuel Pressure
Friday, November 18, 2011 4:48 PM
Quote:

the "vacuum" reference is pre-throttle body on the J-bodies


Depending on how you look at it.

on a LD9 it is on the TB, which depending which port could be considered after the TB.

Usually on Eco's we connect it to the intake manifold.

With the M45 it is also on the charger (boost side).

Quote:

The stock FPR is actually a 1:1 FPR that will increase fuel pressure 1 psi for every 1 psi of boost


I'm glad you posted this. I have also known it to be this way, or read it was. Others say it is not.



FU Tuning



Re: Fuel Pressure
Friday, November 18, 2011 4:53 PM
CaliforniaDomestics wrote:

I suppose I will jump in and answer everyone's questions here haha. Yes, if you increase the fuel pressure at the injectors the IDC usually drops down under closed loop conditions. Now it's been several years since I have played with the GM reflash so I can't remember what the open loop power enrichment AFR was set at (Not that it really matters for this discussion). I remember it being rather rich but my point is that the IDC will typically go to those set parameters (PE AFR) to achieve that AFR with much less IDC change in reference to fuel pressure. Essentially in open loop (WOT) the IDC is typically less affected than it would be in closed loop. Did I lose you yet haha? The GM 36lb injectors are actually rather impressive when it comes to handling extreme fuel pressures (trust me I have put them to the test). They do not clip even well beyond their max duty cycle limits however, this is not to say that the spray pattern doesn't begin to deteriorate with extreme pressures and max duty cycle. You can get those GM 36s to perform pretty close to 42lb norms with fuel pressure manipulation.

In regard to the normal fuel pressures it will range from 50psi to 60psi (idle) depending on several factors but primarily the pump itself.

The stock GM FPR has a 10 psi sweep in vaccum from absolute to atmospheric however, have you guys ever noticed where the stock vacuum line runs on the factory intake system? This is where it gets somewhat comical and I haven't entirely determined if this was a design hiccup or what but the "vacuum" reference is pre-throttle body on the J-bodies. In other words there is NO vacuum reference to the stock FPR even though there is a dummy vacuum line connected. One might ask, well if that's the case perhaps the stock air box is just so restrictive that perhaps it still sees vacuum. Well this is not the case, the stock FPR NEVER sees vacuum on the factory fuel system. The FPR is simply in place to maintain a static pressure at the rail at all times at atmospheric fuel pressure conditions (approx. 60psi) regardless whether you are at WOT or idle. One might now say that can't be! There must be vacuum at that reference under load then, again the answer is NO there isn't. The FPR on a stock J-body fuel system literally does nothing but maintain a static pressure regardless of vacuum reference haha!

Now, for those interested in what the stock FPR does under atmospheric plus conditions (boost). The stock FPR is actually a 1:1 FPR that will increase fuel pressure 1 psi for every 1 psi of boost. I have tested the stock FPR up to 20 psi which actually ends up dead-heading the stock fuel pump. For those not familiar with the term "deadhead" in regard to pressure and fuel pumps, this means the pump reached it maximum pressure limits.

For those who were curious about bigger injectors and the affects on the fuel pump. This is a volume issue when it comes to bigger issues and stock pumps. The stock pump has a limit as to how much volume of fluid it can pump which in turn directly affects rail pressures as well.

Chris the rail and most every other fuel system component I have personally tested over and over to well over 120 psi (not that you would ever need to go that high nor do I condone it but it answers your question as to whether the system can handle the increase). Putting that system at the pressures you want is not an issue.

Hope this helps.



Ive said it time and time again. This guy knows his stuff! Thanks for the advice Alan. I think me and you might be the only two people that will/have taken the Gm flash and made it work how we wanted it. I will be adjusting things this weekend, and hitting a dyno soon.



Re: Fuel Pressure
Friday, November 18, 2011 5:24 PM
Addicted to meth wrote:

Quote:

the "vacuum" reference is pre-throttle body on the J-bodies


Depending on how you look at it.

on a LD9 it is on the TB, which depending which port could be considered after the TB.

Usually on Eco's we connect it to the intake manifold.

With the M45 it is also on the charger (boost side).

Quote:

The stock FPR is actually a 1:1 FPR that will increase fuel pressure 1 psi for every 1 psi of boost


I'm glad you posted this. I have also known it to be this way, or read it was. Others say it is not.


I was only speaking on behalf of the pre 2006 L61 since this was in response to the original posters setup. You are correct though, GM does vacuum reference that same FPR on a ton of other vehicles including even J-bodies with other engine variants. Hence why I have not fully concluded whether this may have actually been an engineering boo boo during production on the pre 06 J L61 engine variant. Regardless in stock form it still serves its purpose to provide a set fuel pressure via restriction in the return line. The LN2 is also vacuum referenced but FPRs can be used in a bunch of different ways and since GM is notoriously a parts bin production mentality this may have just been one more way to save a penny by using the same exact FPR in a bunch of different ways.



ASE Master Certified Automotive Technician

Re: Fuel Pressure
Wednesday, January 11, 2012 12:40 PM
Sorry if i missunderstanding but so wut i gathered ill be fine on the stock fpr for lets say 8psi


*****BLAME IT ON THE ALCOHOL*****
Re: Fuel Pressure
Wednesday, January 11, 2012 12:42 PM
Just trying to see if i need one right now


*****BLAME IT ON THE ALCOHOL*****
Re: Fuel Pressure
Wednesday, January 11, 2012 1:23 PM
john wrote:

Sorry if i missunderstanding but so wut i gathered ill be fine on the stock fpr for lets say 8psi


need more info on what your build consists of



Re: Fuel Pressure
Wednesday, January 11, 2012 1:30 PM
Sorry havent updated profile but vulcan turbo manifold ,garrett 50 trim , 255 fuel pump, ss/sc injectors will only be pushing 8 psi max till my build


*****BLAME IT ON THE ALCOHOL*****

Re: Fuel Pressure
Wednesday, January 11, 2012 1:32 PM
And its a 02 ecotec stock


*****BLAME IT ON THE ALCOHOL*****
Re: Fuel Pressure
Friday, January 13, 2012 3:45 PM
Proper tuning, yes you will be fine.



Re: Fuel Pressure
Saturday, January 14, 2012 12:17 PM
Thanx.


*****BLAME IT ON THE ALCOHOL*****
Re: Fuel Pressure
Tuesday, January 17, 2012 12:59 AM
Addicted to meth wrote:

Quote:

the "vacuum" reference is pre-throttle body on the J-bodies


Depending on how you look at it.

on a LD9 it is on the TB, which depending which port could be considered after the TB.

Usually on Eco's we connect it to the intake manifold.

With the M45 it is also on the charger (boost side).

Quote:

The stock FPR is actually a 1:1 FPR that will increase fuel pressure 1 psi for every 1 psi of boost


I'm glad you posted this. I have also known it to be this way, or read it was. Others say it is not.


If you want I can make a video showing that infact it is boost referenced I have electronic auto meter pressure guage and it does add 1:1



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