Tech tip
Safety Car
Joined: Sep 2001
Posts: 4,150
Likes: 552
From: Glenbeulah Wi
What Agent 86 said.
It's bogus. When you got to WOT (and have no vacuum in the intake) you end up w/the same fuel pressure anyway. You DON'T get better gas mileage. The display may show a better reading but that's b/c the ECM doesn't know about the mechanical change that's been made. If you did your gas mileage using long devision after a fill up, I'm sure that you'd find that this "mod" decreased your actual fuel milage.
Waste of time.
It's bogus. When you got to WOT (and have no vacuum in the intake) you end up w/the same fuel pressure anyway. You DON'T get better gas mileage. The display may show a better reading but that's b/c the ECM doesn't know about the mechanical change that's been made. If you did your gas mileage using long devision after a fill up, I'm sure that you'd find that this "mod" decreased your actual fuel milage.
Waste of time.
The computer does not know that the fuel pressure has been changed so its caculation to determine fuel used wil be off by 15% with a 10 LB pressure change. The 02 sensor will detect a rich condition and shorten the fuel pulse but the higher pressure will add the fuel back in. So the end result is the same amount of fuel will be used but the computer will show better mileage.Fuel flow through the injectors is changes by the square root of the percent pressure change so going from 30 to 40 PSI at cruise will change the actual flow rate at the same pulse width by +15%.
Last edited by bjankuski; Jan 21, 2010 at 07:26 AM.
Le Mans Master
Joined: Feb 2000
Posts: 9,715
Likes: 27
From: Kempner Texas
I heard about this years ago, 20? The theory was that the increase in pressure would change the injector spray by better atomizing the fuel. I have no idea if that's BS or not. I doubt the story's claim are accurate. I can't imagine you'd see that much difference unless the higher pressure was compensating for some other issue.
The ECM can adjust the injector on-time (within its capability) to adjust the A/F ratio to try to maintain 14.7:1 as it always does, but the improved atomization remains. Remember, the mod only effects part throttle operation.
As vacuum goes down (as when at WOT and/or removal of the vacuum hose) fuel pressure increases. To really know by how much, you'll need to install a fuel pressure gauge.
I played around with fuel pressure settings several years ago when I had a 415 in my 86. My experience when increasing the fuel pressure is the engine ran cooler and throttle response was crisper. Basically what I do is let the engine tell me what it likes and what it doesn't.
Best advice is to give it a try under controlled conditions - same ambient temp, same roadway, same speed, etc. If it works, it works. If it doesn't, . . . . Nothing ventured nothing gained.
Jake
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
Kidding aside - I think the newer injectors (FIC rules!) that are available out there these days that do the "atomization" thing at lower pressures a whole lot better than the old stock injectors did back in the day. I do think you might have a slight benefit only at WOT with having the higher rail pressures by removing the vacuum hose from the FPR. Like Jake said...if you don't try it then "Nothing ventured nothing gained".
Drifting
Joined: Sep 2002
Posts: 1,642
Likes: 0
From: Suwanee,Ga
I've owned a copy of the booklet/magazine...it also said the "air foil" was a worth while mod too....they had charts to back it up so it must be true
Kidding aside - I think the newer injectors (FIC rules!) that are available out there these days that do the "atomization" thing at lower pressures a whole lot better than the old stock injectors did back in the day. I do think you might have a slight benefit only at WOT with having the higher rail pressures by removing the vacuum hose from the FPR. Like Jake said...if you don't try it then "Nothing ventured nothing gained".
Kidding aside - I think the newer injectors (FIC rules!) that are available out there these days that do the "atomization" thing at lower pressures a whole lot better than the old stock injectors did back in the day. I do think you might have a slight benefit only at WOT with having the higher rail pressures by removing the vacuum hose from the FPR. Like Jake said...if you don't try it then "Nothing ventured nothing gained".
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
Kidding aside - I think the newer injectors (FIC rules!) that are available out there these days that do the "atomization" thing at lower pressures a whole lot better than the old stock injectors did back in the day. I do think you might have a slight benefit only at WOT with having the higher rail pressures by removing the vacuum hose from the FPR.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
Ok, here's what nobody touched on so far...............
normally, the fuel pressure at cruise, idle, and decel is 38-40 PSI.....that is because you have manifold vacuum. GM figured that for good fuel economy, lower fuel pressure would pump out less fuel because it wasn't needed.
on acceleration, increased fuel pressure would help acceleration if it was bumped up a little.
Soooo when you accelerate, the throttle plates are opened, and the vacuum goes to almost nothing.
That in turn, reduces the vacuum pull on the pressure regulator diaphragm, allowing the spring to push all the way, resulting in a 4-5 PSI increase in fuel pressure. The fuel pressure regulator is still regulating, but at a rate of 5 PSI higher.
The increased fuel pressure allows for increased fuel atomization, which according to FIC, increases horsepower and mileage because the increased fuel vaporous state will tend to mix with the incoming air a lot better, and give more even burning which increases more complete combustion, which yields better utilization from the same drop of fuel.
So, just buy an adjustable fuel pressure regulator, and increase yours a few pounds, and reap the benefits.
normally, the fuel pressure at cruise, idle, and decel is 38-40 PSI.....that is because you have manifold vacuum. GM figured that for good fuel economy, lower fuel pressure would pump out less fuel because it wasn't needed.
on acceleration, increased fuel pressure would help acceleration if it was bumped up a little.
Soooo when you accelerate, the throttle plates are opened, and the vacuum goes to almost nothing.
That in turn, reduces the vacuum pull on the pressure regulator diaphragm, allowing the spring to push all the way, resulting in a 4-5 PSI increase in fuel pressure. The fuel pressure regulator is still regulating, but at a rate of 5 PSI higher.
The increased fuel pressure allows for increased fuel atomization, which according to FIC, increases horsepower and mileage because the increased fuel vaporous state will tend to mix with the incoming air a lot better, and give more even burning which increases more complete combustion, which yields better utilization from the same drop of fuel.
So, just buy an adjustable fuel pressure regulator, and increase yours a few pounds, and reap the benefits.
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Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
Ok, here's what nobody touched on so far...............
normally, the fuel pressure at cruise, idle, and decel is 38-40 PSI.....that is because you have manifold vacuum. GM figured that for good fuel economy, lower fuel pressure would pump out less fuel because it wasn't needed.
on acceleration, increased fuel pressure would help acceleration if it was bumped up a little.
Soooo when you accelerate, the throttle plates are opened, and the vacuum goes to almost nothing.
That in turn, reduces the vacuum pull on the pressure regulator diaphragm, allowing the spring to push all the way, resulting in a 4-5 PSI increase in fuel pressure. The fuel pressure regulator is still regulating, but at a rate of 5 PSI higher.
The increased fuel pressure allows for increased fuel atomization, which according to FIC, increases horsepower and mileage because the increased fuel vaporous state will tend to mix with the incoming air a lot better, and give more even burning which increases more complete combustion, which yields better utilization from the same drop of fuel.
So, just buy an adjustable fuel pressure regulator, and increase yours a few pounds, and reap the benefits.
normally, the fuel pressure at cruise, idle, and decel is 38-40 PSI.....that is because you have manifold vacuum. GM figured that for good fuel economy, lower fuel pressure would pump out less fuel because it wasn't needed.
on acceleration, increased fuel pressure would help acceleration if it was bumped up a little.
Soooo when you accelerate, the throttle plates are opened, and the vacuum goes to almost nothing.
That in turn, reduces the vacuum pull on the pressure regulator diaphragm, allowing the spring to push all the way, resulting in a 4-5 PSI increase in fuel pressure. The fuel pressure regulator is still regulating, but at a rate of 5 PSI higher.
The increased fuel pressure allows for increased fuel atomization, which according to FIC, increases horsepower and mileage because the increased fuel vaporous state will tend to mix with the incoming air a lot better, and give more even burning which increases more complete combustion, which yields better utilization from the same drop of fuel.
So, just buy an adjustable fuel pressure regulator, and increase yours a few pounds, and reap the benefits.
100% with the AFPR comment.I think your comments were where Tom400CFI was headed he just left all the details out for whatever reason. Not sure what exactly the vacuum level is for spun up WOT at a given high RPM but I'm sure there is some level of vacuum present...might affect the FPR diaphram (fuel rail pressure) might not....then there's the better low RPM WOT response thing.....I think I'll let sleeping dogs lie.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
100% with the AFPR comment.I think your comments were where Tom400CFI was headed he just left all the details out for whatever reason. Not sure what exactly the vacuum level is for spun up WOT at a given high RPM but I'm sure there is some level of vacuum present...might affect the FPR diaphram (fuel rail pressure) might not
As for increased FP helping part throttle running, yes, in theory more FP = better atomization...with corresponding TUNING. Simply unplugging the vacuum line to the FPR and calling that a power enhancing "mod" as is the case in this thread, is a joke.
Last edited by Tom400CFI; Jan 25, 2010 at 03:17 PM.