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mywhite89
08-06-2008, 02:01 PM
My car has 60lb injectors, 255lph fuel pump and a kirban adjustable fuel pressure regulator. At idle fuel pressure is at around 32 psi. I took it out for a 3rd gear pull up to redline on the highway and pressure climbs without falling off up to around 51 psi.

Anybody have any thoughts or concerns regarding my fuel pressure? Maybe someone has insight on what their car is set at? I was thinking stock spec pressure at idle is typically 35-45psi but peaking at 51psi seems alright. Doesn't seem to be hurting anything, but I havent checked my plugs for 1,000 miles and sometimes coming down to idle and starting it first thing in the morning causes the car to stumble pretty bad. Car holds fuel pressure after shut-off for a long time, so no leaks.

Chris

P.S. car is tuned at this fuel pressure

S_Mazza
08-06-2008, 02:31 PM
I thought stock fuel pressure was 39 psi at idle. What's your max boost? (The rise in boost pressure should equal the rise in fuel pressure.)

mywhite89
08-06-2008, 03:15 PM
I thought stock fuel pressure was 39 psi at idle. What's your max boost? (The rise in boost pressure should equal the rise in fuel pressure.)

Max boost is 18psi.

David Neibert
08-06-2008, 03:37 PM
Stock pressure at idle is 38-40 psi with vacuum/boost reference line removed. With line connected it normal to see 32-35 psi at idle and the pressure will vary based on how much vacuum your pulling and what type regulator you use. You are supposed to set it to the pressure you want when installing the regulator. My base/static pressure is set a 43.5 psi.

Easy test....drive the car putting only enough load on the motor to show "O" on the vacuum boost gauge..... this should result in same fuel pressure as if hose was disconnected (normally 38-40 psi). After seeing what the number is, then step on the gas...pressure should climb 1 psi for every 1 psi increase in manifold pressure. If you have 40 psi at zero vac/boost, you should have 50 psi at 10 lbs of boost.

If your using a full sweep electric gauge or a manual gauge the pressure changes will be immediate, but if you using a standard $75 electric gauge it reacts to slow to be of any use in testing.

David

fturner
08-06-2008, 03:38 PM
Stock static pressure is 39psi... that is before vacuum hits the regulator. 32psi is about normal for idle with vacuum plugged into the regulator.

For pressure above 39psi, should go up 1 psi per pound of boost.

Frit

S_Mazza
08-06-2008, 03:59 PM
...........

David Neibert
08-06-2008, 04:15 PM
If the gauge readings are accurate, the pressure sounds too low. If base pressure is 39 he should be showing 57.5 psi fuel pressure with 18.5 pounds of boost.

mywhite89
08-06-2008, 04:23 PM
Ah thanks for the information. I do remember setting pressure initally. I set pressure with the engine off by jumping the pump relay, which is the same thing as 0 on the boost gauge(zero vaccum, zero boost). My car idles at 9 inches of vaccum.

Chris

S_Mazza
08-06-2008, 05:40 PM
David, that is a good catch. The increase for boost should be measured from "base" fuel pressure, not idle. So it does sound low.

rzimmerl
08-06-2008, 08:53 PM
1psi of boost = 1 psi of fuel pressure

Ira R.
08-06-2008, 10:32 PM
Two questions. One, if you have an adjustable fuel regulator isn't that the point to be able to adjust it for more pressure? And if so how much?

And secondly, isn't the fuel pressure something that gets adjusted or accounted for when the engine is tuned? Also if larger injectors are installed how do you account for the extra volume needed; just by increasing the flow of the pump and keeping the fuel psi at the recommended 39-40 psi?

And since I read where the stock regulator providing 40psi should be enough for up to 500 horsepower is that accounted for anywhere in this? Oh, and is this a real number, or just figures on an engineer's calculator somewhere??

Ira

atlstang
08-06-2008, 11:40 PM
im not an expert on sc's so correct me if im wrong but i do a lot with the next gen eec's.. the amount of fuel that is injected depends on injector size and fuel pressure.. you can get the same amount of fuel with high pressure and smaller injectors or vice versa.. so say you gain more hp and its maxing out the injectors at that pressure.. adjusting for higher pressures is one way of adding more fuel for more hp.. The reason the fuel pressure is basically variable is usually due to idle problems with large injectors since the injectors have a slight lag time to open and close, for small amounts of fuel it cant inject precisely simply because it opens and closes to fast, so you lower the pressure at idle, the bigger injectors stay open longer to inject the correct amount of fuel.

the eec for sc im familiar with, but that gen varies a lot with the stage of tech on the car. but it should be able to adjust for fuel mix with different fuel pressures just fine.. so higher pressure would theoretically increase max hp within a safe duty cycle on the injectors without changing them out.

Duffy Floyd
08-07-2008, 12:03 AM
You are correct....amount of fuel is related to injector size and fuel pressure.....BUT.....it is differential fuel pressure across the injector which is of concern. Also raising fuel pressure for a given size injector does not continue to raise the amount of fuel that will pass through the injector infinitely nor is the relationship linear. The problem with forced induction engines is the pressure at the injector tip continues to vary. When in vac. you have a greater differential pressure...when in boost less differential pressure across the injector (assuming fuel pressure is fixed). So the variable fuel pressure regulator in an SC is set to provide a constant differential pressure regardless of boost or vac. in the intake manifold.

While the adaptive strategy of the EEC will compensate for some variation its "range" is not enough to compensate for a big change in fuel pressure. Also jacking around with fuel pressure changes the amount of fuel across the entire spectrum of possible intake manifold conditions. Not something you want to do without tuneability of the EEC. Leads to driveability issues galore.

atlstang
08-07-2008, 12:41 AM
ah yeah that makes more since.. especially for supercharged applications. but true nothing is really ever linear with flow.. but i mentioned the idle adjustment problem since im curious if the idle problems in the original post may be attributed being that he has 60lb injectors? seeing how the boost is somewhat bipassed at idle. wondering since im working on adding an sc to a 3.8 for a project car i have in the works with a bud.

mywhite89
08-07-2008, 07:31 AM
but i mentioned the idle adjustment problem since im curious if the idle problems in the original post may be attributed being that he has 60lb injectors? seeing how the boost is somewhat bipassed at idle. wondering since im working on adding an sc to a 3.8 for a project car i have in the works with a bud.

I would say my idle down and intial start-up problem has to do with with the injectors, the cam, parameters for tuning that havent been discovered or utilized, etc. Unless I have a problem that I havent found, which is possible. It's definitely not something that gives me problems, but if someday there is an answer for it then I would take care of it. With a car thats been modified so much and uses old technology, you can't expect it to run like a brand new car. I keep getting closer though.

Chris

David Neibert
08-07-2008, 08:53 AM
Chris,

If the car was tuned with a wide band o2 to run at a safe A/F ratio at whatever pressure you have, I wouldn't worry about what fuel pressure your seeing. What kind of fuel pressure gauge are you using ?

David

Ira R.
08-07-2008, 08:58 AM
Chris,

If the car was tuned with a wide band o2 to run at a safe A/F ratio at whatever pressure you have, I wouldn't worry about what fuel pressure your seeing. What kind of fuel pressure gauge are you using ?

David

So essentially what everyone is saying all comes down to this - our measuring stick and level of safety is tied to the A/F ratio??

Ira

mywhite89
08-07-2008, 12:46 PM
Chris,

If the car was tuned with a wide band o2 to run at a safe A/F ratio at whatever pressure you have, I wouldn't worry about what fuel pressure your seeing. What kind of fuel pressure gauge are you using ?

David

I was using a manual pressure gauge. I'm not worried about my tune. I was just checking things out, looks like they all check out fine.