Legacy turbo fuel system questions...pressure, injector duty

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czo79
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Legacy turbo fuel system questions...pressure, injector duty

Post by czo79 »

Hola!
I got some questions about legacy's fuel system. What kind of fuel pressure does it have stock? I actually have a fuel pressure gauge under my hood, but I only know what it reads at idle, I don't know if it produces more under boost, does anyone know what it maxes out at? Also, at stock fuel pressures, with stock injectors and such, at what kinds of HP do you start to hit 80-90% injector duty cycles? It won't really apply to my car, cause I got a 2.0 with 440cc's, but I'm just trying to get some numbers to give me an idea, a place to start. I also wonder, to people with walbro fuel pumps, does having just the pump increase fuel pressure? Does having just increased pressures reduce injector duty cycles? Does one need a fuel pressure riser to get higher pressures with a fuel pump, or will just the pump do it? Basically, I'm trying to figure out what I will need to do to my fuel system as I do everything else in pursuit of, what else? more power!!! Oh yeah, one more...does anyone know if it would be easy for me to drop one of the new wrx performance aftermarket fuel rails and sti injectors on my 94 wrx ej20?
Much thanks all!
Micum
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Post by -K- »

It better raise fuel pressure under boost. If the fuel pressure starts at say 35psi under boost it will go up, with 1psi boost you get 36psi fuel pressure, 10psi boost 45psi fuel pressure, and so on.. The FPR (fuel pressure regulator) works by letting some of the fuel return to the tank, bleeding the pressure down. So the more fuel you use in the engine the less gets bled back to the tank. Now at some point you will be using all of the fuel the pump can put out, no gas gets sent back. After that the pump can't put out more, the pressure drops and the engine leans out. Now you get the big pump and when you're just driving around, a lot more fuel gets bled off to keep the pressure normal. But when you get on it all that extra fuel can get to the engine.
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Post by Brat4by4 »

This doesn't overwork the FPR like say what happens if you turn your boost too low and the turbo uses the wastegate too much which can't flow enough? Is there any possible issue of wearing your FPR out?
1993 WMP BC6 5MT EJ22T 9psi 3.9:1 213k 205/55R16

62.6 m/s @ 0.66 bar. Gotta love boost. :)
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Post by czo79 »

Thanks K,
That stuff is good to know... So if I understand, a upgraded fuel pump sort of will eliminate the need for a fuel pressure riser or something like that? Unless at the stock fuel pressure levels, my 440cc injector are starting to get to max duty cycles? Then I would have to either get bigger injectors, or increase fuel pressure so my injectors can put out more fuel at the same duty cycle? I'm new to trying to understand the fuel system, so I might have this wrong.
Thanks
Micum
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Post by -K- »

No, the pump will not make up for maxing out the injectors. But it will get you enough fuel to do other things when needed. And not worry about it now. I'm worried to keep up the boost in my car too long, 160k on the pump and who knows if it is still putting out the same as it was new. mile hi said they maxed the pump out befor the injectors so you have bigger ones it's going to max the pump out sooner. What all do you plan on doing to the engine?
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Austin
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Post by Austin »

Your fuel pressure regulator sets a static fuel pressure of 43.5psi. 43.5psi is 3 bar.

Static means referenced to atmosphere. The FPR is referenced to manifold pressure/vacuum. At idle, when there is 15-18-ish inches of vacuum in the intake manifold, fuel pressure is lowered by that much.

The stock FPR is a 1:1 rate.

29.92 inches = 14.7psi = 1 atm. 1 bar = 14.5psi, so it's almost a 1:1 conversion.

At idle, your fuel pressure should be 35-38psi, which is 3bar(43.5psi) + manifold pressure.

Under boost, fuel pressure is more than 3 bar. Boost pressure + 3 bar = fuel pressure.

If you're at 14.5psi boost (which is 1 bar boost), then your fuel pressure should be 4 bar, or 58psi.

Injectors are rated at 3 bar. The stock injectors flow their rated amount at 3 bar fuel pressure.

With a 1:1 regulator, the differential pressure across the injector (fuel rail pressure - manifold pressure) remains at a constant 3 bar. Because of that, the only variable to determine fuel delivery is injector on time.

Having a larger fuel pump will do nothing to increase fuel pressure, unless you are physically exceeding the bypass flow capabilities of your fuel pressure regulator. You don't want increased fuel pressure if your ecu (or whatever you're using to control fuel delivery) is expecting normal 3bar static fuel pressure.

Any system is only as strong as it's weakest link. If you're running out of fuel, as in fuel pressure is dropping towards redline where there is more total fuel flow demand, then you need a larger fuel pump. If you're running out of injector, as in injector duty cycle in nearing 90/100%, then you need larger injectors.

Is that good enough?
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czo79
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Post by czo79 »

Ok, so I have a VDO mini press. gauge hooked up to a T between the filter and fuel rail. It bounces bvery rapidly between like 25-30 psi, and when I pull on the throttle to rev it, it doesn't go up, although maybe it wasn't high enough for boost to come on. Does this sound like a problem to anyone? Any ideas?
thanks
Micum
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Post by Austin »

Pull the vacuum line off your fuel pressure regulator. If your fuel pressure gauge is showing less than 43psi, then either your gauge is off or you need a new FPR.
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Post by Legacy777 »

Austin wrote:Pull the vacuum line off your fuel pressure regulator. If your fuel pressure gauge is showing less than 43psi, then either your gauge is off or you need a new FPR.
If at idle you pull the vacuum line off the FPR, you should see pressure increase. If you pull the line off and it doesn't move, depending on the starting pressure it could be fpr, or could be pump.
Josh

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Post by Aaron's ej22t »

well, if you had a bad fuel pump, you wouldn't have any fuel pressure.
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trac
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Re:

Post by trac »

czo79 wrote:Ok, so I have a VDO mini press. gauge hooked up to a T between the filter and fuel rail. It bounces bvery rapidly between like 25-30 psi, and when I pull on the throttle to rev it, it doesn't go up, although maybe it wasn't high enough for boost to come on. Does this sound like a problem to anyone? Any ideas?
thanks
Micum
You won't get any boost revving the engine without any load on it.

Also, according to my FSM (for EUDM 2.0), at idle the fuel pressure should be 26-30 psi (1,8 - 2,1 bar). So I think you're good.

In D-check you should see 34-38 psi when the pump is powered (2,4 - 2,7 bar).
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Re: Re:

Post by mike-tracy »

trac wrote:
czo79 wrote:Ok, so I have a VDO mini press. gauge hooked up to a T between the filter and fuel rail. It bounces bvery rapidly between like 25-30 psi, and when I pull on the throttle to rev it, it doesn't go up, although maybe it wasn't high enough for boost to come on. Does this sound like a problem to anyone? Any ideas?
thanks
Micum
You won't get any boost revving the engine without any load on it.

Also, according to my FSM (for EUDM 2.0), at idle the fuel pressure should be 26-30 psi (1,8 - 2,1 bar). So I think you're good.

In D-check you should see 34-38 psi when the pump is powered (2,4 - 2,7 bar).

Check the dates of the thread ;)
1992 Legacy SS 5mt, build in progress
Josh Colombo wrote: Mon Jan 14, 2002 10:23 am Wait....I'm confused now.
cj91legss
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Re: Legacy turbo fuel system questions...pressure, injector

Post by cj91legss »

We've all done it at some point in time Mike :-D
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Re: Legacy turbo fuel system questions...pressure, injector

Post by mike-tracy »

True, but, this has got to be a record, over a decade later ;)
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Josh Colombo wrote: Mon Jan 14, 2002 10:23 am Wait....I'm confused now.
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Re: Legacy turbo fuel system questions...pressure, injector duty

Post by Rex Calibvr »

Never too late to post here.
This helped me out
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