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I just found out an interesting little fact from some factory service manuals...

GD/GG WRX stock fuel pumps are specified to flow at least 130 liters per hour at 43.5 psi with a 12 volt power supply.

BC/BF Legacy Turbo stock fuel pumps are specified to flow at least 150 liters per hour at 43.4 psi with a 12 volt power supply.

How about 2.2NA fuel pumps?

Yep,
Even detuned our cars are from the era when Subaru overengineered the crap out of everything. Initially, I have no plans of changing my fuel pump, just the injectors. I would not be at all surprised if our fuel pump was identical to JDM STi's of that generation...

BC/BF non-turbo fuel pumps are rated to flow at least 80 liters per hour at 36.3 psi with a 12 volt power supply. Significantly less.

Matt Monson wrote:Yep,
Even detuned our cars are from the era when Subaru overengineered the crap out of everything. Initially, I have no plans of changing my fuel pump, just the injectors. I would not be at all surprised if our fuel pump was identical to JDM STi's of that generation...

Depending on the extent of your modifications I think you'd still want to upgrade to a larger pump. I remember reading a post from someone on this board (I can't remember his name but he was from CO also) who found that the limiting factor for fuel in the turbo legacy was the fuel pump rather than the injectors.

I'm gonna have to search for that post again.

Yeah, keep in mind that a 9-12 year old pump won't be running 100%. The brushes will wear, the seals will leak a little, the internal friction may be increase, et cetera. Even switching to a new stock Legacy Turbo pump would be an improvement, but the cost of Walbro 255-liter-per-hour pumps is similar.

Other thing to know is the fuel pump on the WRX is run by a fuel controller, which from what I can tell varies voltage.

From what I can tell the legacies are just straight wired to battery juice.

If the WRX system works the way I think it sends reduced power to the fuel pump under normal driving and full 12 volts when under heavy throttle. My Supra did this by using a set of large resistors.

Yeah, you guys are right. The WRX's fuel pump is powered through the fuel pump controller, which has two signal lines coming from the ECU. Presumably it either contains a monster resistor or performs pulse width modulation.

The only provided info about the pump's performance is its flow at 12 volts and 43.5 psi, though.

A little more: the EJ257 STi fuel pumps are specified to flow at least 145 liters per hour at 43.5 psi with a 12 volt power supply.

I've got a semi related question that's been sort of bugging me. On AWD cars there's a second pump type dealer called a fuel sender in the driverside part of the tank. Is this an electrical pump or a mechanical pump, like does it flow at the same rate or is it controlled through vacuum?

There are two separate things going on here:

For one, there are fuel level senders on both sides. The signals are averaged or something.

The other thing is that there's a passive pump called a jet pump that moves fuel from the passenger side to the driver side. Fuel velocity in the return line is used to pull fuel from the passenger side. The jet pump unit is attached to the return line on the driver side and has a line coming from the passenger side which ends in a sock just like the fuel pump.

vrg3 wrote:The other thing is that there's a passive pump called a jet pump that moves fuel from the passenger side to the driver side. Fuel velocity in the return line is used to pull fuel from the passenger side. The jet pump unit is attached to the return line on the driver side and has a line coming from the passenger side which ends in a sock just like the fuel pump.

This is what I'm talking about, so it's similar to a vacuum type scenario?

Yeah, it's based on the venturi effect.

Like this
http://www.tenecor.com/images3/amturbo5000.jpg

Imagine fuel traveling past the outside of the upper cone pulling fuel from the inside of the upper cone as it passes.

what happens when u slow down the rate of gas going to the return line? does it give the motor more gas or less gas.

Well, you increase the pressure so the same duty cycle releases more fuel. The point of a rrfpr

ciper what is a rrfpr? and is this good or bad to do cuz when i do it i get flames out the exhuste, i pluged in a gauge and its only up 10psi more

rising rate fuel pressure regulator

thanks thawa

It increases fuel pressure along with the increase in boost pressure.

Basically it keeps the fuel pressure equal to the boost increase.

If the fuel pump was set to always output 43 psi, then at idle you would get 43 psi of fuel and at 15psi of boost you would get 27 psi of fuel!

Uh... kind of...

The fuel pump isn't "set" to any particular pressure. It just flows fuel through the lines which go up to the engine bay and then back to the tank. The engine relies on a restriction made in the lines to create pressure. A simple way to think about it is like a thumb over a garden hose; without the thumb, there's virtually no gauge pressure.

The stock fuel pressure regulator creates this restriction. It's basically a valve controlled by a spring-loaded diaphragm. On one side of the diaphragm is the fuel in the rail. On the other side is manifold air plus a spring. So fuel pressure pushes against both the spring and manifold pressure. The spring alone would require 36.3 psi of fuel pressure to defeat.

So, as long as fuel pressure is below manifold pressure + 36.3 psi, the valve is closed, and pressure builds. As soon as pressure reaches manifold pressure + 36.3 psi, the valve opens and pressure starts to fall. It reaches an equilibrium where fuel pressure is always 36.3 psi greater than manifold pressure. That way you always inject the same amount of fuel for any injector pulse width, irrespective of manifold pressure.

A rising-rate fuel pressure regulator is a neat gizmo that works the same way except there isn't a 1:1 ratio between fuel pressure and manifold pressure; it raises fuel pressure by some multiple of manifold pressure. For example, a 1 psi increase in manifold pressure would translate to 2, 4, or 8 psi increase in fuel pressure. By putting an aftermarket RRFPR after your stock FPR, you can achieve richer mixtures on boost, because you inject more fuel for the same injector pulse width.

What he said. I was trying to explain it in simple terms

If you read the threads about the LINK ECU problems with some WRX you will find out that the WRX does use a large set of resistors, mounted in the rear of the vehicle. With the old coding on the LINK ECU it would overheat the resistors under very rare conditions and the car would stall (scaring the crap out of some owners).