The Walbro 255lph fuel pump (also known as the 500hp unit or hi-pressure unit) is in most cases an excellent replacement for your Mitsu in-tank fuel pump without the need for the clutter and smell of an external pump and surge tank setup. I found this info whilst reading DSM forums and thought you boys might like to have a read of it, especially the part about twin intank units and the Walbro pressure relief valve opening at 85psi etc. 
Here is AMS' AMS Ultimate Fuel Pump Test
April 2007
Well to put things to rest we've taken the initiative and built our own fuel pump test rig and conducted a series of tests. If anyone has any pumps to test let me know and we'll have Chewbacca run it on the test rig.
We're basically using a variable voltage power supply to run 12V and 14V to different fuel pump configurations. The pumps feed into a fuel pressure regulator and we measure the bleed-off at that pressure. We fill a graduated & calibrated container and track how much time it takes to fill, then with a simple calculation you get flow in liters per hour (LPH).
For starters the base fuel pressure in a stock fuel system is around 42psi, then it goes from there. For every 1psi of boost you add 1psi of fuel pressure. So at 20psi you're running 62psi of fuel pressure. At 28psi boost, 70psi fuel pressure. 40psi of boost, 82psi fuel pressure, and so on.
We learned a few interesting things, especially with the EVO VIII & IX pumps. The EVO VIII pump has a pressure relief valve that opens up at around 65-68psi and the flow output drops dramatically after that, eventually going to zero flow around 75-80psi. The IX pump flows a little more than the VIII pump at lower pressures but doesn't dump the flow at higher pressures. That's the reason you can make decent power at higher boost levels with the IX pump.
We also found that if feed one pump into another (series), the flow only drops slightly through the pressure range. The first pump is feeding the second pump and the second pump doesn't have to work as hard.
The best choice was the twin Walbro 255lph HP external pumps in parallel, meaning both pumps feeding the regulator.
The twin Walbro 255lph HP in-tank pumps in parallel was interesting. If you notice the flow starts dropping off at higher pressures and actually starts to get close to the performance of just one 255lph HP external pump over 90psi. The in-tank 255lph HP Walbro pumps have a pressure relief valve that starts to dump flow at roughly 85psi and the output flow drops.
________________________________________________________
David Buschur says:
Hi everybody.
This is going to be a long one
Well we have been doing a lot of testing over the last 6 months. This is the longest we have waited to actually get the information out there from any testing we have done. Today I finished up the last of what I needed to do on the fuel systems for the EVO.
As anyone that actually knows anything about us knows, we pride ourselves on selling you what you need and what works more than just selling crap that isn't needed or won't work as well as something else we know exists. That's how we want treated and that's how we treat our customers.
With that said, we have done quite a bit of testing and such on the EVO fuel system.
First let's talk about the fuel pumps. Most every EVO shop that sells a Walbro pump for an EVO sells the GSS342 pump.
Here are the specs on that pump at boost levels that are important. This is flow testing the pump at 13.2 volts.
40 psi the pump flows 410 lb/hr
60 psi the pump flows 360 lb/hr
70 psi the pump flows 332 lb/hr
80 psi the pump flows 285 lb/hr
100 psi the pump flows 160 lb/hr
So the first place to look is about 40 psi which is close to EVO rail pressure at idle, or your base fuel pressure. This really doesn't matter either as the car isn't going to run at 40 psi for long. As the boost comes up the fuel pressure raises 1 psi for each 1 psi raise in boost pressure.
To make this easier I am going to break it down in another "chart":
At 60 psi this pump is good for 600 flywheel horsepower
At 70 psi this pump is good for 553 flywheel horsepower
At 80 psi this pump is good for 475 flywheel horsepower
At 100 psi this pump is good for 260 flywheel horsepower
As you can see, as the pump falls off up top you can't make as much HP with it. The higher the base fuel pressure is set and the higher the boost goes the less the pump flows. So for a high horsepower, high boost EVO it just dies up top.
What happens is people end up putting in a big injector thinking they are out of fuel injector and as the pressure is falling the duty cycle is being ramped up higher and higher. We are guilty of this as much as anyone. Not anymore
I was going to go into what happens with you bump the voltage of the pumps from 13.2 to 15 volts but most of you are not interested in doing that and it is going to make this post so long nobody will read it. You can figure about 17% gain in flow. That will get you close. So it is substantial.
With the base pump out of the way now we can go onto our Modified High Output Walbro pump.
0 psi the pump flows 510 lb/hr
40 psi the pump flows 410 lb/hr
60 psi the pump flows 362 lb/hr
70 psi the pump flows 340 lb/hr
80 psi the pump flows 310 lb/hr
100 psi the pump flows 250 lb/hr
This makes this pump good for:
At 60 psi this pump is good for 603 flywheel horsepower
At 70 psi this pump is good for 566 flywheel horsepower
At 80 psi this pump is good for 516 flywheel horsepower
At 100 psi this pump is good for 416 flywheel horsepower
As you can see, if you are running high boost this pump really flows up top compared to a standard Walbro 255.
My brothers car and my car both ran out of fuel. I was able to run my EVO RS up to 542 whp on our dyno with the single Modified High Output pump we sell. I had 1200 cc injectors in the car running them at 100%+ duty cycles. The car ran a 9.97 at 142.32 mph like that. I was running our adjustable fuel pressure regulator kit on the stock fuel rail. This is where the testing started as my brother and I were both having the same troubles. Datalogging fuel pressure it was simple to find the problem. We were out of fuel pump.
Neither of us wanted to cut up our perfectly good EVO's to put in fuel cells. Neither of us wanted to run AN lines, I do not like AN lines. They dry out and crack and are prone to leaking after about 5 years. They are heavy, there is no real nice way to install them perfectly on a car. They are expensive and mostly, they just aren't needed.
My quest for a great double pumper(twin intank pumps) set up began. After careful study of the tank assembly and knowing I didn't want to hack into the factory wiring and didn't want to loose the factory fuel gauge I came up with an idea. First I needed more research. I called the best in the industry and got some flow rates of what the factory fuel lines could handle. Pushing fuel through a 5/16" fuel line you can supply enough fuel for 1,000 flywheel horsepower. This is exactly what I wanted to hear. No street EVO that I am going to want to actually drive is going to make that much power. An EVO making that much power is laggy as hell, no fun to drive and is more than likely dedicated for track. At that point, throw the stock fuel tank out, put in a lightweight fuel cell. My main concern was ease of installation and I wanted to be able to keep using as many stock components as I could.
I built the first double pumper set up and installed it in my RS. It worked absolutely amazing. The car went so rich that the car would barely run at WOT. After dialing in the AFR's again, back to where they were, the duty cycles dropped from 113% down to 65%!! At this point I had the adjustable regulator still on the car with the base pressure set at 50 psi. I was also still running the stock fuel rail. At this point it was obvious I didn't need a set of $700 1200 cc injectors in my car. I pulled them out and put in our $310 1,000 cc injectors.
The duty cycles of course went up a little, no problem. Then I decided to put the stock regulator on and see if I could still get by. No problem. The car was now making 590 whp (up from 542 when it ran 9.97) on 1,000 cc injectors, stock fuel rail, stock fuel lines, stock fuel pressure regulator!! The duty cycles were 84%. That's freaking great!
Here is the picture of our double pumper kit:
As you can see the kit is complete. It is a direct plug in. Non of the factory wiring is tapped into. It plugs in directly. We are using the same wiring bulk head fitting that is used in some Formula One fuel cells. The kit includes a pressure switch that is wired in for the 2nd pump only. At 16 psi of boost the 2nd pump is triggered to come on. For those of you that don't know the EVO doesn't have an external fuel filter. So it cannot be changed unless you buy a new fuel pump assembly. This is stupid. Our double pumper comes with a stainless steel inline 5 micron fuel filter. They can be bought seperately so you can change filters as needed in the future.
More data:
Our double pumper with two standard GSS342's in it will flow:
At 60 psi the twin pumps are good for 1200 flywheel horsepower
At 70 psi the twin pumps are good for 1106 flywheel horsepower
At 80 psi the twin pumps are good for 950 flywheel horsepower
At 100 psi the twin pumps are good for 533 flywheel horsepower
Our double pumper with two of our Modified High Output pumps in it will flow:
At 60 psi, 1206 FWHP
At 70 psi, 1133 FWHP
At 80 psi, 1033 FWHP
At 100 psi, 833 FWHP
Now, a note. There is some loss running two pumps together into a common line. I am going to eventually send these pumps out, for my own curiosity, and have them flowed through the Y that we use.
Right now what is important is we have a solution for 99% of EVO owners to go well into the 9's with minimal changes with a very high level of safety as the majority of the factory components are used.
Here is AMS' AMS Ultimate Fuel Pump Test
April 2007
Well to put things to rest we've taken the initiative and built our own fuel pump test rig and conducted a series of tests. If anyone has any pumps to test let me know and we'll have Chewbacca run it on the test rig.
We're basically using a variable voltage power supply to run 12V and 14V to different fuel pump configurations. The pumps feed into a fuel pressure regulator and we measure the bleed-off at that pressure. We fill a graduated & calibrated container and track how much time it takes to fill, then with a simple calculation you get flow in liters per hour (LPH).
For starters the base fuel pressure in a stock fuel system is around 42psi, then it goes from there. For every 1psi of boost you add 1psi of fuel pressure. So at 20psi you're running 62psi of fuel pressure. At 28psi boost, 70psi fuel pressure. 40psi of boost, 82psi fuel pressure, and so on.
We learned a few interesting things, especially with the EVO VIII & IX pumps. The EVO VIII pump has a pressure relief valve that opens up at around 65-68psi and the flow output drops dramatically after that, eventually going to zero flow around 75-80psi. The IX pump flows a little more than the VIII pump at lower pressures but doesn't dump the flow at higher pressures. That's the reason you can make decent power at higher boost levels with the IX pump.
We also found that if feed one pump into another (series), the flow only drops slightly through the pressure range. The first pump is feeding the second pump and the second pump doesn't have to work as hard.
The best choice was the twin Walbro 255lph HP external pumps in parallel, meaning both pumps feeding the regulator.
The twin Walbro 255lph HP in-tank pumps in parallel was interesting. If you notice the flow starts dropping off at higher pressures and actually starts to get close to the performance of just one 255lph HP external pump over 90psi. The in-tank 255lph HP Walbro pumps have a pressure relief valve that starts to dump flow at roughly 85psi and the output flow drops.
________________________________________________________
David Buschur says:
Hi everybody.
This is going to be a long one
Well we have been doing a lot of testing over the last 6 months. This is the longest we have waited to actually get the information out there from any testing we have done. Today I finished up the last of what I needed to do on the fuel systems for the EVO.
As anyone that actually knows anything about us knows, we pride ourselves on selling you what you need and what works more than just selling crap that isn't needed or won't work as well as something else we know exists. That's how we want treated and that's how we treat our customers.
With that said, we have done quite a bit of testing and such on the EVO fuel system.
First let's talk about the fuel pumps. Most every EVO shop that sells a Walbro pump for an EVO sells the GSS342 pump.
Here are the specs on that pump at boost levels that are important. This is flow testing the pump at 13.2 volts.
40 psi the pump flows 410 lb/hr
60 psi the pump flows 360 lb/hr
70 psi the pump flows 332 lb/hr
80 psi the pump flows 285 lb/hr
100 psi the pump flows 160 lb/hr
So the first place to look is about 40 psi which is close to EVO rail pressure at idle, or your base fuel pressure. This really doesn't matter either as the car isn't going to run at 40 psi for long. As the boost comes up the fuel pressure raises 1 psi for each 1 psi raise in boost pressure.
To make this easier I am going to break it down in another "chart":
At 60 psi this pump is good for 600 flywheel horsepower
At 70 psi this pump is good for 553 flywheel horsepower
At 80 psi this pump is good for 475 flywheel horsepower
At 100 psi this pump is good for 260 flywheel horsepower
As you can see, as the pump falls off up top you can't make as much HP with it. The higher the base fuel pressure is set and the higher the boost goes the less the pump flows. So for a high horsepower, high boost EVO it just dies up top.
What happens is people end up putting in a big injector thinking they are out of fuel injector and as the pressure is falling the duty cycle is being ramped up higher and higher. We are guilty of this as much as anyone. Not anymore
I was going to go into what happens with you bump the voltage of the pumps from 13.2 to 15 volts but most of you are not interested in doing that and it is going to make this post so long nobody will read it. You can figure about 17% gain in flow. That will get you close. So it is substantial.
With the base pump out of the way now we can go onto our Modified High Output Walbro pump.
0 psi the pump flows 510 lb/hr
40 psi the pump flows 410 lb/hr
60 psi the pump flows 362 lb/hr
70 psi the pump flows 340 lb/hr
80 psi the pump flows 310 lb/hr
100 psi the pump flows 250 lb/hr
This makes this pump good for:
At 60 psi this pump is good for 603 flywheel horsepower
At 70 psi this pump is good for 566 flywheel horsepower
At 80 psi this pump is good for 516 flywheel horsepower
At 100 psi this pump is good for 416 flywheel horsepower
As you can see, if you are running high boost this pump really flows up top compared to a standard Walbro 255.
My brothers car and my car both ran out of fuel. I was able to run my EVO RS up to 542 whp on our dyno with the single Modified High Output pump we sell. I had 1200 cc injectors in the car running them at 100%+ duty cycles. The car ran a 9.97 at 142.32 mph like that. I was running our adjustable fuel pressure regulator kit on the stock fuel rail. This is where the testing started as my brother and I were both having the same troubles. Datalogging fuel pressure it was simple to find the problem. We were out of fuel pump.
Neither of us wanted to cut up our perfectly good EVO's to put in fuel cells. Neither of us wanted to run AN lines, I do not like AN lines. They dry out and crack and are prone to leaking after about 5 years. They are heavy, there is no real nice way to install them perfectly on a car. They are expensive and mostly, they just aren't needed.
My quest for a great double pumper(twin intank pumps) set up began. After careful study of the tank assembly and knowing I didn't want to hack into the factory wiring and didn't want to loose the factory fuel gauge I came up with an idea. First I needed more research. I called the best in the industry and got some flow rates of what the factory fuel lines could handle. Pushing fuel through a 5/16" fuel line you can supply enough fuel for 1,000 flywheel horsepower. This is exactly what I wanted to hear. No street EVO that I am going to want to actually drive is going to make that much power. An EVO making that much power is laggy as hell, no fun to drive and is more than likely dedicated for track. At that point, throw the stock fuel tank out, put in a lightweight fuel cell. My main concern was ease of installation and I wanted to be able to keep using as many stock components as I could.
I built the first double pumper set up and installed it in my RS. It worked absolutely amazing. The car went so rich that the car would barely run at WOT. After dialing in the AFR's again, back to where they were, the duty cycles dropped from 113% down to 65%!! At this point I had the adjustable regulator still on the car with the base pressure set at 50 psi. I was also still running the stock fuel rail. At this point it was obvious I didn't need a set of $700 1200 cc injectors in my car. I pulled them out and put in our $310 1,000 cc injectors.
The duty cycles of course went up a little, no problem. Then I decided to put the stock regulator on and see if I could still get by. No problem. The car was now making 590 whp (up from 542 when it ran 9.97) on 1,000 cc injectors, stock fuel rail, stock fuel lines, stock fuel pressure regulator!! The duty cycles were 84%. That's freaking great!
Here is the picture of our double pumper kit:
As you can see the kit is complete. It is a direct plug in. Non of the factory wiring is tapped into. It plugs in directly. We are using the same wiring bulk head fitting that is used in some Formula One fuel cells. The kit includes a pressure switch that is wired in for the 2nd pump only. At 16 psi of boost the 2nd pump is triggered to come on. For those of you that don't know the EVO doesn't have an external fuel filter. So it cannot be changed unless you buy a new fuel pump assembly. This is stupid. Our double pumper comes with a stainless steel inline 5 micron fuel filter. They can be bought seperately so you can change filters as needed in the future.
More data:
Our double pumper with two standard GSS342's in it will flow:
At 60 psi the twin pumps are good for 1200 flywheel horsepower
At 70 psi the twin pumps are good for 1106 flywheel horsepower
At 80 psi the twin pumps are good for 950 flywheel horsepower
At 100 psi the twin pumps are good for 533 flywheel horsepower
Our double pumper with two of our Modified High Output pumps in it will flow:
At 60 psi, 1206 FWHP
At 70 psi, 1133 FWHP
At 80 psi, 1033 FWHP
At 100 psi, 833 FWHP
Now, a note. There is some loss running two pumps together into a common line. I am going to eventually send these pumps out, for my own curiosity, and have them flowed through the Y that we use.
Right now what is important is we have a solution for 99% of EVO owners to go well into the 9's with minimal changes with a very high level of safety as the majority of the factory components are used.
