Walbro Inline Fuel Pump combo w/ Stock Pump

[gotsomegetsome]

yes or no; comments




I need to know if this is fine. I keep running into threads that end up talking about other pumps. Maybe i'm not inputting the right keywords but I decided to do this. I would like to connect the Walbro after the stock pump if there is nothing wrong with the stock pump. But, I would like to know if this is something worth doing or if I should stick to the simple Walbro in tank 255lph

supports:
Lex AFM
680cc siemens injectors
AFPR from driftmotion

 

[91Supra313]

Well, if you put the 255 after the stock pump then you are only going to get the stock pumps pressure. I would just put the 255 in there and take the stock one out. The way you are talking about setting it up is going from a small opening to a large opening. Think of it like pipe work. You will only get the pressure of the smallest one. So if you have the stock one then the 255, if the stock one flows 200 ( just an example ) then you will only get 200. Someone feel free to correct me on that one but that is the way I see it. I am going to be putting my 255 in there as soon as I get new straps since I don't want to take the chance of snapping my old ones and then be screwed.

 

[IJ.]

Well, if you put the 255 after the stock pump then you are only going to get the stock pumps pressure. I would just put the 255 in there and take the stock one out. The way you are talking about setting it up is going from a small opening to a large opening. Think of it like pipe work. You will only get the pressure of the smallest one. So if you have the stock one then the 255, if the stock one flows 200 ( just an example ) then you will only get 200. Someone feel free to correct me on that one but that is the way I see it. I am going to be putting my 255 in there as soon as I get new straps since I don't want to take the chance of snapping my old ones and then be screwed.

bzzzz wrong

Pump #2 would normally see an inlet pressure of ambient+ tank residual pressure so 14.7+ maybe 1>2 psi so can only flow xx volume based on what it can draw in based on the 15>16 psi acting on the inlet.

It then supplies this volume to the rail/injectors/FPR and any unused excess is seen as "pressure"

In the compound pump set up you use pump #1 to supply the inlet of pump #2 at 40+ Psi, net result is a lot more "flow/volume" at the injectors.

Eventually you'll run into a hard limit of what the fuel lines can carry and will need to either add a second line or a single larger line to make use of the increased flow potential of the pumps.

 

[91Supra313]

Well I stand corrected then. Thanks IJ for correcting me. I was under the impression that he would only get the pressure of what the smallest pump would put out. But that is why I said for someone to correct me so. At least now I know.

 

[IJ.]

Well I stand corrected then. Thanks IJ for correcting me. I was under the impression that he would only get the pressure of what the smallest pump would put out. But that is why I said for someone to correct me so. At least now I know.

Same

Always looked at it as "a system will only flow as much as the point of greatest restriction" (fluid dynamics thing)

Did a bit of research into it the last time this issue was raised and that's what I found, think of it along the lines of compound turbocharging and it makes sense.

 

[gotsomegetsome]

I.J., Just so I get this correct, what your saying regarding this: "Eventually you'll run into a hard limit of what the fuel lines can carry and will need to either add a second line or a single larger line to make use of the increased flow potential of the pumps", is that the volume of fuel passing through the series pumps is greater than the realistic capacity of the stock fuel lines to supply the fuel rail. Is this because the stock fuel lines will burst under the increased pressure / flow? and if the fuel lines would hold adequately with the series pumps being used, is it that the fuel lines would be restricting the increased flow provided by the two fuel pumps in series, thus negating some usable increased flow. The last statement I just wanted to double check my understanding.

p.s. Thank you Ian

 

[IJ.]

One of the bigger Walbro's would be close to maxxing out the stock lines flow potential, no you won't "burst" anything as pressue will still be set by the FPR.

xx diameter line can only flow yy at zz pressure, so for arguments sake say you have 500cc injectors running flat out pumping 2550cc per minute and all is well, fit larger injectors say 1000cc we now have to supply double the fuel to the rail at the same pressure which the compound pumps may be able to do but there just isn't the room in the line to physically flow that volume of fuel so the engine leans out.

This is way simplified just to show what I mean, pressure is resistance to flow.

Yes if you increase pressure you get more flow through a given size hole BUT in this case increasing pressure ISN'T an option as the FPR will maintain the set base pressure by bleeding off the additional pressure.

Another "got ya" with this setup you're thinking of is all this increased flow when you don't need it HAS to go somewhere, in this case it'll be through the FPR/return line to the tank and will in a short time heatsoak ALL the fuel in the tank and you then run into tune issues.

 

[gotsomegetsome]

Ah I see, okay thank you very much.

I'm a little confused about the general calculation. For 500cc/min injectors, wouldn't it be ~3000cc/min? or is that just rated, dependent on fuel pressure.

Then for 680cc/min -> 680cc * 6 = 4080cc/min -> 4.08Lpm * 60 = ~245 Lph, so the Walbro 255 should be good

 

[IJ.]

Calculate it on 85% duty cycle as that's accepted as may as well be flat out wide open.

Injector "control" above 85% is sketchy at best.

My fuel system for 500+rwhp is 2x -6 lines 2x Mk4 Denso's 1x -6 return

Pump #2 is boost activated and pump #1 runs at half speed at cruise/idle so it's not heat soaking the fuel.

 

[gotsomegetsome]

AHH that makes so much more sense. ......And thats where you got 2550cc haha okay durrr

Thank you very much! I do believe I have a solid plan now.

 

[IJ.]

Welcome Dude

 

[91Supra313]

yeah.....can someone hand me a map...... I'm beyond fuckin lost with what the hell you said. All I need to know is that my 255 pump will have no issues with the 550 injectors. Since I know this I'm good.....damn IJ.... I need this stuff Barney style..... simple and easy brother. lol.

 

[IJ.]

LMAO it get's a LOT more complicated if you do ALL the math, I know roughly what I need to make xxxrwhp then I just double it all to be safe and have plenty of overhead, 1 leanout during a hard pull WILL ruin your entire day, the fuel system isn't a place to skimp on

 

[gotsomegetsome]

yeah.....can someone hand me a map...... I'm beyond fuckin lost with what the hell you said. All I need to know is that my 255 pump will have no issues with the 550 injectors. Since I know this I'm good.....damn IJ.... I need this stuff Barney style..... simple and easy brother. lol.

lol...

well a ton of people have done 550's with the walbro 255Lph pump and it should be fine.

The calculation that was done earlier was basically this, you take the rated flow of the injectors, multiply by 6 injectors (because thats how many you have), then multiply by 85% duty cycle, which is how long the injectors will be on vs. off (in other words the fuel flow is not perfectly 100%, and a typical rating would be 85% flow). Once you have that value, you can compare it to the flow rate of your Walbro pump.

For your 550cc injectors:

550cc/min * 6 = 3300cc/min TOTAL.

3300cc/min * 85% duty cycle = 2805cc/min REALISTIC flow

2805cc/min / 1000cc in one liter = 2.805 Liters per minute

2.805 Liters per minute * 60 minutes per hour = 168.3 Liters per Hour.



If your Walbro pump flows 255 Liters per Hour, and your injector/fuel setup will consume a max realistic flow of 168.3 Liters per Hour, then you will be perfectly fine.

You have just under 100Lph of flow overhead (safety) for your setup, which is plenty. Ian mentioned that he would not want to do a WOT run with a minimized overhead for fear of running lean, and thus destroying an engine.

In a nutshell, 255lph for 550cc injectors will be fine. Just make sure its a genuine walbro.