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Old 13th June 2016, 09:55 PM   #4 (permalink)
K_arlstrom
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Join Date: Apr 2016
Location: Sweden
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Copy/paste from what I wrote on another forum.

Quote:
Originally Posted by skywalker View Post
it doenst matter what boost levels you are running at all...
Well, allow me to complete that statement. As pump pressure increase, pump flow decrease.
Unless you run static fuel pressure, it will increase together with the boost you run and therefor reduce the flow from the pump
You are correct in that the fuel & air required for the same power is pretty much laws of nature.

Different pumps are affected more or less by this, look at this graph for reference.


I spent a lot of time trying to read up on this during the winter, to know what and why I should buy this or that.
Can't promise that everything I learned is correct, but I hope someone will tell me if I make a mistake!


With some pressure/flow curves like those above, lets make some calculations, pressure and flow is at the fuelrail. Had a hard time deciding how to do this but here is something...

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*Stock fuel line is 6mm ID
*About 3 meters from pump to fuelrail
*I assumed optimized bend and hose flange dimensions for the "minor losses"
*I didnt account for the different materials, as in rubber hose vs steel pipe
*I didnt account for any fuel filter, lets assume that it's clean and free flowing

Here is FLOW vs PUMP PRESSURE (stock lines)
200 lph - 4,8 psi pressure loss
250 lph - 7,4 psi pressure loss
300 lph - 10,6 psi pressure loss
350 lph - 14,4 psi pressure loss
400 lph - 18,7 psi pressure loss

Now, if you look at the chart I posted earlier, when would you say that the fuel system pressure loss would start to make a difference?

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Lets make an assumtion and a simple calculation, 15 psi boost and something like walbro 255 drop in.
43,5 psi fuel rail pressure + 15 psi FPR + 7,4 psi loss from the system = 65,9 psi fuel pump pressure.

Repeat that for the aeromotive trying to push 350 lph and with the boost increased to 22 psi and you end up with 80 psi at the pump!!

-> AND PLEASE REMEMBER THAT THE RETURN LINE IS EQUALY LONG AND NARROW, YOU WILL HAVE THE SAME BACKPRESSURE AFTER THE FPR AT IDLE. <-
Dont blame the FPR for your car running rich at idle/low load with a humongous pump, when it's designed for a much lower backpressure...

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Imho something like the bosch 044 with it's 300 lph and 10psi loss is where you should consider replacing the fuel lines. 8mm inside diameter (~AN6) is MUCH bigger, even if it doesent sound like it.

300 lph, 8mm ID - 2,4 psi pressure loss
400 lph, 8mm ID - 4,2 psi pressure loss
500 lph, 8mm ID - 6,5 psi pressure loss

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While I'm at it, 10mm ID (about AN8).
500 lph, 10mm ID - 2,1 psi pressure loss
750 lph, 10mm ID - 4,6 psi pressure loss

Over and out!
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Last edited by K_arlstrom; 13th June 2016 at 10:52 PM..
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