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| Return line? https://mail.slantsix.org/forum/viewtopic.php?t=61105 |
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| Author: | sandy in BC [ Mon Jun 18, 2018 1:29 pm ] |
| Post subject: | Re: Return line? |
The return line on my EFI system is Zero pressure...its a return line after a fuel pressure regulator. With zero pressure it doesn't give squat about where it leads to. ….garden hose in a toilet bowl. It is returned to the sump where the strainer for the pump is located. no aereation. No "sloshing" Any loss of volatiles is most likely when you are fueling....or when you don't have venting straight. Return line is the easiest/most worried about part of EFI installs. Pressure regulation deserves more attention. |
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| Author: | jcc [ Mon Jun 18, 2018 4:21 pm ] |
| Post subject: | Re: Return line? |
Quote: It is returned to the sump where the strainer for the pump is located. no aereation. No "sloshing" Any loss of volatiles is most likely when you are fueling....or when you don't have venting straight. Return line is the easiest/most worried about part of EFI installs. Pressure regulation deserves more attention. Not mentioned yet, is most high pressure EFI fuel pumps/systems are designed to supply some percentage of volume constantly over what the engine requires at max output, that not used at that moment is sent to the return. Do the math, the entire tank is flushed every few minutes at low speeds/low demand. There are fuel pump electronic speed/cycle regulators available at additional/cost/complexity/reliability to mitigate this issue. Everyone has a different set of priorities, best to be informed fully before picking your poison. |
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| Author: | kesteb [ Mon Jun 18, 2018 9:01 pm ] |
| Post subject: | Re: Return line? |
And some people over analyse stuff to the point of.... My system will use the tried and true low pressure lift pump to a "swirl pot", then to a high pressure pump, which will fed the injectors. The regulator will feed back to the swirl pot. Excess fuel will be sent back to the fuel tank. A well documented and easy to build system. There will be less then 3' of high pressure line in the system. Some would consider this a plus. The low pressure pump is a modified BBM mechanical pump rated 33gph at 7psi. There is no regulator in the low pressure side. The low pressure side only needs enough volume to keep the swirl pot full. The swirl pot fills from the bottom and exits from the top. The swirl pot is just 3" x 9" of exhaust tube with end plates and 4 3/8 NPT bunges welded on. . The high pressure pump is the standard external 255lph style. This is enough to feed the injectors. It is feed from the bottom of the swirl pot, then feeds thru a fuel filter, to the injectors and back to the regulator. The regulator then drains back to the top of the swirl pot. Fuel lines are 3/8". The reason for all this is that it leverages the already installed parts. I really don't like buying stuff twice. And this setup will not be the best for all people. |
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| Author: | ntsqd [ Tue Jun 19, 2018 5:27 am ] |
| Post subject: | Re: Return line? |
The return line should be nearly zero pressure. I say nearly because truly zero pressure won't push the returning fuel back to the tank. The lower that pressure can be, the less it affects the pressure in the fuel rail(s). It is one to one, if the return line pressure is 10 psi it will increase the fuel rail pressure that same amount. This has been demonstrated to be the case on another forum by many others. If the return line pressure varies and the system is open loop or in open loop mode then the AFR is going to vary. If it is in closed loop mode and the computer doesn't have enough adjustment range then the AFR is going to vary. Do what you want, but I'm going to try to eliminate as many uncontrolled variables as I can. My Valiant's system uses the OEM 5/16" line for the return, I added a 3/8" supply. There is no way that engine needs a 3/8" supply, those sizes were chosen by the maker of the pick-up assembly. No swirl pot/"accumulator", no low pressure pump, simple. I want the feed line under pressure, more pressure is better. That is the best way to avoid vapor lock. Here in the Lower Left Coastal Desert that is a big concern. On the Bronc-up I do have a low pressure pump, but that is because I'm using it to push fuel thru a water-separating 10 micron marine fuel filter, partly because the aftermarket tank has no filter sock. When that truck goes EFI (if it doesn't go diesel) it will have the high pressure pump right after the marine filter, and then a 4 micron filter just before the fuel rails. Again, I want the feed line under pressure to head off vapor lock. |
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| Author: | jcc [ Tue Jun 19, 2018 7:52 am ] |
| Post subject: | Re: Return line? |
By your comment above, seems like reversing the return/feed lines would better achieve your goals better as i see it, in that a 3/8" return line would would almost never have a greater restriction then a 5/16" feed line, and as stated, motor could never require the capacity a 3/8" would allow anyway. |
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| Author: | ntsqd [ Thu Jun 21, 2018 9:43 am ] |
| Post subject: | Re: Return line? |
Quite possibly true, but I figure that the fuel consumption, even at idle, offsets the difference. The Bronc-up has 3/8" supply and return lines since I had to plumb the whole system. As I bought it the fuel line was 25' of rubber hose...... |
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| Author: | jcc [ Sun Jun 24, 2018 10:08 am ] |
| Post subject: | Re: Return line? |
FWIW, I did the math on fuel lines @.024" thickness, difference in CSA between the two is approx 54%. I assume flow difference is similar. |
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| Author: | ntsqd [ Tue Jun 26, 2018 1:16 pm ] |
| Post subject: | Re: Return line? |
Could do the fuel consumption calcs based on HP to find the actual GPH and compare that to the ~10% boundary layer occluded flow of each to determine the flow over-head of either. Or could say "3/8" will support X HP @ y psi and I'm at 52.7% * X HP so there's roughly a 47.3% flow over-head" and not worry about it more than that. Which is where I am. a 5/16" line will flow way more than enough fuel for my 170's output, so using it as a return means that there is even less demand on it. |
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