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Technical Fuel line size effect on pressure?

Discussion in 'The Hokey Ass Message Board' started by c-10 simplex, Jun 14, 2019.

  1. c-10 simplex
    Joined: Aug 24, 2009
    Posts: 1,331

    c-10 simplex
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    Fuel line size and it's effect (if any) on pressure and volume?

    Please discuss freely.
     
  2. Mr. Sinister
    Joined: Sep 3, 2008
    Posts: 1,039

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    The bigger a pipe, the higher potential volume. Raising pressure does not increase volume, the opposite actually happens.
    When you buy a fuel pump, look for the flow rating at specific pressure, as some will mislead you by stating flow rate at zero psi.
    If you take a 3/8 fuel line and reduce it to say 1/4, you're increasing pressure by creating a restriction because of a lack of volume.
     
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  3. Mr. Sinister
    Joined: Sep 3, 2008
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    Additionally, I'm a huge fan of running EFI fuel pumps on carbureted engines. When set up properly for your combo, you have a constantly refreshing supply of cool fuel that will never lose pressure at any practical RPM. It requires a spendy fuel pressure regulator and a return line to your tank, plus EFI rated fuel line, but it's well worth the money and effort to not have to worry about fuel starvation issues. The regulator does all the work, supplying the carb with a steady flow of fuel at all times and at a dead nuts consistent pressure, from idle to WOT. You may not NEED this setup, but once you run one on a performance build, you'll never go back.
     
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  4. Hnstray
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  5. squirrel
    Joined: Sep 23, 2004
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    pressure at which end of the line?
     
  6. Hnstray
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    Hnstray
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    from Quincy, IL

    ;) ........this oughta be good!
     
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  7. c-10 simplex
    Joined: Aug 24, 2009
    Posts: 1,331

    c-10 simplex
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    pressure at which end of the line?

    i pretty much had a hunch that would come up......

    i'm mostly concerned about pressure at the carb.


    So, if we theoretically did go from 3/8" down to 1/4" in the above example, would pressure increase or decrease at the carb?

    i ASSume volume would go way down, but pressure goes up? Not that it matters if the volume is now small?
     
    Last edited: Jun 14, 2019
  8. alanp561
    Joined: Oct 1, 2017
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    Squirrel has it right. You need to read up on basic hydraulics. Unless you've raised the pressure on the 3/8 line, you're not going to get any more pressure on the 1/4 line. You're just going to get the same pressure in a reduced volume. I've been involved with a great number of hydrostatic leak tests and I can guarantee you a stream of water from a large diameter pipe, say 2 ", at 250 psi, will knock you down but it won't cut you. .A stream of water at the same pressure coming from a small diameter tube with an .010 ID will cut your skin.
     
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  9. Mr. Sinister
    Joined: Sep 3, 2008
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  10. Mr. Sinister
    Joined: Sep 3, 2008
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    Mr. Sinister
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    I get what you’re saying. But why when I pinch the return line off my pressure regulator, fuel pressure goes up at the gauge on the carb? Honest question, not being a smart ass. I’m effectively reducing the pipe diameter, and the effect is raised line pressure.
     
  11. alanp561
    Joined: Oct 1, 2017
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    Your entire system is pressurized and when the fuel in the return line hits the end of the line, it is no longer under pressure.
     
  12. alanp561
    Joined: Oct 1, 2017
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    If you pinch off the return line, you will get a slight rise in pressure but it stops, doesn't it? The entire line is at the same pressure up to the point you pinched it off.
     
  13. Mr. Sinister
    Joined: Sep 3, 2008
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    Right, where it dumps back into the tank. But for the length of the return hose it’s still under pressure. So the restriction in the system causes pressure to rise, regardless of where it is in the system. So in the OP’s case, going from a bigger line to a smaller line won’t make a difference in pressure, only volume. But necking it down from 3/8 to 1/4 at a given point will. A regulator makes it all moot either way.
     
  14. Mr. Sinister
    Joined: Sep 3, 2008
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    Several psi, actually. The harder I squeeze, the higher the pressure goes. Not dangerously, but higher than a Holley carb can handle. If I think of it tomorrow, I’ll get a quick video.
     
  15. tubman
    Joined: May 16, 2007
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  16. The way I look at it, Installing a return line decreases the pressure because it creates another outlet. When you pinch the return it increases back to deadend pressure. When adding a return line I believe some adjustment to the regulator has to be in order. Try pinching the main line, I bet there is no pressure difference. This of course could all be utter nonsense.:)
     
  17. Mr. Sinister
    Joined: Sep 3, 2008
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    Mr. Sinister
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    I believe you’re right about pinching the feed line.
    My bypass regulator and the more common dead head style regulator both regulate pressure to the carb, but the bypass does have the added benefit of the return where the typical dead head style regulator does not. I don’t know what the dead head style regulator does to pressure in the supply line from the pump though.
     
  18. Good point! The regulator is in a way pinching the main line. I'm hoping it's a little more to it than that. Some one will chime in and straighten me out.
     
  19. gene-koning
    Joined: Oct 28, 2016
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    If you have a fuel system designed with a pressure regulator, a fuel return line, and a pusher fuel pump, if you close off the return line, the system will increase in pressure until the system has reached the pressure capacity of the fuel pump. It will do this because there is no way to relieve the pressure if the fuel can not return to the tank. The regulator has no control over the amount of pressure the pump can deliver. Excess pressure will overcome the regulator if the fuel can not be bled off.

    if you deadhead a fuel pump, it will build pressure until the pump fails, the lines fail, or the pump bypasses. Gene
     
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  20. Rich S.
    Joined: Jul 22, 2016
    Posts: 273

    Rich S.

    That’s because, when you go from 2” to .010” you’ve increased the velocity.


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  21. patterg2003
    Joined: Sep 21, 2014
    Posts: 526

    patterg2003

    As mentioned that it is best to learn about hydraulic flow. A small line can have high pressure and low to no volume. Fluids have ideal velocities as as the velocity increases above the ideal velocities the resistance increases consuming the pressure to make it flow. The cross sectional area of the 1/4" tube is about 44% of the 3/8 tube so the rough math is that the velocity would have to be 2-1/2x faster in the small tube for the same volume rate of the larger tube. The velocity may be excessive so the flow resistance becomes the flow restriction. The tube should accommodate the max flow rate + and if it has a properly sized pressure regulator then there is always adequate pressure through the full flow range.
     
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  22. The law of hydraulics says increasing or decreasing the fuel line size will not change pressure. But it will change volume.
     
  23. Rich S.
    Joined: Jul 22, 2016
    Posts: 273

    Rich S.

    According to the Bernoulli Equation, when you increase velocity, pressure goes down.


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  24. What’s the question again,,,

    Since were taking fuel,,,, that means we’re burning it,,,, to create energy,,,

    How much energy (power) are you trying to make ??? That determines the volume of fuel

    All the fuel you need or will use in your 1/2” line or any size line down nothing needs to fit thru the needle seat orifice size,,,, is there just 1, or is there more 2 maybe 3, or is there 8 needle and seat orifices? Is it a simple buoyancy vs gravity controlled device or is it something else? That limit on control has established the maximum pressure. The power levels desired has established the minimum volume.
     
  25. Rich S.
    Joined: Jul 22, 2016
    Posts: 273

    Rich S.

    The original question was “Fuel line size and it's effect (if any) on pressure and volume?”


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  26. Ebbsspeed
    Joined: Nov 11, 2005
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    Lets assume your electric fuel pump can generate a maximum pressure of 20PSI. If you have no pressure regulator, or a deadhead regulator, the feed line pressure will be 20PSI, UNLESS the needle valve(s) in one or more carbs are open. There will be a very slight drop in pressure at the regulator or carb inlet for that period of time that fuel is flowing. In the case of a bypass regulator, the feed line pressure will be just slightly above the setting of the bypass regulator, and this increment is determined by the resistance to flow (size and length) of the bypass return line to the fuel tank.

    There are also considerations based on maximum volume the pump can move at 20PSI, line size and length, etc. The above explanation is as simple as I can make it without getting into fluid dynamics, viscosity, temperature, voltage at the pump, static air pressure, whether or not there are lumps of chicken shit in the fuel lines, etc.
     
  27. Mr48chev
    Joined: Dec 28, 2007
    Posts: 25,382

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    Not gas or fuel line but along the same line. https://www.irrigationtutorials.com/using-a-smaller-pipe-to-increase-water-pressure/
    I can remember reading someone's theory years ago that the further you went on a line you needed to reduce the size of the line to maintain the same pressure but that may or may not work.

    It still comes down to fuel line sized to the volume of fuel you need.
    I remember working on a friend's car years ago that he pulled a six out of and put in a pretty stout V8 and it would basically run out of gas when he went though the gears because the manufacture put a small diameter fuel line on the six cylinder cars. We ended up changing the line from pickup tube in the tank to the fuel pump and then it ran like Jack the bear.
    Pretty much the same thing happened with my 57 Chevy panel with the real stout 327 in it. Truck would launch great but the 2 WCFB's on Corvette intake ran out of gas just about the time you shifted and ran out again about mid way though second. It had an electric pump at the tank but who ever put it together had put a brass fitting designed for natural gas with a 1/8 orifice in it line at the carbs. Probably dug out of the spare brass fitting can when they put it together for it's correct in and out fitting sizes.
     
  28. Right !!!
    It’s not gonna matter much as long as the minimum volume is met without exceeding the maximum pressure.

    Minimum volume value is set by desired power out put and maximum pressure limit is set by the pump and outfeed control device. All of which not mentioned here.

    Never get more pressure out than you put in, via the pump. it also needs to include (very important) a factor of time. It’s not just gallons, it’s gallons per hour or minutes. And again nothing mentioned here.
     
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  29. jimmy six
    Joined: Mar 21, 2006
    Posts: 4,547

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    All I know is a 57 Chev 270 HP and FI had a 3/8" line and the other engines had a 5/16" line. Not sure on the mechanical fuel pumps. Chev 6's had a 1/4" in the early 50's
     
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  30. Truckdoctor Andy
    Joined: Jan 13, 2017
    Posts: 795

    Truckdoctor Andy
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    Good point! Let the factory engineers figure it out and I’ll just use what they used.


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