Register now to get rid of these ads!

Technical Re: fuel bypass return

Discussion in 'The Hokey Ass Message Board' started by 550Coupe, Apr 6, 2021.

  1. I'm installing a fuel pressure regulator with a bypass return. I was of the understanding that the return line always went into the top of the fuel cell. I have a standard five gallon plastic fuel cell which only has one opening on top for the vent and the two openings on the bottom. I was watching a Summit you tube video where the speaker said you could use one of the bottom openings for the return. Now that would make things a lot easier, not having to drill a hole and mount a bulkhead fitting but I think this guy was wrong. Comments?
     
  2. lostone
    Joined: Oct 13, 2013
    Posts: 2,820

    lostone
    Member
    from kansas

    The gas doesn't care where it returns to the tank.
     
  3. jimmy six
    Joined: Mar 21, 2006
    Posts: 14,802

    jimmy six
    ALLIANCE MEMBER

    I’d rather have it at the top but the return line is under pressure and the head pressure in your 5 gallon tank is negligible. Give it a try
     
    Elcohaulic and 550Coupe like this.
  4. Thanks guys. I guess I just wanted a second opinion.
     

  5. Since liquid always seeks its own level, if return line enters at the bottom there could be a lot of fuel in that line?
    Seems if return line runs along chassis rail it should ride up above the fuel cell height and the drop back down to the bottom connection on cell to minimize fuel in that line.
     
  6. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,589

    Blues4U
    Member
    from So Cal

    I would prefer to have separation between the return and the outlet on the tank, preferably on opposite sides of a baffle, to allow any heat in the fuel to dissipate, or any contaminants to settle, or air to dissipate, before being picked up. This is really more of a concern with hydraulic reservoirs than fuel tanks, but I'd still like to follow that practice if possible. Phil's concern about the return port being lower than the fuel level could be an issue when having to pull the line, or the regulator or anything else that is in series with the return line and below the fuel level in the tank; requiring you to have to pinch off the flow somehow. So it'd be better to come in at the top to avoid it siphoning off. But if that's all you have, I guess it will have to do.
     
    ekimneirbo and pprather like this.
  7. Well, I after giving it some thought I think the Summit tech was wrong and I'm going to run the return into the top of the fuel cell. Just makes more sense. Thanks for the input.
     
  8. Mike VV
    Joined: Sep 28, 2010
    Posts: 3,029

    Mike VV
    Member
    from SoCal

    Yes, the "return" line MUST be free breathing..!
    ANY amount of restriction (below the fuel level !), the system will not work correctly.

    Mike
     
  9. Hollywood-East
    Joined: Mar 13, 2008
    Posts: 1,993

    Hollywood-East
    Member

    Absolutely, Correct me if wrong, but a return that's pushing fuel, in to fuel, Is going to have a different reading than coming in (back) the top..Rite?
     
  10. Hollywood-East
    Joined: Mar 13, 2008
    Posts: 1,993

    Hollywood-East
    Member

    You beat me... What I thought
     
  11. I had an incident years ago where the vehicle that I was following suddenly juked to avoid a tire tread from an 18-wheeler in our lane. I had no time to react and the thick tread rolled under my car like an alligator on 'Swamp People', pulling loose the braided fuel return line that the PO had installed. Fortunately, it didn't separate. It just broke all the clamps and ties. For this reason, I'm gun-shy about any part of the fuel system being low or on the bottom. I prefer the lines high and the return at the top for safety issues.
     
  12. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,230

    Mimilan
    Member

    If your car has a propensity for vapor lock, run the return below the fuel level. [any returning vapors cool]
    If you are using a bypass regulator , there will be slight pressure in this return line anyway.[it will be getting pumped back]

    If you have a serious vapor lock issue, use a high [ish] pressure electric fuel pump near the tank,
    Run a fuel line loop to the carb and back to the tank again.
    Tee off a dead-head regulator near the carb.
    AND use an adjustable pressure "check valve" on the return near the tank [too hold vapor pressure]

    The idea of this ^^^^ is to hold fuel line pressure above boiling point [ whereas a bypass regulator lowers the line pressure to the bypass setting]

    Having the return going below fuel level reduces aeration.

    As an experiment! fill a bucket with water .
    But hold the hose about 3 feet above and watch the water aerate!
    Then do this again with the hose end under the water level.
     
    Last edited: Apr 7, 2021
    Beanscoot and rod1 like this.
  13. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,719

    1971BB427
    Member
    from Oregon

    There wont be any restriction since every time the return line is moving gas, the supply line is sucking it at a higher rate than it returns. So it will have a negative pressure on the tank, and wont matter where the line is attached.
     
    tb33anda3rd and jimmy six like this.
  14. Mike VV
    Joined: Sep 28, 2010
    Posts: 3,029

    Mike VV
    Member
    from SoCal

    Yea...not quite so much..!
    For this comment to be true, the fuel in the tank would need to be levitated..! NOT gonna happen. The gasoline has weight, period. No amount of NORMAL fuel pump "suction" (even for pull in an electronic fuel injection system), is going to provide the gasoline with zero weight.


    BUT...while the bypass line should still come in from the top of the tank, so that the fuel weight does not hamper the incoming gasoline. It also helps to insure that there will be no backflow (toward the carburetor / fuel injector) in the return line.
    The line should end below the fuel level, near the bottom of the tank. This is so that there will be no aeration, or bubbles that would be formed as the gas drops down from the top of the tank to the fuel level surface. Bubbles in the fuel system are as bad as bubbles in the oil pan.
    The returning fuel only sees a small amount of sitting fuel weight this way and does not hamper the incoming returned fuel.
    At low RPM, the standing fuel weight is no big deal as the return pressure is high. BUT, during times of high RPM, there will be less return pressure to try to overcome the standing fuel weight.

    The return line - In from the top, all the way to near the bottom of the tank.


    Mike
     
    Hnstray, pprather, rod1 and 1 other person like this.
  15. saltflats
    Joined: Aug 14, 2007
    Posts: 12,594

    saltflats
    Member
    from Missouri

    I would want it in the top but not pointing to the pickup.
     
  16. 61SuperMonza
    Joined: Nov 16, 2020
    Posts: 489

    61SuperMonza
    Member

    I added a fuel return line on my 61 Corvair. It is plumbed in with a tee fitting in the fuel filler tube. The return line will function as intended from the top of tank.
    When I installed the turbo motor I felt the return line was necessary although the 62-63 turbo motors didn't have the return.
     
  17. Happydaze
    Joined: Aug 21, 2009
    Posts: 1,904

    Happydaze
    ALLIANCE MEMBER

    On the fuel return line ending below the fuel level matter, I read somewhere that this helps reduce the buildup of static electricity. Apparently fuel free falling from the top of the tank creates static. I can't comment on the validity of this information though but have my returns ending toward the bottom of the tank anyway.

    Chris

    Sent from my SM-T515 using The H.A.M.B. mobile app
     
  18. Back pressure is the same, top or bottom

    Ben
     
  19. The setup as described by @Mike VV is the way it is done on factory fuel tanks as well as the better quality aftermarket tanks.
     
    Hnstray likes this.
  20. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 4,217

    ekimneirbo

    Have you asked the tank manufacturer why they put the return in the bottom to begin with? Also, how do you plan to cap the return hole ? Not saying its right or wrong, just wondering.
     
  21. First, this is a nostalgia gas car, not a street car. I'm using a five gallon JAZ/ Summit fuel cell. I comes with two 8AN openings on the bottom (supply for two fuel pumps) and one 6AN fitting on top (vent) . The problem was I watched a Summit you tube video and in it the tech said that one of the 8AN fitting could be used as a return. I have decided that's not the way to go and will just drill a hole in the top and install a bulkhead fitting for the return.
     
    Blues4U and ekimneirbo like this.
  22. Budget36
    Joined: Nov 29, 2014
    Posts: 13,174

    Budget36
    Member

    I don’t see a reason why you couldn’t put a fitting in the bottom, use a tube with a 90 to keep it low or the same as the pickup.
     
  23. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 4,217

    ekimneirbo

    That makes a lot more sense now. Definitely don't want any shortage of fuel in a drag car. Your Avatar is awsome, and I never use the word awesome. Whats the tank made out of........steel/plastic/aluminum?
     
  24. The tank is plastic.
     
  25. ekimneirbo
    Joined: Apr 29, 2017
    Posts: 4,217

    ekimneirbo

    Since it is plastic, I would consider making the return line hole near a corner of the tank where it has more strength and having a frame/body bracket just ahead of it to lessen vibration/pulsation.
     
  26. Yep, it's going in the corner.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.