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Fuel Line Size - 3/8" ID or 1/2" ID

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  • #16
    There is little point in considering 1/2" lines, as much of the head loss comes from the fittings and in-line parts (selector, filter, gascolator, pump, transducer, etc.) - most of which you cannot buy with a 1/2" internal diameter.

    We found 3/8" to be more than enough, and we have a lot of "obstructions" in the fuel system. I think using the Gold Cube is worthwhile for anyone with an O-540, the Red Cube does cut down the head quite a bit at those higher flow rates.


    • #17
      IIRC, EI says the gold cube is for gravity fed systems (all types) and the red cube should only be used with systems that have boost pumps... I do know that the gold cube is what they told me to use (and is working just fine) for my gravity-fed Citabria (65 7ECA with O-200 power).
      Jim Parker
      Farmersville, TX (NE of Dallas)
      RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)


      • #18
        The red cube is also more prone to obstructions blocking fuel flow.


        • #19
          Battson nailed it. The best thing for clean flow is minimize the number of fittings and make the flow friendly. The less restrictions in fittings & piping the better. I did not catch it soon enough & installed tees at the front and rear tank intersection at the doors. Wyes are more flow friendly friendly so better to have used wyes at those intersections. I bought a wye from Jegs and put it at the left right intersection before the gascolator. Sweeps are better than tight bends or elbows. We used two full port ball valves for the L & R tank fuel valves that have zero restriction. One valve handle was modified so both cycle on & off with the same rotation. The gascolator is a bottle neck where all the flow passes through a single gascolator. Other than weight two gascolators could improve flow as only half the amount of flow is passing through & resistance decreases with reduced velocity. The slower flow would be less turbulent reducing resistance & water in slower flow will drop out better.

          When we designed piping we used velocity to determine if we were in the correct range for the fluid. Normall targeted 10 - 12 fpm for water flow as over 15 fpm the flow resistance begins to rise so much that the pump pressure needs to increase to overcome the resistance & it is just burning expensive horsepower to force flow. There are books like the Cameron Hydraulics manual and others that give numbers for velocity vs. resistance. IMHO If the velocity through the tube is in the comfort range to meet the certification flow tests then its normal flow velocity will be in the lazy flow range which is better.

          There are a lot of Bearhawks flying with Lycoming 540's using 3/8" tube that had to meet the flow tests for certification. The Continental fuel injected engines are a different beast as the engine draws more fuel for the mechanical injection system than it burns with the excess returned to a header tank or main tank. That fuel lines for a Continental IO have to flow more than an engine using a carburetor.

          This is from a previous discussion & the flow numbers are comforting.



          • #20
            Fuel line 3/8 verses 1/2
            If you plan to install fuel flow system most of them have a 1/4 opening
            Just a though.


            • #21
              I used 3/8 tube in Three Sigma which runs a 260HP O-540. When I ran the fuel flow test, I was not able to get the 150% of the expected takeoff fuel flow, which I blamed on the use of a fuel flow transducer. I used the low pressure drop sender recommended by JPI for gravity flow systems. Eric Newton was running similar tests on Miss'ippi Mudbug at the same time with similar results. His solution was to remove the fuel flow sensor. My solution is to run a 5 psi Facet pump during takeoff and climb. The pump was originally installed just to provide pressure for the solenoid controlled primer system. The pump is turned off during cruise flight.

              Don't just arbitrarily add a fuel pump to a carbureted engine. I purchased a carburetor set up for gravity flow pressures. If I run the fuel pump I can't lean down to an appropriate cruise fuel flow before the mixture control hits idle cutoff. If you want an engine driven fuel pump you will need to have the carburetor modified to accept the higher fuel pressure.

              I'd really be surprised if a bigger tube made a difference if all of your components are built for 3/8 line, since most of the pressure drop will be in the components.
              Russ Erb
              Bearhawk #164 "Three Sigma" (flying), Rosamond CA
              Bearhawk Reference CD


              • #22
                My BH had a 540 LYC and I was making 260hp+ never had fuel flow problems.


                • #23
                  In this thread in fuel line diameter there are a mixture of references to inside diameter, and outside diameter. For clarification, where 3/8 inch aluminum fuel line is specified, is this inside diameter or outside diameter?

                  Aircraft Spruce seem to reference all of their tubing by OD.
                  Last edited by Nev; 10-08-2021, 04:53 PM.
                  Nev Bailey
                  Christchurch, NZ


                  • Bcone1381
                    Bcone1381 commented
                    Editing a comment
                    3/8 OD is typically used and it has a .035" wall thickness, but .028 wall is also commonly used.

                  • davzLSA
                    davzLSA commented
                    Editing a comment
                    I believe tubing is measured by OD and fuel hose is measured by ID. You should refer to the 43-13 to be sure.

                • #24
                  While I used 3/8' aluminum hard line I had to go to 1/2' hose from the firewall to the carburetor to meet the flow test requirements for gravity flow to my 0-540.


                  • #25
                    I read every fuel flow report of fuel flow tests the BH that I could find. 3/8" seems adequate, if a bit marginal to fully meet the 150% test. I was also questioning whether the protocol should be 150% for gravity fed, or 125% for a pumped system. I am still not convinced which one should be used. Our local tech counselor/DAR is a ex Boeing engineer that did lots of fuel systems stuff and usually gives tech forums at Oshkosh. I will ask him.

                    If you have an injected system that has return fuel, either Conti or EFI, and you return fuel to the tanks, I think I would go with 1/2" fuel lines for sure. I don't know how much fuel a Conti IO-470 pumps at full gallup, but the EFI stuff flows 40-45 gph continuously. FWIW I think the worst restriction in my fuel system is the Newton fuel valve. But I think it is good enough.


                    • #26
                      I originally put 1/2" lines in my BH. Then an IA who is well respected came by and said the lines look nice but you will have multiple leaks in about 150 hours since you exceeded the bend radius in several places. Stick to the plans.

                      Anyone want to buy some 1/2 fittings?