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Detroit fuel supply / return hose size

Discussion in 'Engines' started by Capt Ralph, Oct 29, 2012.

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  1. Capt Ralph, Oct 29, 2012
    #1
    Capt Ralph

    Capt Ralph Senior Member

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    The old fuel system on our Ole Bertram can use some updating. Currently a single pick up from the glass tanks, down a 1 inch hose, to a T in the engine room. Then 3/4 inch lines to the tank selector valves. Then 1/2 inch lines to the Racors & mains.
    Funny part, I think the selector valves are only 3/8 inch port size.

    The mains are 650hp, Detroit 12v71TIs. The on engine fuel hose size appears to be ¼ & 3/8” hose.

    The return lines seem to be 3/8 “ to the selector valve, ½” to the T, 5/8” back to the tank.

    Wow, what a hard ware mess.

    Some of the original 34 year old hoses are still in use.

    Questions are, (where do I start?) what size hoses does a 12v71ti really need?
  2. Capt J, Oct 29, 2012
    #2
    Capt J

    Capt J Senior Member

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    That depends on the length or run or how many feet it is to the tank from the engines. Also the height of the run has a little to do with it as well. The motor's only need 3/8", but running 3/8" more than 5-10' will cause a restriction because of the length of the run. How many feet is it from the tank to the engine fuel selector valves? etc? I would say 3/4" would be sufficient on the supply side, and 5/8" on the return like it has. DD's move a ton of fuel so the return size needs to be almost as large. I remember I transfered (purposefully) over 200 gallons in 10 minutes with a pair of 12v71's at 800 rpms, just by putting the supply on 1 tank and returns on another.
  3. Bill106, Oct 30, 2012
    #3
    Bill106

    Bill106 Senior Member

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    Your 12V71's probably don't need quite as much fuel as the 900 hp versions I've installed but if I remember correctly we used -16 (3/4") on the supply and -12 (5/8") on the return. As CaptJ said, it is a nightmare reducing down but it needs to go full size (except for a brief reduction to -12 at the Racors due to their fitting size) all the way to the fuel inlet block where it goes to 3/8". There may also be a restriction fitting intentionally installed in the return line to maintain pressure in the rack, when you're replacing lines make sure you save and reuse that as well.
  4. Capt Ralph, Oct 30, 2012
    #4
    Capt Ralph

    Capt Ralph Senior Member

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    Aft tank supply is about 18 feet to the T, 5 & 7 to the selector valve, return line 5 & 7 from the valve, then the T and 5 feet back to the tank. Return restrictors are on the engines.
    Forward tank is in the ER. say 8 feet for supply and return to the selector valves.

    Vertical lift is not much, aft tank is 26 inch tall, top of tank is near level with selector switch. About three feet up from that is the fuel pump.
    Forward tank is 38 inches tall. Selector valve is about mid tank height, then almost 3 feet to the fuel pumps.

    I never liked the idea of the same pick up used for both mains. But I guess I'm not going to worry.
  5. Capt J, Oct 31, 2012
    #5
    Capt J

    Capt J Senior Member

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    Most of the time, most people traditionally ran 1 engine off the foward tank and the other engine off the rear tank.
  6. Jimbo1959, Nov 1, 2012
    #6
    Jimbo1959

    Jimbo1959 Member

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    #10 hose would be more than adequate for the suction and #6 or #8 for the return.
    Jim