3 fuel tanks, one fill up point?

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Originally Posted by Jitterbug

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By 'connecting all the tanks together' what do you mean? Do you mean just connecting them with hose to, in effect, make one large tank? if so wouldnt you see all the fuel rushing to whichever was the lowest if you are on a hill? I guess check valves only allowing flow to the tank with the fuel pump would solve this to some extent.

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Yes the fuel would flow to the lowest point and possibly below the fuel pick up, where ever that is.

Not that it is of interest here but the additional tanks fitted to the D3/D4 do exactly that, fit a second tank between the filler and the original tank. the second tank sits higher than the main and the fuel keeps the main full until the second tank is almost empty.

I will take some measurements this weekend and work out the hight differential, length of hose and friction/losses to see what dia pipe i would need to match the flow rate of a high flow fuel pump, as well as seeing if there is a viable route for the pipe that doesnt hang below a chassis rail.
I am sure i still have my fluid mechanics book somewhere!

I generally dont worry about it, with all tanks connected at the filler just put the filler in and let it go as its feeding all 3 tanks at once you'll get a pretty even fill rate if you plumb the manifold in such a way that the furthest tank gets the most effective bite at the fuel.

as for using the fuel leave one tank in place as the running tank and then just pull from the supply of the others and T into the breather for the main running tank.

In the event that you overfill it with all the tanks interconnected via the fillers it'll just circulate back to the tanks that are empty.

For the TDI fuel system I like to plumb additional tanks so the fuel flow goes

tank-lift pump-sedimentor-fuel return line. most fuel pumps are positive displacement so include a one way valve and most sedimentors have 2 inlets and outlets so you can simply slice the return line open plug it into the 2 outlets of the additional sedimentor and you dont need to faff around with anything else.