Gravity

[Felix]

In the picture below if tank B was feeding an engine and was being filled by tank A would B fill to the brim or would its A to B fill pipe need to be higher? A will be the one getting filled at the pump and for argument sake both tanks are 50lt so will i have 100lt when the pump nozzle starts to click?

[David]

If that is vaguely to scale (and assuming you connect the tanks!) your problem will be that the tank B won't fill at the rate you chuck the fuel in as the breather is small.

Other than that, yes, it will fill to the brim (and up the breather to the final level in tank A).

[Count Steer]

Is that a vent/overflow on top of B? I think that the levels will equalise ie B could overflow. Can they be put at the same level? (I have a series of water butts daisy- chained together and I don't think it matters how high the connectors are. ie if the connectors were half way up they first would fill half way, then the second etc then they'd all gradually fill evenly. As it is the connectors are at the top so the first fills, then the second etc. If the last one was lower than the others I think it would overflow and the others fill no higher than the top of the that one. Once the level of the connector on the vessels is reached the liquid level in them stay equalised...I think.

[Mr. Dazzle]

So long as air can get out of the top of B (which it can, via what I assume is a vent already shown?) it'll brim - but only when A is about half full. The 'top' of the liquid will always find a level regardless of how many tanks you've got (assuming air has an route out), so A will start filling before B is finished IYSWIM. The level in A would reach pretty much to the cross bar of the "A" by the time as B is full.

[Cousin Jack]

Just make sure that the vent for B is as high as the vent for A, and that the link pipe can flow at least as fast as you can fill A.

[Horse]

Just make sure that the vent for B is as high as the vent for A, and that the link pipe can flow at least as fast as you can fill A.

... and that the link pipe can fill B faster than the engine can empty it.

[Mussels]

Tank A would be full before fuel starts coming out of tank B's breather. I'm not sure I'd trust a breather not to leak after a while.

[Mr. Dazzle]

Tank A would be full before fuel starts coming out of tank B's breather.

Only just with the way it's currently drawn

[Felix]

Quick skive between jobs so i should explain a but more. B is my standard (To small) tank for my camperbus. They dont make the 105lt tanks no more. I have been offered another identical tank (65lt) so as far as i know one has to be higher than the other for this to work. The top pipes are the filler holes bus yes also breathers. Fuel flowing up B's filler is a worry though. The link pipe will be thicker than the engines feed pipe so fuel flow when driving should not be an issue. Was keeping them close to allow me to fill B what A is full rather than wait for gravity to fill B then top up.

[Mr. Dazzle]

If they're level it'd work too...but you essentially just need to ensure you're always adding the fuel at teh highest point, otherwise you won't get the benefit.

You could probably plumb tank B's filler back into A's? Or maybe put B's into the top of tank A.

[Taff]

If the breather of tank b vents into tank a or joins the vent in tank a then the filler cap of b can seal when a is brimmed

For supply, you could have both tank pumps running into the single fuel line through a T piece, maybe through a restrictor if the engine pump is sensitive to the supply pressure.

[Mussels]

As long as A has a breather you should be able to fill it through B just fine, the fuel will just flow back into A until B's filler tube is full and the pump switches off.
So just fit A slightly higher or level, use a fairly large bore connection between the two and have some sort of breather in A. If you fit A too high (above the filler cap) then you just won't be able to fill it completely, if the connection is too small you'll keep having to wait for it to equalise before continuing to fill.

[Le_Fromage_Grande]

Get a couple of Jerry cans

[Felix]

OK i was reading it all wrong. They can be the same height. Fill A and due to a small link pipe B will take an age to half fill so keep both filler necks so i can jump to the other. 90% in each tank would be plenty for my goal. As said B is the engine feed and link pipe wider than engine/pump feed pipe. B also has fuel gauge sender unit and it will also still tell me when half full although telling me both tanks what is great as they will be almost 350 miles before i need to start looking for a fuel station.

[Count Steer]

I had a car with 2 tanks but you could switch between them with a doohickey on the dash.

[Felix]

I had a car with 2 tanks but you could switch between them with a doohickey on the dash.

What an old Jag? Its about the only car i can think of with two tanks. Having said that it may have been one but just a convenient extra filler on the other side.

[Count Steer]

I had a car with 2 tanks but you could switch between them with a doohickey on the dash.

What an old Jag? Its about the only car i can think of with two tanks. Having said that it may have been one but just a convenient extra filler on the other side.

Correct! 5.3l V12.

PS they were saddle tanks in each rear wing and not connected although it appears some owners modded them to connect them up so only one fuel pump was needed.

[Mr. Dazzle]

Old Alpina BMs had an optional 'touring' fuel tank too I believe.

EDIT, like this:

https://www.r3vlimited.com/board/forum/ ... -dashboard

[Big Red]

Old series landrovers also had twin tanks.
Although might have just been old military vehicles.
I had a series 2a military landrover with twin tanks under the seats.
there was a valve below the dash next to the transmission tunnel that switched from tank to tank.
It also switched the tank units over for the petrol gauge.
Found one but they are not cheap.
https://www.johncraddockltd.co.uk/52678 ... tanks.html

[Mr. Dazzle]

Bollox to the twin tanks, if you only get 350 miles on 100 litres I'd be looking for a new vehicle