Plumbing twin tanks for EFI?

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Denon Osterman
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Plumbing twin tanks for EFI?

Post by Denon Osterman » Fri Feb 15, 2019 9:32 am

So I've decided to get EFI for my boat, which makes plumbing the fuel system a bit of a headache...especially with twin tanks. Ideally I'd like to draw from (and return to :? ) both tanks equally, which I know is tough. In a traditional system I would have just pulled from both tanks with equal length hose and physics would have kept things relatively level. But with electric pumps and return systems...it gets much more complicated :(

Another issue is that the electric pumps required only like to "push" fuel - they don't like to "pull" it. The bottom of my tank is very near to the bottom of my boat, and I'm weary of mounting the electric pump so low in the bilge just to have to pump fuel back up to the throttle body.

Since I already have the mechanical pump I was thinking of getting a very small "sump tank" and using it as a sort of accessory for the efi system. The mechanical pump would pull equally from the two main tanks and pump fuel into the sump tank (would I need a float valve?). The Electric pump would then pull from, and return to, the sump tank. This would ensure the EFI is always primed, eliminate the twin tank issues, etc. I know it's a bit of a ridiculous proposal and I welcome any other thoughts to try and get an EFI system working nicely with two tanks!

Thanks,

Denon

Ozzieboat
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Re: Plumbing twin tanks for EFI?

Post by Ozzieboat » Sat Feb 16, 2019 9:31 pm

Denon
I take it you are incorporating an EFI system to a previously carburettored engine. USCG requirements state that you cannot have a pressurised fuel line more than 14" from the engine, so most marine engine manufacturers incorporate a fuel cell. The HP EFI pump draws fuel from the cell and the cell is fed by a LP pump mounted on the outside of the cell from the tank/s. Most manufacturers use a Carter rotary vane pump as they have good lift capabilities. Return fuel from the engine is fed back to the inlet of the cell, so you don't need a complicated system to return the fuel to the selected tank. I would incorporate a tank selector valve between the tanks. Also a N.O. oil pressure switch to power the LP pump is needed for safety reasons. The EFI manufacturer should have addressed the HP pump in their design for the same reason.

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jenko
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Re: Plumbing twin tanks for EFI?

Post by jenko » Sun Feb 17, 2019 12:37 am

try one of these in each tank and no return most of the newer efi cars don't run return lines now the pressure regulator is with the pump in the tank you will need custom tanks made but they come with fuel sender and swirl pot :D
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Bill Edmundson
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Re: Plumbing twin tanks for EFI?

Post by Bill Edmundson » Sun Feb 17, 2019 2:01 pm

Denon

I run 2 tanks. I prefer the manual valve. My thought is to run one down then switch. Then fill the empty tank. This way I keep fresher fuel and if I get bad fuel someplace It is only in one tank.

Bill
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Denon Osterman
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Re: Plumbing twin tanks for EFI?

Post by Denon Osterman » Sun Feb 24, 2019 4:13 pm

Hi all,

Thanks for the replies / ideas!

Bill, I understand why most people prefer two tanks. The thing with my scenario is that my hull is quite light and the tanks are port / starboard, so I want to try and keep the weight relatively level. I also don't have any issues with bad gas as I only ever get gas from one marina. That being said, the worst imbalance I would ever see is 100 lbs...and I weigh more than that. So this might be what I end up doing.

Alan, a little "sump tank" right by the engine is exactly what I was thinking. The issue is I can't find any that are coast guard rated no matter how hard I try. Do you happen to know what brand / make I should be searching for, or where I could find one?

Thanks!

Denon

Ozzieboat
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Re: Plumbing twin tanks for EFI?

Post by Ozzieboat » Sun Feb 24, 2019 10:39 pm

Denon
They are quite expensive $560 - $590 so I made my own. That said we don't have to worry about approval for these type of things "down under" for private use. I did a quick search and came up with "marinepartssource.com You could probably shop around the dealers to see if they have a second hand one or be lucky enough to find one at a boaties swap. In the picture you see four hoses as my return fuel goes back to the fuel tank via the cell because I previously had the EFI fuel pump in the tank with a return line, but I could see the pump was too hard to get to so I incorporated the fuel cell.
DSCF9146 (1024x768).jpg

Denon Osterman
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Re: Plumbing twin tanks for EFI?

Post by Denon Osterman » Mon Mar 18, 2019 2:32 pm

Well after doing a lot of thinking and digging and research I've discovered how the OEM's plumb their EFI systems without having to return to the tank, and still meet all of the requirements for fuel starvation prevention, coast guard compliance, etc.

I'm not sure I'm going to go this route yet as Paul Kane raised a number of good points, so I'm still considering if this will be worth all the trouble or not. That being said, the "solution" isn't actually as difficult (or expensive) as I thought it would be.

Basically, I went through the Mercury part system diagrams and found that on their big block EFI engines they basically use one of two setups (with no real rhyme or reason between them). One is a premade module that incorporates the individual parts...the other is using the parts off the shelf. In both cases the actual routing is the same, and shockingly simple / clever IMO. Basically, they use a fuel / water separator as a surge tank. In other words, a low pressure mechanical pump pulls from the tank and goes to the Fuel / Water separator. A high pressure pump then pulls from the separator , feeds through a cooler / the high pressure fuel rails, etc...a bypass regulator then dumps / returns back into the Fuel / Water separator. See below pic.
525efi01.jpg
at <$200 all in for a fuel / water seperator, and cooler, this doesn't seem to be a bad way to get cooled gas fed to your (EFI) engine in a coast guard compliant way - plus you don't need a return line to your tank or otherwise, and the added benefit of there being a reserve for the pump to always draw from is really nice as well. I'm not sure if this will apply to anyone else's scenario, or even my own if I decide to stick with carb, but I do think it's a pretty clever way to get the job done!

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Mr Hot Rod
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Re: Plumbing twin tanks for EFI?

Post by Mr Hot Rod » Mon Mar 18, 2019 3:13 pm

I see that you're sorting this out !

Here's the info I emailed to Denon :

Here are some alternatives :
  • Not USCG rated, but a very clean way to augment a low pressure system to high pressure for EFI applications :
    Image

    Adjustable Universal EFI Sump Fuel Kit - 67gph (35-90 psi)
    Download the installation Instructions to see what's involved. With this fuel sump mounted at no more than 14" from the engine, only that section would be high pressure. The rest of the system would be at low pressure supplied by a USCG marine fuel pump. Will require the installation of a vent line from the sump overflow vent port to the fuel tank.
To make the system USCG compliant :
  • Mount fuel sump less than 14" from the engine
  • Upgrade hose and clamps to EFI standards
  • Install shut-off valve at tank output, fuel-water separating fuel filter inlet and outlet, USCG low pressure electric fuel pump inlet and as close to the engine as stated in the USCG regulations.
____________________

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Kane Custom Boats Ltd.
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