Passive fuel subtank? (1 Viewer)

[mct75]

Looking at the height of the main fuel tank, it looks like the upper and lower fuel levels of the main tank are inclusive of the heights of most sub tanks. As such, is there a reason why everyone uses a pump to explicitly move fuel from the subtank to the main? Why not just connect the two tanks with a 1" diameter hose, and vent the subtank appropriately? As the main is filled and drained, fuel will just flow to and from the sub. This avoids having to deal with pumps, wiring, dual fillers, switches, new sun visors etc. While I like doing things the OEM way, factory subtank retrofits are phenomenally expensive.

 

[greentruck]

With fuel, you can't be too careful, especially with OEM installs with lawyers and regulators look things over closely. If connected in that simple fashion, a failure of the connecting line would dump most of the fill from both tanks on the ground -- not good.

I do remember a past discussion where other issues about this were raised, but don't recall what they were offhand.

 

[mct75]

True, but the same caution has to be applied to any fuel-carrying hose in the system, and plenty of hoses carrying fuel run throughout the truck.

To me, a simple hose would be way safer than an external electric fuel pump, and people here use those all the time without fireballs.

 

[lugueto]

I believe having gravity-filled aux tanks in the states may be trouble. IIRC it was discussed somewhere in these boards.

Venting that fuel tank "properly" might also be an issue.

Agreed, I wouldn't want to drain both tanks due to a hose failure.

I'll stick with a pump, personally, but I can't fault you for looking for a simpler approach.

 

[mct75]

Functionally, it's the same as a larger aftermarket gas tank. Just in this case the larger tank is made of two tanks connected by a hose. The types of hazards that would damage a hose mounted to the side of the main tank would puncture that tank first. The subtank would have to have the hose mounted lower (to drain all the way) but I do not think it's a hazard that can't be avoided. A skid plate coming off the tank would guard it just as well as any OEM application would.

Teeing into the charcoal canister line should allow both tanks to vent to the evap system, and also balance between each other.

I'm just wondering what the "catch" is. I'm not brave enough to try this on my own!

 

[gnarlytoes]

I see no reason it would not work.

I've owned several dump trucks with dual tanks and gauge and fuel pickup in just one. The connecting line runs along a piece of channel attached to the frame.

The only problem was parking on a side slope. If full, the fuel would try to drain out the vented cap of the lower tank. If low fuel, it could all drain to the tank without fuel pickup if it was on the lower side.

Of course these are diesel fueled, so not as much danger of fire if a leak occurred.

 

[FastCarGuy]

Assuming the auxiliary tank is mounted to the rear of the main tank, and the sharing line is also mounted towards the rear of the main tank, then if going up a long steep grade all the fuel would run to the rear tank leaving the pickup tube in the main tank with possibly no fuel to suck...JMO

 

[mct75]

The rear would have to be a bit higher than the main (otherwise it would be a rock magnet) so the hose going to the main tank could be placed about midways up. At least that's what I'm thinking in my head.

 

[math]

A check valve (even one manually operated by cable) would solve the "lose all the fuel" problems, wouldn't it?

 

[serenity]

Yep, a simple check valve would prevent fuel from siphoning back into the second tank during steep hill climbs or while parked on a slope.

 

[mct75]

A check valve might impede filling the subtank, though.

How about having the port installed on the front or side of the main tank? Then, in a steep climb fuel cannot flow back into the tank.

Perhaps on the inboard side, so the tunnel for the driveshaft can offer some protection? I'll need to crawl around under mine and see how much clearance there is.

Another possibility, is mount the large diameter tube way up high, so that the main tank only "overflows" to the sub tank in the last stages of filling, then use a smaller line mounted around front for transfer while driving. I would think that even in the most extreme fuel consumption scenarios (3FE towing up a mountain), you would only need a gallon transferred every 5 minutes or so.

Maybe too (I'll have to check) there would be enough clearance to replace the drain plug of the main tank with a thin banjo fitting connected to the subtank. Banjo fittings are used all the time with fuel systems. This would be plug-and-play without even having to remove the main tank. Filling would take a long time, though, as the banjo can't flow a whole lot. Still, just brainstorming.

 

[george_tlc]

... or maybe just add a transfer pump eh?

Why deal with all the 'what if' scenarios when a transfer pump scheme is well proven for this kind of application and works well?

Yes, you need the dual neck filler, but that really is the correct way to do the job so that anyone can fill your tank(s) safely and quickly.

I was probably one of the first people to install an OEM subtank in my 80 in the US. It has been reliable for the 15+years it has been installed. I think I've recouped any extra $$ I had to spend over that 15+ years versus some kludged gravity scheme. I can trust my wife or my kids to use, fill and transfer fuel without any special provisos.

cheers,
george.

 

[math]

... or maybe just add a transfer pump eh?

Why deal with all the 'what if' scenarios when a transfer pump scheme is well proven for this kind of application and works well?

Yes, you need the dual neck filler, but that really is the correct way to do the job so that anyone can fill your tank(s) safely and quickly.

I was probably one of the first people to install an OEM subtank in my 80 in the US. It has been reliable for the 15+years it has been installed. I think I've recouped any extra $$ I had to spend over that 15+ years versus some kludged gravity scheme. I can trust my wife or my kids to use, fill and transfer fuel without any special provisos.

cheers,
george.

Ostensibly, it should be even easier to use a well-designed gravity-feed subtank. You don't even need to pull the knob or run the transfer pump. There's absolutely nothing wrong with the dual neck filler and transfer pump, except that they're hard to find and not cheap.

 

[george_tlc]

Yes, but the gravity scheme is illegal (for safety reasons if nothing else)... So why not do the job correctly?

Yes, you will need to find the dual neck filler - but it's not an impossible task. Fuel transfer pump can be non-oem so not an issue to source.

cheers,
george.

 

[mct75]

So, a giant gas tank is legal, but two gas tanks connected together breaks the law? What if I make a single gas tank that is shaped like two gas tanks connected by a 2 foot pipe? Is a gas tank with an appendix illegal?

I also don't see how an electric fuel pump in open air is safer than a pipe. I must be really missing something but to me anything with arcing motor brushes already has a head start on a length of pipe as far as "danger" goes. An external fuel pump setup also has at least twice as many hose connections to leak from, not to mention the housing itself. I know they are designed to be intrinsically safe, but a pressurized fuel spray onto a hot, electrified fuel pump in open air gets you a lot closer to "fireball" than a pipe dripping onto the ground.

 

[jfz80]

So as to not enter the argumemt ...

The floor is level...... your sub is going to have to be shallower by a decent margin to get gravity in your side, no?

 

[george_tlc]

So, a giant gas tank is legal, but two gas tanks connected together breaks the law? What if I make a single gas tank that is shaped like two gas tanks connected by a 2 foot pipe? Is a gas tank with an appendix illegal?

I also don't see how an electric fuel pump in open air is safer than a pipe. I must be really missing something but to me anything with arcing motor brushes already has a head start on a length of pipe as far as "danger" goes. An external fuel pump setup also has at least twice as many hose connections to leak from, not to mention the housing itself. I know they are designed to be intrinsically safe, but a pressurized fuel spray onto a hot, electrified fuel pump in open air gets you a lot closer to "fireball" than a pipe dripping onto the ground.

You've made up your mind. Feel free to follow your path to a solution since you've apparently justified your setup with various two tank/1 pipe scenarios.

I'll make ONE point. Your gravity system requires the hose/connection to the aux tank to be at the lowest point. If a leak occurs it will leak and leak till you have 50L (at least for the factory sub) on the ground. The fuel pump transfer feeds into and from the TOP of both tanks. Turning off the fuel pump will prevent continuous fuel leakage when detected. What are you going to do with the gravity scheme if you're out away from a large 'bucket'?


cheers,
george.

 

[dogfishlake]

I have done my own custom sub tank and used the same pump and concept as the one posted by @LandCruiserPhil. The dual filler was the pricey part for me, which you can get through @Akella , or build your own simple split style. I am very happy with my filler even though it wasn't cheap. I get your gravity idea, just not sure it will work. That's the way it goes with a new idea, you are the pioneer and we are the naysayers waiting to be proven wrong . Do your thing and you'll be king.

For me, the custom tank, proven Holley blue pump, and some custom plumbing has made for a sweet system. But then again, who knows, your idea may be a great way too. I would say that a one way valve, or manual valve to isolate the tanks seems wise. Good luck.

 

[dogfishlake]

My sub tank install starts on page 13 if it helps. You may be able to do your set up idea without the pump, not sure.
Beaten, built, 80 series rebuild thread

 

[woodsman 44]

Just get a MAF long ranger tank and be done with it....