Gas tank modification for larger fuel lines

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I am gathering and preparing to upgrade my fuel system using a RobbMC 1100 hp mechanical fuel pump and regulator system and to use 1/2 lines.

After considering all my options I have come up with the following plan to modify my tank.

The return line will go through the original stock fuel lines and sender.  The new line will be routed by removing the return fitting from the top of the tank (It is held in just like the sending unit) cutting off the barbed fitting and using a 90 degree bulkhead fitting for the start of my fuel lines.  On the bottom of the bulkhead fitting I'll use a tube nut and a section of 1/2" aluminum line that is bent so as to sit towards the rear of the tank at the lowest point or the rearmost point (I'm undecided here, but leaning towards lowest)

By going this route, my tank can remain unaltered, I don't have to run a return line more than about 6 feet, I won't have any fuel line in the transmission tunnel and I can get 1/2 line run front to back without interference with the tail pipes.

I am currently thinking this idea is pretty darn practical, however, what has seemed like a good idea in the past has from time to time turned out to have an issue I had not considered.

Does anyone see something I am missing?

 
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Would the pump have any difficulty pulling the fuel up to the top of the tank before starting to the motor? In a normal setup you are using gravity and fuel weight to get the fuel up to the front and then the pump has only a short distance to pull the fuel. Normally I would think that you would need an in-tank pump for the setup you are describing. I have not tried this so my only concern is how effective the pump would pull the fuel up to the top before going to the front of the car. I would also think that there is a possibility to get air bubbles in the line every time you start the car. Just my initial thoughts.

 

turtle5353

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Would the pump have any difficulty pulling the fuel up to the top of the tank before starting to the motor?  In a normal setup you are using gravity and fuel weight to get the fuel up to the front and then the pump has only a short distance to pull the fuel.  Normally I would think that you would need an in-tank pump for the setup you are describing.  I have not tried this so my only concern is how effective the pump would pull the fuel up to the top before going to the front of the car.  I would also think that there is a possibility to get air bubbles in the line every time you start the car. Just my initial thoughts.
+1 That's what I was thinking also. I know the car should fire up with the fuel that's in the carb bowl. So it may not be an issue once its up and running. Not sure if an air pocket would form or if that big old mechanical pump would just pull the fuel right on through. That's a beefy mechanical pump, so you may not have an issue. If you do end up having a problem with an air pocket, you could possibly install an inline check valve right after the 90 at the top of the tank, then run your 1/2" fuel line?? Then you would only have to worry if your tank is almost empty cause the line inside the tank will be as full as the tank is at any given moment.

But im just guessing here!! Im sure you will make it work!! ::thumb::

 
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Jeff73Mach1,

I am with jbojo on his concerns. If you would look at my FiTech thread, I installed the Tanks Inc. PA-4 in the tank electric fuel pump. Another concern to the lowest point, is when you take a corner, with low fuel, you may running out gas temporarily.

The PA-6 High Performance 400Liter Per Hour Pump (500 - 1000 Horsepower) would be the one to consider.

106 GPH @ 40 PSI

99 GPH @ 50 PSI

91 GPH @ 60 PSI

I know you would be totally satisfied with the setup. If you do got the electric fuel pump, make sure you get a Ford Fuel Shutoff Inertia Switch. I got one off a Ford EFI parts car and mounted it in the trunk.

https://lmr.com/item/LRS-9341B/1981-89-Mustang-Fuel-Shutoff-Inertia-Switch?year=1986&gclid=CIW-54GjktMCFVU7gQodIiUDcA

Just my recommendations. Please post some pictures on your progress of your fuel project.

mustang7173

 

TommyK

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The new line will be routed by removing the return fitting from the top of the tank (It is held in just like the sending unit) cutting off the barbed fitting and using a 90 degree bulkhead fitting for the start of my fuel lines.  On the bottom of the bulkhead fitting I'll use a tube nut and a section of 1/2" aluminum line that is bent so as to sit towards the rear of the tank at the lowest point or the rearmost point (I'm undecided here, but leaning towards lowest)
I think you might be better off to jettison the original vapor return gizmo and just use a piece of flat sheet metal to run your bulkhead fitting through. It should seal just fine with the factory o-ring and retainer provided the thickness of the sheet metal is similar to the thickness of the rim of the factory fitting. There isn't much room between the top of the fitting and the trunk floor.

 
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Thanks for all the thoughts. The bolt in pick up is 3/8ths, I'm moving to 1/2 so it won't work, but the idea is sound. Part of the equation is to run it so it does not interfere with the stock sending unit.

Using a sheet metal blank makes sense and I have plenty of appropriate scrap laying around.

As to clearance, I don't mind if I have to do trunk floor mods to get the additional clearance-easy peasy,; a little cutting, a little forming and a little welding all while the tank is out.

I don't know if it would be a problem lifting the fuel that little bit more, but the stock set up lifts the fuel about 1/3 the tank height and the system I am putting together has two large fuel filters that should provide some additional reserve capacity if the tank line does lose prime. I don't know if it will when the system is sealed up tight, I would think it would not-I'll do a little research. That being said, mechanical pumps pull far better than electric pumps which are usually pusher pumps by design. They have no problem pulling fuel from the front of the vehicle and this pump is a substantial improvement over stock and most other aftermarket mechanical pumps.

You have given me some ideas and I will consider and research a bit before I cut anything-one thing I do like about my plan is that if it doesn't work as designed, moving the lines, (Swapping positions) while it will cost me my gas gauge, would restore the general design back to one that is already relatively proven.

Now I'm wondering if a gas gauge mounted in the sender hole could be made to work?

I'm not interested in running an electric pump with a carburetor. My set up may indeed not work

 

Don C

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I wouldn't worry about coming out of the top of the tank, as soon as fuel is pulled out of the tank and the line fills it will act as a siphon.

My only concern is in using aluminum tubing for the pickup. The end that runs down into the tank will not be supported and aluminum tubing doesn't always respond well to vibrations, tending to crack. I would go with steel or flexible, like this:

https://www.summitracing.com/parts/fsy-stk-1002/overview/

 
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I wouldn't worry about coming out of the top of the tank, as soon as fuel is pulled out of the tank and the line fills it will act as a siphon.

My only concern is in using aluminum tubing for the pickup. The end that runs down into the tank will not be supported and aluminum tubing doesn't always respond well to vibrations, tending to crack. I would go with steel or flexible, like this:

https://www.summitracing.com/parts/fsy-stk-1002/overview/
Good call on the aluminum tube Don.

When I mentioned air bubbles in the line from drawing from the top I was thinking how fluids tend to maintain the same level.  When the tank is full the levels in the tank and pick up tube would be the same but as the tank level gets to say around a quarter of a tank after time the pickup tube would eventually equalize to the same level as the tank.  In a perfect vacuum this should not happen but I can see the levels being the same at some point especially if the car sits for some time.  So I think that an air pocket may develop between the top of the tank and the fuel level in the tube.

I found this picture that illustrates what I was thinking though its not correct to the gas tank it shows how liquids equalize.  If you remove water from one barrel the levels will equalize.  If you put a check valve at the bottom of the pick up tube I think it would fix the problem.

Do in tank pumps have a check valve?

u9z9S.png

 

keiths71

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A friend of mine Ken at Kidd Darrins custom cars in melb.fl. built me a stainless 1/2" sending unit out of a stock one. super nice job on it. I believe robb mc now offers one. I am planning on running the return line out the top but away from the tunnel

 

Don C

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I wouldn't worry about coming out of the top of the tank, as soon as fuel is pulled out of the tank and the line fills it will act as a siphon.

My only concern is in using aluminum tubing for the pickup. The end that runs down into the tank will not be supported and aluminum tubing doesn't always respond well to vibrations, tending to crack. I would go with steel or flexible, like this:

https://www.summitracing.com/parts/fsy-stk-1002/overview/
Good call on the aluminum tube Don.

When I mentioned air bubbles in the line from drawing from the top I was thinking how fluids tend to maintain the same level.  When the tank is full the levels in the tank and pick up tube would be the same but as the tank level gets to say around a quarter of a tank after time the pickup tube would eventually equalize to the same level as the tank.  In a perfect vacuum this should not happen but I can see the levels being the same at some point especially if the car sits for some time.  So I think that an air pocket may develop between the top of the tank and the fuel level in the tube.

I found this picture that illustrates what I was thinking though its not correct to the gas tank it shows how liquids equalize.  If you remove water from one barrel the levels will equalize.  If you put a check valve at the bottom of the pick up tube I think it would fix the problem.

Do in tank pumps have a check valve?
Air will also be drawn into the fuel line when the fuel level is low and fuel sloshes around. This usually isn't a problem because the fuel pump draws more fuel, along with the air bubbles, before the fuel bowls in the carburetor run dry, and the air bubbles are just vented out of the top of the carburetor. However, air bubbles in the fuel line, between the tank and pump, may exacerbate vapor lock problems on carbureted engines.

Your diagram is factual, but only applicable when all points in the system are exposed to the same air (atmospheric) pressure. If one of the barrels has more air pressure in it than the other one the water level in it will be lower than the other, as would happen if one barrel had a tight lid on it. If the downspout has less air pressure in it, pushing down on the water, the water level in it will be higher than in the barrels. This what the fuel pump does, in two ways, one it pulls a vacuum into the fuel line and atmospheric air pressure in the fuel tank pushes fuel into the fuel line. When the fuel enters the fuel pump it now pumps the fuel and pulls the fuel along. If the fuel pump pulls too hard it will vaporize the fuel, causing vapor lock. Because the fuel pump has at least two check valves in it, the fuel will not return to the tank nor equalize levels.

Yes, in-tank fuel pumps have a check valve, which keeps the fuel line pressurized, unless an injector or a return system regulator leak.

 
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