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Kreb
I strongly recommend that when you fabricate your fuel tank, that you design the tank with two internal baffles and divide the tank into thirds, even if you are building a road car. If making a tank for racing, you should add a recessed sump for the fuel pump pickup. I would suggest that even if building a road car, you should consider a fuel tank sump, because even if you are building a road car, you will probably consider entering it into a local autocross. A racing tank will also need additional access holes on the top surface, or a removable top, so you can install the foam in each section if required by the regulations or track.
If your seven is going to be used mostly on the track, design for a capacity of 5 or less gallons and for road design for a range of 7 to 10 gallons, depending on available space. The tank is best made from corrosion resistant material. These specs are the minimum thickness, and if the tank is a road car, then you should increase the gauge slightly. For aluminum you need 1.6mm [16 gauge 5005 or 5052] for stainless steel 1.mm [20 gauge 303 or 304] and for mild steel 1.2mm [18 gauge]. If you use plain low carbon steel by chance, your best option is cold rolled steel Vs hot rolled, which usually has a heavy scaled surface and could cause injector issues if it is not cleaned well. In all cases a good fuel filter needs to be incorporated into the fuel injection system. If you go with aluminum the tank should weight about a 1/3 less than the steel design.
You have two options for mounting rings for the fuel pump. You can tack weld the mounting rings to the inside surface of the tank or try using several puddle welds from the top of surface of the tank to attach each ring. You could try placing the ring on top of the tank surface and weld around the perimeter, but it may be difficult to weld it with that much local heat without distorting the surface. Of course, you will need to grind any welds on the mounting surface for a good gasket fit and seal.
Position the fuel pump mounting ring as close to the front wall, i.e., the lowest section in the center of the tank as possible, for the best low fuel level pick up capability. The fuel gauge sending unit should be fitted to the very end of the tank so you can fit the longest possible float arm in the tank. You will still have limited float travel. This requires that you modify the arm length and bend it, to get maximum travel i.e., complete sweep of the gauge. But do not be surprised, if there is not enough space in the tank for an accurate full travel fuel tank float. Using a shorten float arm, reduces the leverage and total travel which may cause inaccurate readings because there is not enough leverage to overcome the variable resistor’s friction. If you have the option, get an after-market donut ring type float fuel sending unit, which would be much easier to install. Last, check the function the sending unit, before welding the ends on the tank.
Before welding in the tank baffles, you need to cut the top two corners, for a relief, so fuel can pass between the sections during refueling. You only need a very small ¼” cut at 45* across each lower corner for ease of fitting in the formed corners. Allow about a one inch 45* cut in the upper corners of the fuel tank. If you plan on any auto-x, you may want to add a lowered sump in the center of the tank with hinged gates in the center section, to prevent fuel starvation. A couple 3/8” holes behind each hinge in the lower section of the baffles will be sufficient. The fuel return tube needs to extend all the way to the bottom of the tank, so the discharge is below the fuel level to prevent aeration of the fuel. You may also want to consider adding a drain plug to the sump.
Mount the fuel tank on top of rubber strips to prevent metal to metal contact and install pads on the upper corners of the tank where the hold down brackets or strips contact the fuel tank corners. You will need a least one grounding wire for the gas tank. I installed two ground wires, one directly from the tank to the chassis and a second at the electrical connection for the fuel pump.
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