It's just a fuel tank. Simply a receptacle for holding the precious juice from which horsepower is made. The stuff isn't in there very long, how can you go wrong? Just build a box, put a cap on it, a bung on one end and we're headed for the dragstrip!
On a fuel injected drag vehicle, many factors involving the fuel tank should be thoroughly considered. Design it carefully the first time or you may end up building several tanks or making modifications as you learn things the hard way.
The first consideration is volume...how big should it be? If you're running a large motor on methanol, you'll use at least several gallons for each burnout/backup/stage/run cycle. If you plan to drive your car to the line and back from the top end, consider a 3-4 gallon capacity the bare minimum. Due to increased fuel volume consumed, an injected nitro or supercharged methanol application will consume even more. It's less likely that those vehicles will be driving to the lanes and back to the pits however, so 4-5 gallons is in the ballpark. Naturally, a blown nitro vehicle may need 2-3 times this much capacity depending on boost and other factors.
A good rule of thumb is to figure out how many gallons per minute your fuel system is delivering into your motor at 8000 RPM. Round this number up to the nearest gallon and use that for your total tank capacity. For example, a motor may only use 60% of that 6.2 GPM fuel pump (rated at 8000 engine RPM) meaning 3.72 GPM is delivered to the motor. Round this up and build a 4 gallon tank to ensure you have plenty for a pass. Remember, you don't want to use it all - there better be some left in there after a run! Sucking air at the top end because the tank is a bit small and you just decided to do a half track burnout could mean melting down in the lights.
Some folks build huge tanks figuring they need ballast on the car anyway. Their thinking is that they might as well make the ballast useful. I can't go along with that for several reasons. The first is safety: the less fuel you have on board in the event of an accident, the better. The other is that fuel only weighs around 6 pounds per gallon...it isn't very efficient ballast. Plus, fuel tanks are very seldom clear out on the nose of a car. To be efficient ballast and have the most effect while adding a minimum of overall weight to the vehicle, a fuel tank several feet back from the nose isn't a good plan.
The shape given to your fuel tank will of course vary with your application. If you can, it would be nice to design the shape of your tank to direct or funnel the fuel at the outlet when under acceleration. It can be tricky getting the shape you want and the volume you desire at the same time. Building a mock-up tank out of cardboard will allow you some cheap trial and error while providing a nice flat pattern to transfer to the real material when you're ready to start cutting metal. If you're building a tank with a consistent cross section in at least one direction, figure out how many square inches you'll have in one "slice" - draw a grid and count the squares. Then multiply that by the length to get volume in cubic inches. Divide the volume by 231 to find out approximately how many gallons your proposed creation will hold.
Aluminum is the material of choice for fuel tanks that require fabrication. It's light, easy to machine, bend, weld, and polish. Consider .080" the minimum thickness for this purpose. This is still very workable but not as likely to split, dent or wear through at contact points over time like thinner stuff. Having the inside of your aluminum tank anodized is a terrific touch to prevent corrosion and ensure long life. The process involves completely filling the tank with an electrolyte solution and inserting a lead cathode through the filler hole. A large DC current is applied to the tank and hard anodic cells form on the inner surfaces of the tank. The tank must be tilted such that the hydrogen given off during the process does not build up in the tank, displace the electrolyte inside and create an explosion hazard.
The outlet size on your tank will be determined by your application. A motor drinking gasoline will only need a -8 line to supply the pump. A small, naturally aspirated motor on alcohol can do with a -10 line just fine. Blown alcohol situations should consider -12 the minimum. Can it be too big? The larger the hose you use on the inlet side, the harder it will be to prime your system when first starting up for the weekend. A large hose will contain a lot of air that must be displaced and expelled by the pump. If your pump is higher than the level of fuel in your tank, you'll have an even harder time starting with a large hose as compared to a smaller one. Also, the larger the hose, the greater the possibility of it sucking flat while under heavy demand. Using a piece of hardline with just enough rubber hose to join it to the tank and to the pump will prevent that possibility. Stainless steel spring-like material can also be purchased and inserted into suction side hoses to keep them open.
Just as important as the size of the hole letting fuel out, is the size of the hole letting air into the tank as the fuel leaves. You'll never want any kind of vacuum in the tank or this will effect your overall tuneup and cause consistency problems at best. A good rule of thumb for this is to have at least HALF as much vent area as outlet area. Why only half? Well, air flows through a hole much easier than fuel, so having equal area is not really required. Having a huge vent in the tank can also create a problem with fuel sloshing out. If the guy at the starting line sees any fuel dripping onto the track from your car, he will shut you down - as well he should. After backing up, stopping and pulling forward into the beams, your fuel will be sloshing back and forth in the tank if there is any air space inside at all and the vent could dribble. Be careful that you don't setup a siphon situation with your venting. Once fuel begins to run out of a vent, it may continue to siphon out until the fuel is below where the vent meets the tank!
Some organizations require tip-over valves on fuel tank vents to stop spillage in the event of disaster. Check your rule book or just plan to use them. Vented caps are almost universally against the rules everywhere and never provide enough vent area anyway. Plan your venting strategy around a sealed cap and consider your outlet size.
Fuel injected vehicles usually return some fuel to the tank, although more and more folks are returning their fuel to the inlet side of the pump instead. This method works fine and actually serves to decreases the demands on fuel exiting and air entering the tank. It does tend to increase fuel temperature rather quickly however, and consistency can suffer as a result. Also, any air in the system (such as is trapped behind a high-speed check valve) can get recirculated many times when the check valve opens before being completely ingested by the engine. This too can cause consistency problems. The imporant thing about returning fuel to the tank is to do it in such a way that air bubbles do not reach the outlet and get sucked in instead of fuel. Introduce the returning fuel as far from the outlet as possible to give the bubbles time to settle down. The best way to get rid of bubbles is not to make them at all! If the fuel return port utilizes a tube that extends into the tank below the level of the fuel, it won't be blasting a stream of fuel through the air space into the liquid creating a bubbly froth in the tank.
A very common fuel tank related problem is fuel starvation. This serious issue is easiest to solve during initial design, so think hard on this before you start welding things up. If you don't plan for it now, chances are you will have to deal with it later. Generally, the symptom is a motor that quits when the car comes to a halt after the burnout. When the brakes are applied and the car begins backing up, the fuel moves forward in the tank and the pump sucks air. One big gulp is all it takes to kill the motor and restarting can be very difficult. The driver should be "in the zone" at this point and not worried about the motor starving for fuel. One way to almost guarantee a crappy reaction time and a lost round of competition is a tense and harried restart half way back to the starting line!
A couple of strategies can be employed to prevent starvation. The simplest is to ensure that there is as little air space as possible in the tank during this critical time. Fill the tank as full as possible before heading to the lanes. Remember that warming the car before the burnout and roasting the hides to half track uses a lot of fuel. This translates to air in the tank as you back up from the burnout.
Feed the fuel pump from as low as possible in the tank so that even if fuel moves forward, the air space remains above the outlet. Place the outlet fitting right at the bottom edge of the back of the tank. You might even extend a tube into the tank from the outlet with a cap on the end and put an adequately sized slot in the very bottom of the tube to pull fuel from as low as possible.
Another method is to utilize a bulkhead near the back of the tank with a swinging door. The vertical bulkhead should have several large holes in it near the bottom. A piece of aluminum piano hinge on the rear of the bulkhead attached to a flapper will cover the holes on deceleration and keep fuel around the outlet. It will swing backward to allow fuel to the outlet while accelerating. The flapper needs to remain very loose, so the snug hinge pin on the flapper should be removed and replaced with a loose fitting piece of aluminum welding rod to allow free motion at all times. The ends of this new hinge pin should be left long and bent such as to prevent the flapper from swinging up past 90 degrees. The flapper bulkhead can be assembled separately and easily inserted up into a slot in the bottom of the tank.
Since the fuel tank is a fairly heavy component when full, it is a typical victim of tire shake. Cars can shake so hard, they easily split the fuel tank or tear off the mounting tabs welded to it. Instead of rigidly mounting your tank, consider building a nest for it to sit in and capture it with metal straps to let it "float" in place. Be sure to use rubber, plastic or something similar between contact points to prevent wearing through due to vibration and flexing.
While building your tank hold-down system, you might want to make it easy to detach and remove the tank. Draining the tank completely can be a big benefit when putting the car to bed and this is much easier when you can lift it out of the chassis. You may also decide to change the fuel recipe and run a different percentage of nitro for a round or two. Removing the tank to pour it all back into a jug for quick remixing and checking may make life much easier between rounds.