You want to have a constant supply of fuel under hard accelleration, such as when you launch at the strip. Before the sump is soldered to the tank you drill several large holes throught the bottom of the tank to fill the sump portion. That's why there's a fuel feed line coming off of the sump. It then feeds to a quality filter, then to the electric pump. I'm installing a second filter in the engine compartment just before my carbs. I'll plug the fuel outlet on my stock sending unit and hook up the wire for the stock fuel gauge. The only modification you'll make to the stock sending unit is to provide a seperate ground from the sending unit to the frame. The stock set-up uses that clip which connects the sender to the fuel line, which is connected to the frame, which in my opinion, is a poor design to start with. Too many places for a bad connection, and a faulty fuel gauge reading. The second outlet on the sump is if you use a return line sysem, or a seperate supply for Nitrous. I just capped mine closed. It also gives you an easy way to drain the tank if need be.