In a full-flow system, the pump still pushes fuel up to the regulator, but it is designed to bypass all fuel not used back to the tank by way of a separate fuel line routed back to the tank. This system offers several advantages including a more dynamic versus a static system that reduces unwanted pressure drops. In a dead-head system, the check valve (regulator) is located between the fuel pump and the carburetor. This means line pressure is generally higher up to the regulator and reduced going into the carburetor. This higher system pressure is necessary to overcome g-forces during the launch and to push the fuel through a restrictive regulator because the pump must start that column of fuel moving from a static position. In a full-flow system, the regulator is downstream of the carburetor and is configured so that the pressure to the carburetor is the same as line pressure. The full-flow system still has to overcome g-forces, but the pressure required to do this is less because the fuel is already moving in the system. Another advantage of a full-flow system is that with the engine at idle and the charging system at full efficiency, you can also set the fuel pressure more accurately and expect it to remain constant regardless of load. That doesn't happen with a dead-head system.
The downside to a full-flow system is more lines and fittings and increased expense, since this requires a more complex regulator as well as a separate line back to the tank where a return must be built into the fuel tank. Another critical point is that high-volume carbureted fuel-flow systems demand a large return line. For a big pump like a BG King Demon, Aeromotive A-1000, or any large pumps for a carbureted application, the recommendation is no less than one size smaller return line than the feed line. This is due to the large volume of fuel returned to the tank at idle at carbureted fuel pressures. If you cannot set the line pressure low enough to suit your needs with the engine not running (and sufficient voltage to the pump), this is usually an indication of a restrictive return line. EFI systems do not require as large a return line since these systems generally operate at 43 to 65 psi and a slight 4-5-psi return-line pressure will not affect fuel flow.
Flow TestingIf there is one point worth emphasizing with any fuel delivery system it should be to eliminate the 90-degree fittings. Anytime you pressurize a liquid-transfer system, a tight-radius 90-degree change in direction will cause a flow loss. The worst of these fittings are those cheap, brass fittings you can buy at the auto parts or hardware store. Since pumps push better than they pull, it's also best to improve the inlet side as much as possible, usually with a larger inlet line. To test these ideas, we built a bench-top 31/48-inch fuel system with a Mallory 140 pump with six nasty brass 90-degree fittings and measured the time it took to pump four gallons of solvent from one tank to another. Then we tossed all the ugly fittings and replaced them with straight -6 AN fittings. The time difference between the two systems was not dramatic, but we did see a 10 percent improvement in flow. This was not a true fuel-delivery test because we were not running the pump against 5 or 6 psi of pressure, but it does point to the importance of minimizing flow restrictions.