The fuel injection system on the classic 6.9 and 7.3 liter International-Navistar engine used in diesel fords from the 70s to the early nineties is relatively simple, but some confusion still remains about how it functions. The major components of the system are the lift pump, filter head and filter, fuel injection pump, fuel injectors, return system, selector valve, and the fuel tanks.
The fuel pump or "lift pump" (which is a classic pump used since back in the days of carburetion) pumps fuel into the filter at somewhere between 6 and 12 psi which then passes through another line to the injection pump, or "IP". The IP contains a transfer pump which pulls fuel from the inlet and feeds it to the cylinder inside of the IP. If this pump pulls fuel as fast as the lift pump can provide it, then the pressure in the filter head becomes low. If the lift pump provides fuel faster than the IP can use it, that passes out of the line on the filter head that connects to the return system. The fuel pump is a self-priming diaphragm-type pump, and it pulls fuel through an electrically-operated selector valve which can be switched between tanks.
The injection pump is a Stanadyne DB-2. It pumps the fuel at high pressure through a rotary valve which delivers it to the correct injector, through the hard lines to the injectors. The IP is cooled only by fuel so it is necessary to have excess fuel passing through the IP and being returned to the tank for cooling via radiation and contact with air passing over its surfaces.
The fuel injector permits a metered amount of fuel to pass through into the engine. Uninjected fuel is released through two holes on the sides of the upper part of the injector. An O-ring seals the cap above and below these holes. As long as the fuel is not impeded from this point to the tank, the pressure remains low because pressure is caused by resistance to flow. There is some resistance to flow caused by turbulence while passing through the return lines, but there is nothing else of note to restrict the flow. Fuel is also returned to the active fuel tank through the selector valve, which has two inputs and two outputs so that it can switch both the supply and return lines.
The pickup unit (which also holds the fuel level sender) has two hard lines on it, one for pickup and one for return - the pickup line has a rubber foot with a bypass valve on the side in case it gets clogged, while the return line is terminated with an anti-siphon nipple, which is a little piece of rubber with a sort of slit in it that opens up when fuel flows through and more or less closes off when it stops. This is less effective than an actual check valve, but also introduces far less restriction as there is no spring to work against.
When we discuss the pressure induced by turbulence in a hydraulic system (and this is one, of course, as it depends on a working fluid to transmit force to do work, namely opening the injectors and spraying fuel, which is also the working fluid) we refer to it as pressure drop as it affects the pressure coming out of the other end. However, there is no need for pressure in the return, because no work is done there. Increasing the size of the lines in the return system decreases the pressure drop, which is another way of saying that it decreases the back pressure, or the resistance to flow. Fuel delivery was increased for the 7.3 engine and it is increased still more for turbocharged models, so it would make sense to increase the size of the lines. This was done for the 7.3 liter engine. The 7.3 liter factory turbo motor (which carries a detuned version of the ATS 093 aftermarket turbo kit) has slightly different return line routing to accomodate the turbocharger location, but other turbo kits including the earlier ATS 083 and ATS 088 kits utilize the stock routing as the turbocharger has a different orientation.