Fuel line sizing can be a challenging task for both the feed and return side lines. Something to keep in mind as you plumb the car is the engine power level and the impact it has on line sizing.
From a design stand point there are some significant differences between mechanical and electronic system fueling demands. While the engine's appetite for fuel is similar for similar engines and similar power levels the plumbing can differ significantly. Mechanical pumps reach max flow at max engine rpm. Electric pumps reach max flow the moment you turn them on.
This flow difference has a significant impact on the minimum mandated line sizes for the two systems. In the mechanical system at low engine speed the mechanical pump is also turning at low speed and producing, a predictable low fuel volume. As a result even if there is a bypass requirement back to the tank (and there always is) it is small because the pump output is small.
The EFI system with an electric pump is quite different. Electric pumps pump their max flow rate the moment you turn them on. If you have a 250 HP capable fuel system and a 200 HP engine, when you are driving around town using 25 HP to move the car, you actually return 225 HP worth of fuel (250-25) to the tank. That means a prudent fuel system designer would provide a return capability nearly equal to the feed capability.
The larger the engine power level the more significant this problem becomes. If you have a 575 HP engine with a 600 HP fuel system, for a little headroom, while you are motoring around town using 25 HP your fuel system needs to be capable of handling a 575 HP return of fuel to the tank. That 575 HP fuel return requires plumbing identical to that needed to feed a 575 HP engine at max power.
So what happens if you undersize a return line? You end up faking out your fuel pressure regulator creating an unanticipated rich condition for the engine. While it can be tuned around by a sharp tuner, the engine will be more responsive to the gas pedal and easier to tune for the tuner if the return line is properly sized so it is not giving the fuel pressure regulator a head fake.
Aeroquip has an excellent page on fluid velocity (volume) through their hose and how to select the proper hose sizing to optimize this dynamic. I have attached the page from their tech section with the nomograph that they use for this along with the formula they used to build their nomograph and how to use both of them.
Alas and alack! The Club Cobra vBulletin software has been set up to limit the size of uploaded pdf files to 39K. The Aeroquip page even when downsized exceeds this limit with a size of 81K.
My recommendation is to go to their site, down load their latest catalog which is the 2013 edition and go to their technical data section page #54. They are speaking to
oil velocities and sizing of
oil lines however, the data is applicable to all fluids.
Ed