and now for the old guy DUM question of the day..........
i have read, and been told many times by many different people, the return line should be LARGER, SAME SIZE, and SMALLER in diameter.
these have been on successful installations, so if it is successful and works, it must be ok.
i like the idea of larger, or possibly the same size. less likely to cause residual heat buildup in the fuel system.
it is also quite possible a smaller line would work if there were not much fuel being returned to the tank.
so, any return line is better than nothing, depending of course, the system design itself ?
Found out it depends on the regulated pressure and pump volume.
Have a setup with the older Mallory 250 GPH pump and by-pass regulator, both -8 AN supply and return.
At idle, the fuel pressure would creep up one or two PSI, go from 7 to 8 or 9 psi.
The pump also has a by-pass, so I set the pumps by-pass to just dump a small amount of fuel back into the fuel cell.
Anyhow, I think High Volume pump and low regulated pressure like 6-7 psi, the return needs to be same size or larger than supply line.
Also, non-boosted carb, the fuel pressure is normally just referenced (or regulated) to atmosphere, so fuel line pressure to vented bowl should stay the same under different loads.
Anyhow, the pressure in the return line has to be lower than the regulated pressure, and pressure is a function on fuel volume, line diameter, and line length. Because the regulated pressure is only a few psi, the return line can't have much pressure in it, or pressure will start to increase.
On my EFI stuff, it is different. The normal 225 lph pump is only about 60 GPH, and "regulated" pressure is 3 or 4 bar = 43.5 psi or 58 psi. By "regulated", that is usually referenced to manifold pressure, so the regulated pressure stays the same across the fuel injector (on return type EFI), but if you have a pressure gauge in the line it only shows the 3 or 4 bar pressure when the engine is not running. Under vacuum the gauge pressure will look lower because the gauge is referenced to atmosphere not manifold pressure.
Anyhow, the largest EFI pump I tested on my car was the Walbro 450 lph (about 119 GPH) at 4 bar with 3/8" supply and 5/16" return and the smaller return line was not a problem.
Because the regulated pressure is way higher, a small amount of pressure in the return line is not an issue.
