Your instincts were spot-on, Bill. I attempted to post this earlier today, but the website was uncooperative.
In the FWIW bucket, for a long time, today included, a school of thought has argued to place the return line near the bottom of the tank where it is impacted by the "g" forces acting on it from braking, turning and acceleration. When you do this, you place a
variable pressure feedback in the return line that is equivalent to the amount of fuel in the fuel tank multiplied by the "g" forces acting on it. The pressure feedback is greatest when the tank is full and least when it is near empty.
Some fuel system architects take an agnostic, 'it doesn't really matter,' point of view. In the bigger picture, it will not turn the engine's fuel delivery system upside down. Still, it will place a variable back pressure on the system's fuel pressure regulator, changing the differential pressure across the injector without notifying the ECU.
The impact is essentially indistinguishable if you are tuning a drag race application. If you are tuning a daily driver or a road race car, then the driveability factor is impacted by the changing back pressure in the return line, and it can not be tuned around because it is a variable based on how much fuel is above the return port in the tank, whether the car is turning left or right, accelerating or braking.
The changing backpressure against the pressure regulator diaphragm modifies the measured pressure differential across the injector. The differential pressure across the injector is essential because it is what the ECU uses (along with several other metrics) to calculate injector pulse width. The injector pulse width calculation is critical to delivering the required fuel volume necessary to meet the target AFR the tuner has commanded for the engine under varying load conditions.
Is it the end of the world? Not at all. The engine will run either with or without the return line back pressure. The back pressure will, however, create a richer than desired fueling condition that the engine would operate better without. It is and becomes a part throttle engine responsiveness and driveability issue.
When you buy EFI fuel pump baskets from the major EFI system and component manufacturers, they will position the return line plumbing on the top of the fuel pump basket hat sometimes for simplicity, sometimes for correct fueling and sometimes for both.
I have included two pics of pump baskets below. One is from AEM, and another is from Fore Innovations. Both pump baskets include the return plumbing at the top of the hat. If you look at any other providers, they do the same thing, for all the reasons we have already covered.
If the fuel system designer does not use one of the commercially available pump baskets and places the pumps external to the tank, the fuel return should still be at the top of the tank. The most commonly expressed concern for this placement is the aeration of the fuel.
Fuel aeration is a red herring! Did you ever try to put the fizz back into a flat beer — it can't be done with the equipment any of us have at our disposal. The same thing with gasoline that you are attempting to add 'fizz' back into — it doesn't happen!
If the fuel system architect elects to use external pumps, it is very easy to put the return line anywhere they desire without welding. You use an AN style bulkhead fitting with gasketed washers from McMaster, Grainger, or one of the other industrial supply sources. The gasketed washers will withstand 1000 psi and more, so there are no leaks to worry about and no welding to do on a tank with gas fumes potentially still inside it.
Here is a link to a Fore Innovations fuel basket / hat,
Go to this URL =>
https://www.foreinnovations.com/Cobra_fuel_hat_p/55-800.htm and click the "
Larger Photo" button in the middle of the page below the picture to see the top side fuel return used by Fore Innovations. You will clearly see the return port on top of the fuel pump basket.
The pic below is the Aeromotive approach to plumbing the return fuel line. Again you will find it on top of the fuel pump basket.
Is it a show stopper if you do not plumb it to the top — absolutely not! Will it improve your tune and the car's driveability at other than wide-open throttle? Absolutely, yes!
The in-tank pump, while difficult to implement in an FIA style tank, is the preferred pump location for feed, temperature control, vibration, and noise. Unless there is something I am missing about the FIA tank, your implementation will need to be outside the tank. It still works fine; Detroit plumbed their early systems this way for ten years or more before moving the pump inside the tank.
