Return fuel location

You won’t have a problem.

The return fuel will be close to the same pressure as the fuel rail. Approx mid 40psi.

Ed

The 912iS installation manual states that the return line fuel pressure must not exceed 7.25 PSI.  This is measured at the return line after the engine mounted fuel pressure regulator.  This is so that the regulator has the required pressure drop across the diaphragm to function properly, and this demonstrates the need for minimum 3/8” return line.

I don’t see the same language in the 915 installation manual, but they are both injected engines running the same fuel rail pressure.  I don’t know if this requirement was omitted in the 915 IM by mistake or if it does not apply.  At any rate, the small head pressure added by the rise to the wing tanks would not put you anywhere close to that number, but it would be a reassuring test that you could do. I did measure my return line pressure (912iS) and it was less than 1 PSI.  My fuel returns to the wing tank using a dual selector valve so the fuel is returned to the selected tank.  The return line should come in higher than the fuel supply outlet to avoid vapor bubbles from being drawn back into the fuel system.  

Assuming a decent size return line, the pressure will be the same either way.  Remember that the header tank sees the head pressure from the height of the fuel in the wing tanks.  This is the same as the head pressure a fuel line running into the wing tank will see.  The only difference is the head pressure due to flow resistance in the return line.  If the return line to the header tank is 8’ long and the return line to the wing tank is 12’ long, then you will see slightly higher head pressure due to the resistance to flow of the extra 4’ of line, but that will be minimal at modest flow rates.  You might want to consider upping the return line size by 1/8”, but even that is likely unnecessary.

Hi all

The injected engines use a mechanical fuel pressure regulator placed after the fuel rail just over cylinder number 4.  After it exits the regulator the pressure drops since it travels back to the supply without further restriction.  (other than the distance of travel and if it has to go up from the engine rather than down and the required check valve in return to tank)

Cheers

excerpts from 916 installation manual