The original intent was to use an Aeromomentum engine which required a return line. That didn't pan out.
Ahh.
A classic PWM port injection has a pressure regulator in the return flow path. The regulator holds the injector supply at a specific pressure above atmospheric regardless of engine demand. Think of it as a
constantly variable restrictor. An alternative introduced later is a regulated pressure electric supply pump and no return line. Given a known pressure and a known injector flow rate at that pressure, the EFI computer merely calculates required injector open time. Injector flow is start/stop/start/stop.
A Bendix RSA type is a constant flow system. That includes Avstar and Precision (both near clones), and Airflow Performance (not a clone, but similar in how it regulates fuel flow). They meter using a pair of connected diaphragms further connected to a ball valve, which feeds the divider. The air diaphragm has venturi pressure on one side and dynamic pressure on the other. The fuel diaphragm has supply pressure on one side and pressure after a jet drop on the other. The combined pressure deltas move the ball valve. Being based on delta, the system is relatively insensitive to supply pressure, and will operate from roughly 15 psi to the limits of its internal seals, possibly as much as 90 psi.
Here's the thing. A Lycoming's engine driven pump is effectively a pressure regulated supply, typically 25 to 30 psi. It's done mechanically by sizing the pump spring. It's also a variable volume pump. An engine cam raises the pump diaphragm to max travel/max spring compression, then releases. With standard RSA plumbing (no return) the spring only pushes the diaphragm down a small fraction of an inch,
in direct proportion to outlet demand. At idle, the movement is tiny. At WOT, it is more. Given a return line with restrictor, and WOT operation, the diaphragm travel would be greater yet. Note that being spring driven, as travel increases, spring force (thus fuel pressure) decreases....not a lot, but it's there.
More critical, note the limited system headroom. Best case, supply is maybe 30 psi, while minimum to run right is roughly 20 psi. If restrictor flow rate plus engine demand approaches pump capacity, supply pressure drops.
All the above is background. Let's return to the previous question.
Went with the Lycoming. The plumbing was there. Avstar recommended the fitting during a discussion with support. I didn't insist on using the return line but it seemed like the guy I talked to liked it. That is certainly speculation on my part from a conversation 10 months ago.
If recommended,
why?
That said, it seems like a system that automatically bleeds down may be beneficial. Is this really different than the AFP purge valve?
Very different. An AFP purge valve is closed during normal operation. There is no return flow with the engine running.