Early 912 ULS and fuel return lines...

Please read this flight safety article regarding the fuel return line.

https://www.rotax-owner.com/en/flightsafety/476-ai476

Yes there is a formula:

Q = 29.81 x C x dˆ2 x √P/S

Q is flow rate in gpm, C is orifice coefficient-use 0.65 for sharp-edged orifice, d is orifice diameter in inches, P is pressure drop across orifice in psi, S is specific gravity of the fluid-use 0.74 for gasoline.

Note: P/S are both under the square root sign. Multiply Q by 60 if you want gph.

I have always thought the Rotax return line restrictor is too small and have seriously considered enlarging mine. I think a greater flow rate, say about 2 gph, would give greater vapor lock protection and could be easily handled by the system.

The only thing good about the very small Rotax restrictor in my view is that it gives a flow rate that is about 10% of normal cruise flow to the engine. This allows me to use a single fuel flow transducer that measures total flow and the 10% error can be easily calibrated out of the displayed value by tweaking with the "K" factor for the Red Cube. I have been using this setup for 7 years and after proper calibration, my fuel flow readout is accurate within about 2%. It would be 100% accurate if all my time was spent at cruise conditions, but there is always a little taxi, takeoff and landing time to throw in the small error which is hardly noticeable.

James Wrote:
" Bill, the return flow is nowhere near 5 gph.... You must be thinking of fuel injected systems where the fuel pressure is far higher... 

You are correct!  My mind gets stuck in the 912is world and I tend to think that anything under 40 psi is low fuel pressure if I don't watch myself.  Too may hats!
But even dumping an additional Half-Gallon an hour into the Balance Tube would have very bad effects.
It would probably double the flow at idle and the Vacuum within the tube wouldn't help either.

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The formual for "Fluid Flow Through an Orifice" is well documented on the internet as are many calculators.