Fuel return Line for a 912UL

Good deal!

I didn't know if you had any experience with the "Jewler's Tools" and didn't want any suprises.

It sound like you are good to go!

Bill,

In my book, more information is always accepted and better than too little.  

My life long goal has always been to learn something new every day.  

Thank you for sharing your knowledge and experience. 

Surprises with expensive aircraft most of the time do not end well. 

And I'll keep posting about this to share the final solution.   

Just out of curiousity I was wondering how that 0.35mm Orifice would work in a 45psi fuel system.

The flow is proportional to the square of the diameter and the square root of the pressure.

45 psi is real close to 16 times your 2.8psi, so we would expect to see about 4x the flow.

4 x 1.875 = 7.5gph.  A litle higher than I would like to see but definitely within the ballpark of reasonable.

What we want is about 2.6x the flow for 5gph, so about 7x the pressure.  ~20psi would be about spot-on for the 0.35mm Orifice.

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In hindsight, I think the driving factor is that 0.35mm is the closet reasonably sized Fuel Jet that already has a part number.

I want to tell you somyhing else but I keep forgetting....

Now I remember!!!

The Engine driven Mechanical pump will have a different fuel pressure from the AUX pump.

Do you know what that pressure is?

We can adjust the flow seen on the AUX pumps so the MECH pump, that is used continuously, Bypasses the nominal flow quantity.

I am very curious about the flow rates you guys are talking about thru the fuel bypass orifice of 0.35 mm (0.014"). The 5+ gph you mention is WAY above what would be calculated thru that size orifice. The engineering formula for orifice flow is:  GPM= 29.81 x Cd x dsquared x square root of (pressure drop/specific gravity. Cd (orifice coefficient) for a sharp edged orifice jet is ).85, dsquared is 0.014" squared=0.000196, pressure drop is lets say 5 psi for a 912uls, specific gravity of gasoline is 0.74. Put all this into the formula and the result is 0.013 gpm x 60 = 0.77 gph. In my case my fuel pressure runs about 3.5 psi during cruise, so fuel bypass flow would be even lower, about 0.66 gph.

I know Jeffery says he actually tested the much higher flow rate, but something is fishy here. Either he had a much larger orifice jet, or much higher pressure at the orifice because the laws of physics must be satisfied.