Fuel Flow Discussion, Moved from Float Mounting

[zkelley2]

That's actually a really good question as the 1/4 npt would be the smallest diameter at that point. The reason I think it would help is because 3/8 lines can flow plenty of fuel, we know that, but it's usually us, the builders that put some combination of too many bends or too many AN fittings with sharp corners in there that restricts the flow. The AN8 fittings and 1/2 bends will flow more. That's my logic behind the 1/2" lines. But given a straight pipe out the fitting and nothing else, then yes, the 1/4 NPT would absolutely be the choke point and there would be no point.

Actually the fuel valve would be the smallest diameter, because though most of them use 1/4 NPT, the routing inside the valve is incredibly restrictive. Something like 1/3 to 1/4 the area of a 1/4 NPT ID. When I saw that I wondered how much better flow I could get with a better flowing valve.

[Mark Goldberg]

Would 1/2" lines really make a difference with the 1/4 NPT fittings of the gascolator and outlets of the tanks? Mark

[zkelley2]

I'm not sure why someone building and wants gravity fed carb wouldn't just go 1/2 lines. Cost is minimally different and when you're installing it, it makes no difference. The AN fittings cost a couple dollars more each. But there's no way you won't get 150%. You don't have to redesign anything.

[Battson]

Yes, Gold is recommended. By the way, the cost of the Gold cube is as the name suggests...
I did wonder whether the transducer alone would make all the difference. The EFII pump system claims it doesn't impede flow when in bypass, but....

[zkelley2]

You're not supposed to use the red cube on a gravity feed system. Gold or I guess black.

[Battson]

I think it’s well established that a single 3/8 line will provide sufficient fuel under gravity feed conditions.

I think this is true for a carburettor engine with a fuel system built as Bob designed it.

We did the above, then added an electric pump with a bypass built-in, and a Red Cube fuel flow transducer. We couldn't get 150% flow with those things in the line, however we did get the 125% required for a continually pumped fuel system (engine driven pump with electric backup).

So I agree - 3/8 line is more than enough for what most people need. If someone was installing a carb engine with an electric pump only, and a red cube transducer - they might have issues.

[Ed.Meyer]

Return is to main ranks via a duplex fuel valve.

[svyolo]

ED;
Do you return to a header, or the main tank(s)?

[Ed.Meyer]

You are right Brooks that the system returns unused fuel. When I first turn it on prior to engine start, I can hear a short burst of bubbling in the tanks as air is purged. I am not sure if the volume pumped stays the same. Fuel pressure varies some with manifold pressure. Higher MAP has higher FP. The same fuel pumps drive the EFII system for an O540 setup so I think the volume being pump is somewhat higher than the engine demand.than our O360.

[Bcone1381]

Having followed this thread and then seeing the safety notice from Bob regarding fuel pumps with his fuel system design I decided to do a test.
...........
My conclusion from this is that there is apparently enough fuel flow capacity feeding from only one tank via 3/8" lines to supply all the fuel the engine needs plus whatever amount the fuel regulator returns to the tank even with the other tank feed open to the vent. I recognize that this is not an exhaustive test but it was enough to boost my confidence in the fuel system as installed.

I think you are right, Ed. You experiment gives good evidence that the first fuel pump in your system is supplied by gravity. Will a change in power change your fuel flow demand? I don't think so, but not sure. I seems to me like your system has a fixed rate of fuel demand out of the tank regardless of the power setting, and system returns unused fuel. Am I right?

[Nev]

I think it’s well established that a single 3/8 line will provide sufficient fuel under gravity feed conditions.

The issue is what happens once a pump is added to the system, and one line then becomes un-ported.
Will the pump suck fuel from the other line, or will it suck air in preference to fuel from the un-ported line, thereby resulting in a stoppage.

[zkelley2]

I think we know, or we all should know that a single 3/8 port and line can feed enough fuel because we've all done the fuel test recommended by the AC and even asked for by most DARs/FAA people.

[Ed.Meyer]

Airplanes that have carburetors have a float bowl that holds enough fuel for a short idle. Ours will not start with the fuel valve off because the pump will not build pressure. Screams loudly in protest. I think the gascolator probably was a factor in supplying a bit of fuel.
I agree that this could be further tested with climb/decent and lower fuel levels. Given what I learned so far, I think that fuel coming from a single port would be enough since each port is a 3/8” line and even with both ports feeding from a single tank, they tee into a single 3/8” line. Doing this test at real low fuel levels I an not willing to do.

[zkelley2]

Your thoughts are what I have experienced Mark. I have started with the fuel selector off on purpose. It does start, and it'll idle for a few minutes, but putting power to it for taxi I can't get 100ft.
If I started the airplane on the runway in position I don't think I could put full power to it before it'd stumble.

[Mark Goldberg]

I have always thought that an engine would start from the OFF position of the fuel valve - maybe. But taxying out and during run up the engine would quit before the plane flies. I have seen this before a few times. But you are probably correct that all planes are different. Mark