Hello Paul,

I am trying to determine the size of the return lines and volume of fuel returned as I plan for my EFI 13B.  I may have to create a header tank to handle the return and the question is, how big does it have to be?  Any rules of thumb?

Thanks,

William Hagood

Most people use AN6 lines for fuel.

The volume of fuel returned will be inversely proportional to the HP
being generated. At idle and taxi it could be 80% of the fuel
pumped. So the header tank will fill rapidly. The question then
becomes what do you do with the header tank overflow?

At take off and climb the fuel pumped is a max so the flow to the tank
may be as little as 10 to 20 percent of the fuel pumped.

Done wrong fuel can be pumped over board in short order through
the tank vents.

The traditional and safest way is use 2 tanks and a  two level
valve and send the return fuel to the tank it is pumped from.

A header tank, if used should be able to flow fuel back
into the tank it came from. One gallon should be sufficient.

Defective fuel systems are a major cause of engine
stoppage and crashes. Particularly in low wing airplanes.
They are tricky to design and they appear simple but are tricky
to do right. The lowly vent system design is very important.

One of the big mistakes is pointing  the vents in the
direction of flight as dynamic pressure above 100 MPH
will push the fuel out of the tank in a direction
you may not want it to go. That is the same pressure fed
to the air speed indicator by the pitot tube.

Another mistake is having the vent lines coming out below
the level of the fuel causing a siphon under certain
circumstances.

A third mistake is using a LEFT, RIGHT, BOTH & OFF
fuel selector valve on a low wing airplane.

Low wing airplanes should use only a LEFT, RIGHT & OFF
fuel selector valve.

The best and safest way is to copy certified aircraft
fuel injection systems if possible.

Paul Lamar