Gravity feed may be used with many high wing installations and some installations which
have a fuselage tank. The fuel pressure requirements of each unit are as follows:
Gravity feed will provide about 1.0 PSI for every 30 inches of head. For example: The
installation of an EFS-2 requires that the bottom of the tank must be at least
60 inches
vertical distance (plus the head loss of the valves, fittings, lines and filters) above
the Throttle Body to provide the minimum required pressure. Remember that this height must
be measured in the steepest climb attitude for aircraft with engines in front, and in the
steepest decent attitude for aircraft with pusher engines. In this worst case
attitude, the fuel system should be flow tested with fuel flowing through the TBI fuel
inlet finger screen, and demonstrate a flow rate 150% of the fuel flow required by the
engine at it's maximum power output, when operating on the last gallon in the fuel tank.
A common installation error made by our customers who wish to install the Throttle
Body, but don't want to install a fuel pump, is to try to design a gravity feed fuel
system where only minimum head in available. This is a dangerous situation because the
Throttle Body will meter fuel adequately at low power settings with fuel pressure well
below the specified minimum, but at high power settings the Throttle Body is unable to
meter sufficient fuel. As a result, the engine will run lean at full throttle.
It is common aircraft practice to design a fuel system which has both an engine driven
pump and a backup pump such as an electrical boost pump, or mechanical wobble pump.
Systems which rely completely on electrical pumps without any backup system should be
avoided.
