Originally Posted by
rickair7777
TYPICALLY for all piston engines (for specifics see your airplane's AFM)...
Carbs: The boost pump and/or gravity supplies fuel to the carb. All you're doing is filling the float bowl at near-ambient pressure.
On a low-wing you need the pump, on a high-wing gravity alone is usually enough. The boost pump on a high wing is a back up to gravity...ie not strictly required unless you fly with sustained low g's, have some kind of flow resistance in the lines/filter to the carb, or experience a total gravity failure.
On some high wings gravity might not provide enough flow through the filter for sustained high-power ops (ie takeoff)...in which case boost pump use is mandatory.
Fuel Injection:The boost pump and/or gravity delivers fuel from the tanks to the inlet of the main fuel pump which requires a slight positive pressure head. The main pump boosts that pressure much higher, probably to about 30-50 psi for the injectors.
A failed boost pump may not stop the engine, but a failed main pump will. The boost pumps or gravity cannot supply more than 2-3 psi, nowhere near the 30-ish required for fuel injectors.
Again, read for AFM.
What you posted may be true for some aircraft but certainly not for all. Electric boost pumps come in all pressures and flow rates. On many aircraft the electric pump will run the engine just fine in the event the engine driven pump fails. In fact in the experimental world many are removing the engine driven pump and going with electric pumps only on injected systems. This approach has some real advantages with vapor lock reduction when using auto fuel.