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EFI Fuel Pump Selection

 

Various factors must be considered when selecting an electric fuel pump for an EFI system, or if you are just trying to determine whether your existing fuel pump can handle horsepower increases to the powertrain.

The most important criterion in sizing an electric fuel pump is calculating the total fuel requirement for the engine.  The formula used to determine total fuel required is similar to the one used to calculate injector sizing.

 

                                              Q = MAXHP x BSFC

 

Using a 400 HP, naturally aspirated engine with a BSFC of 0.45 four our example (.55 BSFC for turbocharged or supercharged engines), we can calculate the total fuel needed to feed this engine:

 

                                               Q = 400 x 0.45 = 180 lbs/hr

Now we need to express this value in volume flow rate such as gallons per hour (gph).  Divide 180 lbs/hr by 6, which is approximately the weight of 1 gallon of gasoline, and the result is 30 gph.  Therefore, we need a fuel pump that can supply a minimum of 30 gph for our example engine.

           

The analysis is not complete.  Fuel pump flow capacity decreases as fuel system pressure increases.  Figure 1 shows flow curves of 2 fuel pumps at various pressures.

                                                       Figure 1

 

When selecting a fuel pump for a particular application, you must know what fuel system pressure the pump will see.  Notice, if you selected pump 1 it would be adequate if you ran your fuel system in the 20-230 psi range.

             

One final consideration in making our fuel pump choice is the pump’s performance at various voltages.  Voltage has a direct effect on pump output.  Apply more voltage and the pump will spin faster, pumping more fuel.  Apply less voltage and the pump will slow down, pumping less fuel.  Figure 2 demonstrates the voltage sensitivity of the fuel pump’s output performance.

                                                     Figure 2

 

For our example, do not choose a pump that flows 30 gph at 13 volts since a slight voltage drop of even 1 volt will cause the engine to run out of fuel at peak horsepower (the worst time to run out).

So when selecting an electric fuel pump, keep in mind the engine’s fuel requirement, system operating pressure and voltage inputs in order to prevent fuel starvation under all driving conditions.

 

 

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