This should only be seen as a basic guide on how to choose a fuel pump. With a little knowledge and data, choosing a fuel pump for your horsepower delivery becomes fairly straight forward.

Weight of fuel:

The average advertised weight of a gallon of premium fuel is 6.34 lb/gallon.

B.S.F.C. – Brake Specific Fuel Consumption

Brake Specific Fuel Consumption or B.S.F.C. is the projected amount of fuel required to produce 1 HP for 1 hour. This means that an engine with a B.S.F.C of .5 will burn 1/2 or .5 lbs of fuel to produce 1 HP for one hour. Determining exact B.S.F.C for a specific engine is complicated and requires an engine dyno.

Based on industry standards the B.S.F.C for:

Normal Aspirated Engines is .45 – .5

Supercharged Engines is .55 – .60

Turbo charged Engines is .6 – .65

Pumping Losses:

Pumping losses is the difference in fuel pressure measured at the fuel pump exit and that of the fuel pressure at the exit of the fuel rail. Friction, fuel line inside diameter and turns or bends in the fuel line is the cause of the pressure difference which can be as high as 4 – 10 PSI.

FPR – Fuel Pressure Regulator:

A fuel pressure regulator or FPR is a piece of equipment that mechanically or electronically controls fuel pressure. Without getting to technical, the FPR’s purpose is to raise or drop the fuel pressure from a preset point by the same amount the pressure in the intake manifold changes. Lets say the FPR is set to a base pressure of 43.5 PSI and the manifold pressure goes up by 10 PSI the FPR now should control the fuel pressure at 54.5 PSI.

Injectors and boost:

Most fuel injector flow rates are advertised statically at 43.5 PSI. This means that fuel flow is measured while the injector is kept fully open and fuel is pumped threw the injector at a pressure of 43.5 PSI into atmospheric. To maintain advertised flow rates the fuel pressure should be raised or dropped by the same amount the pressure in the intake manifold raises or drops. If static fuel pressure is set to 43.5 PSI and manifold pressure raises to 30 PSI fuel pressure should rise to 43.5 + 30 = 73.5 PSI. Max HP supported by an injector is calculated to flywheel power. If you do not raise the fuel pressure under boost the injector will spray less fuel and if you do not lower the fuel pressure under vacuum the injector will spray more fuel.

How to choose a fuel pump

How much fuel do I need for a 4 cylinder turbo charged engine making 500 HP to the crank at 30 PSI Boost?

First we need to calculate total fuel pressure. Lets say the measured pumping loss is 5 PSI

(Base fuel pressure) + (Total Boost) + (Pumping Loses) = Total Fuel Pressure required

43.5 + 30 + 5 = 78.5 PSI

Calculate fuel required:

Target Hp * B.S.F.C. = Fuel required in lbs/hr

500 *.6 = 300 lbs/hr

Most fuel pumps flow rate is advertised in gallons per hour:

Lbs/hr / fuel weight per gallon = gal/hr

300 / 6.34 = 47.32 gal/hr

To make 500 crank HP at 30 PSI of boost we need a fuel pump that will be able to supply 47.5 gal/hr at 78.5 PSI.

From the selection chart on the bottom we can see that a Walbro high pressure 255 ltr/hr at 13.5 volt will supply enough fuel for the HP requirement.

Walbro fuel pump flow rates and current draw at 12 volts

Pressure
(psi)
190 ltr/hr
fuel pump
(gal/hr)
190 ltr/hr
fuel pump
(amps)
“high pressure”
190 ltr/hr pump
(gal/hr)
“high pressure”
190 ltr/hr pump
(amps)
255 ltr/hr
fuel pump
(gal/hr)
255 ltr/hr
fuel pump
(amps)
“high pressure”
255 ltr/hr pump
(gal/hr)
“high pressure”
255 ltr/hr pump
(amps)
0 56 2.6 53 2.8 75 4.7 76 4.4
10 52 3.2 50 3.5 72 5.2 73 4.8
20 48 4.0 48 4.3 67 6.1 68 5.5
30 44 4.8 44 5.0 62 7.2 63 6.3
40 40 5.7 41 5.9 58 8.2 59 7.1
50 36 6.6 38 6.8 53 9.3 54 8.0
60 32 7.6 35 7.8 49 10.6 51 8.9
70 28 8.6 31 8.8 40 11.8 46 9.9
80 22 9.7 29 9.9 29 13.3 41 11.0
90 12 10.8 25 10.9 0 14.6 32 12.0
100 0 12.5 21 12.0     22 13.2
110     17 13.5     11 14.5
120     11 15.0     1 15.8


Walbro fuel pump flow rate and current
draw at 13.5 volts

Pressure
(psi)
190 ltr/hr
fuel pump
(gal/hr)
190 ltr/hr
fuel pump
(amps)
“high pressure”
190 ltr/hr pump
(gal/hr)
“high pressure”
190 ltr/hr pump
(amps)
255 ltr/hr
fuel pump
(gal/hr)
255 ltr/hr
fuel pump
(amps)
“high pressure”
255 ltr/hr pump
(gal/hr)
“high pressure”
255 ltr/hr pump
(amps)
0 63 3.1 60 3.3 82 5.2 85 5.0
10 61 3.5 58 3.9 81 5.9 82 5.2
20 57 4.2 56 4.5 76 6.5 79 6.0
30 52 5.0 52 5.3 71 7.8 73 6.8
40 49 5.8 50 6.2 68 8.6 70 7.5
50 46 6.6 47 7.2 62 9.8 65 8.2
60 42 7.5 45 8.0 58 10.8 61 9.1
70 38 8.4 41 8.8 50 12.0 56 10.2
80 32 9.4 38 9.8 39 13.3 51 11.1
90 22 10.3 34 10.9 11 14.4 43 12.1
100 14 11.5 31 12.0 1 15.0 32 13.2
110   12.7 28 13.2     20 14.3
120     22 14.5     11 15.7
130     18 16.0       17.0