Fuel Pump Calculator

How fuel pump sizing works

At full power the pump has one job: deliver all the fuel the engine burns, at the pressure the system runs, with flow to spare. How much fuel the engine burns comes from the same number used to size injectors, its brake specific fuel consumption (BSFC): crank horsepower × BSFC gives pounds of fuel per hour. Pumps are rated by volume, so the calculator divides that weight by the density of the fuel to get liters and gallons per hour.

Pressure is the other half of the answer. A pump moves less fuel the harder it has to push, so a flow figure only means something together with the pressure it was measured at. With a boost-referenced regulator, fuel pressure rises one psi for every psi of boost, so on a turbocharged or supercharged engine the pump faces base pressure plus boost, plus whatever the filter and lines lose at full flow, at the very moment it has to deliver the most fuel. A regulator that is not referenced to boost holds the same pressure all the time, and the pump only works against the base pressure.

Voltage matters as well. An electric pump spins faster and flows more at higher voltage, so ratings are quoted at a stated voltage, and a pump that sees less at its terminals under load flows less than its chart says.

What to enter and where to find it

• Crank horsepower. The power you expect at the flywheel. BSFC is defined at the crank, so convert a chassis dyno figure with the whp to hp calculator first.
• Aspiration, fuel and BSFC. They work as in the fuel injector calculator: leave BSFC blank for the typical figure for that fuel and aspiration, or enter your own from dyno data. E85 is taken at 83% ethanol, the most fuel-hungry blend sold under that name.
• Base fuel pressure and boost. The regulator setting without boost, and the highest boost the engine will see. Leave boost at zero for a naturally aspirated engine.
• Regulator. Boost-referenced (1:1) adds the boost to the pressure; a fixed regulator does not.
• Headroom. Extra flow on top of what the engine burns. It starts at zero so the result is the bare requirement; see below for why most builds add some.
• Pump flow and its pressure. From the pump’s flow chart. Read the flow at the working pressure the calculator shows if you can. If you only have a single rating, enter it with the pressure it is quoted at, and the calculator will tell you when that rating flatters the pump for your system.

The fuel pump flow formula

Six relationships. The unit conversions are exact; the estimate is in the BSFC, and in how far the pump’s real flow at your pressure and voltage differs from the figure you enter.

1. Fuel flow (lb/hr) = crank hp × BSFC
2. Pump flow (L/h) = fuel flow (lb/hr) × (1 + headroom) × 0.4536 ÷ fuel density (kg/L)
3. Pump flow (gal/h) = pump flow (L/h) ÷ 3.785
4. Working pressure = base fuel pressure + boost (boost-referenced 1:1 regulator)
5. Working pressure = base fuel pressure + rate × boost (rising-rate regulator)
6. Horsepower a pump supports = pump flow (L/h) × fuel density ÷ 0.4536 ÷ (1 + headroom) ÷ BSFC

The densities are 0.74 kg/L for gasoline (US gasoline varies from 0.72 to 0.78 with the blend and the season), 0.781 kg/L for E85 at 83% ethanol and 0.791 kg/L for methanol. As a quick check, a naturally aspirated gasoline engine at a BSFC of 0.50 burns 50 lb/hr for every 100 hp, about 30.6 L/h or 8.1 gal/h.

The last formula is the first two run backward: it turns a pump’s flow into the power it can feed, which is how pump makers arrive at their horsepower ratings. Those ratings assume a fuel, a BSFC and a pressure, so recalculate them for your own combination rather than comparing headline numbers.

Fuel pump flow and pressure

Every pump has a flow chart, usually with one curve per voltage: flow on one axis, pressure on the other, and the flow falling as the pressure rises. One maker’s own figures for a large pump show it losing about a third of its flow between carburetor pressure and the highest pressure on its chart, and gaining more than a fifth at high pressure when the voltage at the pump rises from battery level to what a running charging system supplies.

That is why the calculator compares your requirement with the flow at its own pressure and warns when that pressure is below your working pressure: on your engine the pump will deliver less than that figure, and the true number comes from its chart at the working pressure. Fuel pump flow ratings with no pressure at all, or at a pressure far below yours, are the most misleading of all.

A rising-rate regulator or FMU adds several psi of fuel pressure per psi of boost: working pressure = base + rate × boost, so a 4:1 unit at 10 psi of boost on a 43 psi base pushes 43 + 4 × 10 = 83 psi. Enter that figure as the base pressure and set boost to zero.

Fuel pump size: how much headroom?

The formula gives the fuel the engine burns. The pump usually has to move more than that. On a return-style system some fuel must keep flowing back through the regulator to hold the pressure steady, and the voltage at the pump under full load is often lower than the one the chart was drawn at.

Fuel system and engine management makers advise allowing for this (many engine builders add a percentage to the pump volume, and the real flow injectors and pump must supply is likely to exceed the calculated figure), but they do not agree on a single figure, so this fuel pump size calculator leaves headroom at zero and shows the bare requirement. Enter the margin you want, or read the result as a minimum. The Horsepower tab uses the same headroom, so a pump supports less power there as you add margin.

Worked example

Three typical questions, worked out with the same functions that run the calculator.

Typical fuel flow per 100 hp

Fuel flow scales directly with power, so the quickest way to judge a pump is by how much flow it needs per 100 hp. The table uses the calculator’s default BSFC for each fuel and aspiration (gasoline from published sizing ranges; E85 and methanol scaled from gasoline by their stoichiometric ratios, but never below a fuel-system maker’s published minimum for that fuel) and no headroom. Multiply by your power in hundreds of horsepower and compare with the pump’s flow at your working pressure.

A boosted gasoline engine needs about 30% more flow per horsepower than a naturally aspirated one, and the same boosted power on E85 needs a further 40%. Naturally aspirated, methanol needs 2.1 times the volume of gasoline at the same power. Turbocharged and supercharged methanol engines are the row that stands out. For them the table and the calculator use 1.80 lb/hp·hr, the bottom of the 1.80–2.00 range in a fuel-system maker’s sizing guidance. That maker presents its BSFC figures as observations from experience, and this one reflects boosted methanol engines run very rich, with the extra fuel used to cool the charge and the combustion chamber, as alcohol racing engines often are. It is about 22% above the same-lambda estimate of 1.48, so for a street or track turbo engine run nearer the maximum-power mixture it oversizes the pump. If yours is calibrated that way, enter a BSFC closer to the estimate, or one from your own dyno data.

Common mistakes

• Reading a free-flow figure. A flow number with no pressure, or at a pressure well below yours, overstates what the pump delivers on the engine.
• Forgetting boost. With a boost-referenced regulator the pump in the example pushes 58.5 psi at full boost, not 43.5 psi.
• Using wheel horsepower. BSFC is per crank horsepower, so a chassis dyno figure understates the fuel by the drivetrain loss.
• Treating every fuel alike. E85 and methanol need far more volume for the same power: in the example the same power needs 280 L/h on E85 against 199 L/h on gasoline.
• Ignoring voltage. Charts are drawn at a stated voltage. A pump that sees less at its terminals under load flows less than the chart shows.
• Taking a horsepower rating at face value. A pump’s horsepower rating assumes a fuel, a BSFC and a pressure that may not be yours. Recalculate it from the pump’s flow at your working pressure on the Horsepower tab, and size the injectors from the same fuel flow with the fuel injector calculator.