What the inputs mean
- Target crank power: the power you plan to make at the crank, not at the wheels.
- BSFC (brake specific fuel consumption): pounds of fuel the engine burns per horsepower per hour. It depends on the engine, fuel and tune, so it is an assumption you set. The buttons are common rules of thumb, the same ones the fuel injector size calculator uses.
- Fuel: sets the density that turns pounds into gallons and liters. Gasoline density varies, so 0.74 g/ml is an assumption you can change. Ethanol is 0.789 g/ml at 20 °C. For a blend, the density is weighted by the ethanol volume share.
- Safety margin: extra flow on top of what the engine burns, 20% by default. This is an assumption too, not a figure from a standard.
The formula
fuel flow (lb/hr) = power × BSFC × (1 + margin)L/hr = lb/hr × 0.45359 ÷ density (g/ml)gal/hr = L/hr ÷ 3.7854blend density = e × 0.789 + (1 − e) × gasoline density0.45359 is kilograms per pound, and a density in g/ml is the same number in kg per liter. The blend density treats the volumes as adding up exactly, ignoring the small volume change when ethanol and gasoline mix.
Worked example
A boosted gasoline engine aimed at 500 hp, BSFC 0.60, gasoline at 0.74 g/ml and a 20% margin:
500 × 0.60 = 300 lb/hr burned. With the margin: 300 × 1.20 = 360 lb/hr. That is 360 × 0.45359 = 163.3 kg/hr, and 163.3 ÷ 0.74 = 221 L/hr, or 220.7 ÷ 3.7854 = 58.3 gal/hr.
The same 500 hp on E85 at 85% ethanol and a BSFC of 0.80 needs 480 lb/hr. At a blend density of 0.85 × 0.789 + 0.15 × 0.74 = 0.782 g/ml, that is 279 L/hr or 73.6 gal/hr.
Reading a pump rating
A pump does not have one flow figure. It flows less as the pressure it pumps against rises, and its flow changes with the voltage at the pump. On a boosted engine with a boost-referenced regulator, fuel pressure rises with boost, so check the flow at that higher pressure. The maker's flow chart at your operating pressure and voltage decides whether a pump is big enough, not the number on the box.
The result is maths from the numbers you enter. Check that the lines, filter and regulator can carry the flow too. Running ethanol? Work out the blend in the E85 mix calculator and its stoichiometric ratio in the air fuel ratio calculator. Mixing pump grades? See the octane mix calculator. Estimate power from boost with the boost calculator.
Questions
What size fuel pump do I need for 500 hp?
At a BSFC of 0.60 (boosted gasoline) with a 20% margin, about 360 lb/hr: 221 L/hr or 58.3 gal/hr at 0.74 g/ml. Naturally aspirated at a BSFC of 0.50, it is 300 lb/hr (184 L/hr).
How much more fuel pump do I need for E85?
With the common rule of about 30% more fuel, 500 hp boosted at a BSFC of 0.80 and a 20% margin needs 480 lb/hr, which is 279 L/hr or 73.6 gal/hr for E85 at 85% ethanol.
How do I convert lb/hr to liters per hour?
Multiply by 0.45359 to get kg/hr, then divide by the fuel density in g/ml. At 0.74 g/ml, 360 lb/hr is 221 L/hr. Divide by 3.7854 for US gallons per hour: 58.3.
Why do fuel pump ratings differ from the flow I get?
A pump's flow depends on the pressure it works against and the voltage it receives. A rating taken at low pressure or a high voltage can be well above what the pump gives in your car. Use the maker's flow chart at your fuel pressure.