How this calculator works
A fuel pump must deliver enough liquid fuel to support the engine at full power while holding pressure. Required mass flow is brake horsepower times BSFC, and dividing by fuel density converts that mass to litres per hour. Electric pump ratings fall as pressure rises, roughly with the square root of the pressure ratio, so a pump rated at 255 L/h at 3 bar will not deliver 255 L/h at 4 bar. In power mode the page corrects the rated flow to your system pressure before estimating the horsepower it can support. Ethanol blends and methanol need more volume because they carry less energy per kilogram and also have different densities.
Formula
Required mass flow (lb/h) = hp x BSFC. Volume flow (L/h) = lb/h x 0.45359237 / density (kg/L). Corrected pump flow = rated flow x sqrt(rated pressure / system pressure). Supported power = corrected mass flow / BSFC.
Worked example
A 500 hp naturally aspirated petrol engine at 0.50 lb per hp hour needs 250 lb/h. At 0.745 kg/L that is 152.2 L/h, or about 40.2 US gallons per hour. A 255 L/h pump rated at 3 bar delivers about 220.8 L/h at 4 bar, which supports roughly 510 hp on E85 with a forced-induction BSFC of 0.75.
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Frequently asked questions
Is the rated flow measured at the injectors?
No. Pump ratings are usually free flow at a reference pressure, without the restriction of the filter, lines, regulator and injectors. A real installation delivers less, so choose a pump with headroom above the calculated requirement.
Why does the same pump support less power at higher pressure?
An electric pump moves less volume as the pressure it works against rises. The square-root correction is an approximation; the pump curve from the manufacturer is the accurate source.
Does a returnless system need a different calculation?
A returnless system regulates pressure at the pump or uses a pressure sensor, so the pressure at the injectors may not rise one-for-one with boost. Use the pressure the injectors actually see in the system pressure field.
How much safety margin should I add?
A common starting point is 20 percent above the calculated flow, with more margin for a pump that will run at high duty cycle or feed a future power increase. Add the margin to the flow, not to the engine power only.