How this calculator works
An injector must supply enough fuel for the engine at its highest power, while staying below a safe duty cycle so it does not overheat or lose control near the top of its range. Total fuel flow is brake horsepower times brake specific fuel consumption. Dividing by the number of injectors and by the duty cycle gives the flow each injector must provide while open. The page converts that mass flow to cubic centimetres per minute using a typical fuel density for the selected fuel. Injectors are rated at a reference pressure, usually 3 bar, so the page also shows the equivalent 3 bar rating for the pressure you entered.
Formula
Total fuel = brake hp x BSFC. Injector flow = total fuel / (cylinders x duty cycle). cc/min = lb/h x 453.59237 / (density in g/mL x 60). A 3 bar rating scales with the square root of the pressure ratio.
Worked example
A 500 hp gasoline engine running naturally aspirated at 0.50 lb per hp hour needs 250 lb/h in total. With eight injectors at 80 percent duty, each must flow 39.06 lb/h, which is about 396 cc/min or 295 g/min at a gasoline density of 0.745 g/mL.
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Frequently asked questions
Why not run injectors at 100 percent duty?
An injector held open continuously cannot respond to the next pulse and runs hotter. A duty cycle around 80 percent leaves headroom for rich mixtures, fuel pressure variation and a slightly higher future power target.
Does higher fuel pressure increase injector flow?
Yes. Flow through a fixed orifice rises with the square root of pressure, so going from 3 to 4 bar increases flow by about 15 percent. Raising pressure also loads the pump and can change the spray pattern, so do not use it as the only way to add capacity.
Why does the result differ from an injector catalogue?
Catalogues quote flow for a specific test fluid and pressure, often gasoline at 3 bar, and may use a different density. Use the cc/min and lb/h figures together and check the supplier rating at your actual pressure.
Which BSFC should I choose?
The page uses typical values: about 0.50 for naturally aspirated gasoline, 0.60 for forced induction, and higher values for E85 and methanol because they carry less energy per kilogram. A dyno measurement of your engine is always better.