Fuel Injector Size Calculator (cc/min & lb/h from horsepower and BSFC)

Understanding the fuel injector size calculator — concept and principle

This calculator estimates the flow each fuel injector must deliver for a target engine power, from brake-specific fuel consumption (BSFC), the number of injectors and the maximum duty cycle you allow. It gives the answer in cc/min, lb/h and g/min, together with the engine's total fuel flow. Tuners use it when upgrading injectors for more power.

Each injector must flow 523 cc/min of petrol (0.745 g/cm³) while open 80 % of the time.

How it is calculated

Total fuel (lb/h) = power (hp) × BSFC (lb/hp·h)

Per injector (lb/h) = total ÷ (number of injectors × duty cycle)

g/min = lb/h × 453.59237 ÷ 60; cc/min = g/min ÷ fuel density (g/cm³)

Power in kW is converted to hp (1 hp = 0.7457 kW).
Duty cycle is the fraction of time each injector may stay open.

The principle

BSFC says how much fuel an engine burns for each unit of power per hour; it reflects its efficiency and how rich it runs. Multiplying by power gives total fuel flow. That flow is shared by all injectors, and since an injector only flows while open, it must be sized for the flow it would deliver if it were always open, divided by the share of time it is allowed to be open. Converting mass flow to volume flow uses the fuel density.

Worked example

Default inputs: 300 hp, BSFC 0.55, 4 injectors, 80% maximum duty, density 0.745 g/cm³.

Total fuel = 300 × 0.55 = 165 lb/h, or 74.8 kg/h.
Per injector = 165 ÷ (4 × 0.80) = 51.6 lb/h.
g/min = 51.56 × 453.59 ÷ 60 = 390 g/min.
cc/min = 389.8 ÷ 0.745 = 523 cc/min.

Each point is the calculator re-run with only “Number of injectors” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Number of injectors	Injector flow needed (cc/min)
1	2,093
2	1,046
3	698
4	523
5	419
6	349
8	262

How to read the result

Choose the next injector size up, rated at the fuel pressure you will actually run. Garrett gives typical BSFC values of about 0.50–0.60 lb/hp·h for turbocharged petrol engines; richer tunes and boosted engines sit at the higher end. Staying below about 80% duty leaves room for the injector to open and close cleanly.

Common mistakes

Using wheel power instead of crank power.
Ignoring fuel pressure: injector flow rises with the square root of the pressure difference.
Using petrol density for ethanol blends, which also need much more fuel by mass.
Running injectors near 100% duty, where control is poor.

Keep in mind: This is an educational estimate. Confirm sizing with the injector maker's flow data and have engine tuning done by a qualified professional.

Frequently asked questions

What is BSFC?

Brake-specific fuel consumption, the fuel mass burned per unit of power per hour.

Why limit duty cycle to around 80%?

Injectors need time to open and close; near 100% they lose control and may stay open.

Does higher fuel pressure increase flow?

Yes, roughly with the square root of the pressure ratio across the injector.

Sources: Garrett Motion, "How to Select a Turbo, Part 2" (BSFC values) · C. Shih, "Internal Combustion Engines" lecture notes, FAMU–FSU College of Engineering (definition of BSFC)

Checked 9 Nov 2026. Garrett Motion, “How to Select a Turbo, Part 2”: BSFC values for turbocharged petrol engines (about 0.50–0.60 lb/hp·h); fuel flow = power × BSFC (definition of BSFC, C. Shih, “Internal Combustion Engines” lecture notes, FAMU–FSU). 1 lb = 453.59237 g.

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