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Fuel Injector Size Calculator

Enter your target horsepower, cylinders, BSFC and duty cycle.

How it is calculated

Fuel flow needed is horsepower times BSFC, shared between the injectors and divided by the share of time each can be open: cc/min = (HP × BSFC × 10.5) ÷ (cylinders × duty). HP is the target crank horsepower, BSFC is brake specific fuel consumption in lb per hp per hour, cylinders is the injector count (one per cylinder), and duty is the maximum duty cycle written as a fraction from 0 to 1 (0.85 for 85%). The constant 10.5 converts lb/hr to cc/min for gasoline, so the lb/hr figure is simply cc/min ÷ 10.5. The tool also shows HP per cylinder. Everything must be above zero. The result is per injector, rounded to whole cc/min. Fuel type, fuel pressure and injector dead time are not part of the calculation, so treat it as a minimum and check the injector's data sheet.

Worked example

A 400 HP engine with 8 cylinders, BSFC 0.5 and a 0.85 maximum duty cycle. Step 1: total fuel need = 400 × 0.5 = 200 lb/hr, which is 200 × 10.5 = 2,100 cc/min. Step 2: usable injector capacity per cylinder = 8 × 0.85 = 6.8. Step 3: 2,100 ÷ 6.8 = 308.8, shown as 309 cc/min per injector. Step 4: in lb/hr that is 309 ÷ 10.5 = 29.41 lb/hr. Each cylinder carries 400 ÷ 8 = 50.00 HP/cyl. You would pick the next available size above 309 cc/min.

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Common mistakes

  • Typing the duty cycle as 85 instead of 0.85. The field is a fraction; 85 gives a result of 3 cc/min, a nonsense answer that is easy to miss.
  • Using a BSFC for the wrong engine type. Naturally aspirated and boosted engines burn fuel at different rates (boosted engines usually need a higher BSFC), so using the wrong figure over- or under-sizes the injectors.
  • Entering the wheel horsepower instead of the crank horsepower. BSFC is a crank figure, so wheel HP understates the fuel needed.
  • Using the injector count of a different layout. The tool assumes one injector per cylinder; a staged or dual-injector setup needs the flow shared between them.
  • Choosing the exact result as the final injector. Pressure changes, dead time and ethanol blends all raise the real need, so leave headroom.

About this calculator

Sizing fuel injectors correctly is one of the first steps in any engine build or forced-induction upgrade. The required injector flow rate depends on target horsepower, the engine's Brake Specific Fuel Consumption (BSFC) — how many pounds of fuel it burns per horsepower per hour — the number of cylinders sharing the load, and the maximum safe duty cycle (typically capped around 80-85% to leave headroom and avoid injector overheating).

The result is expressed in cc/min (common for aftermarket injectors) and lb/hr (common on OEM spec sheets), letting tuners and engine builders pick injectors that can supply enough fuel at wide-open throttle without running near 100% duty, which causes fuel pressure instability and lean conditions at high RPM.

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Comments (1)

  • Editorial team

    The result is a minimum, and the BSFC you choose moves it a lot. For 400 hp on eight cylinders at 85% duty, a BSFC of 0.5 gives 309 cc/min per injector, while 0.65, closer to what boosted engines tend to need, gives 401. Enter the duty cycle as a fraction, such as 0.85. What BSFC figure are you using for your build?

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