How do I calculate the injector size I need for a horsepower target?

Multiply the target horsepower by BSFC to get total fuel flow in lb/hr, then divide by the number of injectors and the maximum duty cycle. For 400 hp at 0.60 BSFC on 8 cylinders at 80% duty, each injector needs 37.5 lb/hr, about 394 cc/min.

What BSFC should I use?

Typical starting values are about 0.50 lb/hp·hr for naturally aspirated gasoline engines, 0.60 for turbocharged, 0.65 for supercharged, and 0.80 for E85. They are starting points. The right value depends on the engine and the air-fuel ratio, and should be confirmed on a dyno.

What duty cycle should I design for?

Leave headroom. Around 80% at peak power is a common design ceiling, which keeps the injectors from running open and leaves room for the engine to need more fuel than predicted.

How does fuel pressure change injector flow?

Flow through a fixed orifice scales with the square root of the pressure difference. An injector rated 37.5 lb/hr at 43.5 psi flows about 43.3 lb/hr at 58 psi. Under boost, the useful pressure is the fuel pressure minus the manifold pressure.

How do I convert lb/hr to cc/min?

Multiply by about 10.5 for gasoline. The exact factor is 453.6 divided by 60 times the fuel density in g/cc, so denser fuels like E85 give a lower factor, near 9.7.

Fuel Injector Size Calculator

Fuel injector flow needed for a target horsepower from BSFC, cylinder count, and duty cycle, in lb/hr and cc/min, with a fuel pressure correction.

Scratchpad (not saved)

Save to create a persistent, shareable workspace.

Target Crank Power

Target power at the crank, not the wheels.

Power Unit

Unit for the value entered above.

Number of Injectors

One per cylinder for port injection.

BSFC (lb/hp·hr)

Custom BSFC

lb/hp·hr

Used only when BSFC is set to Custom.

Maximum Duty Cycle

A fraction between 0 and 1. 0.80 is a common ceiling.

cc/min per lb/hr

10.5 for gasoline. It depends on fuel density: 453.6 / (60 × density in g/cc).

Injector Rated Pressure (optional)

Pressure at which the injector rating is stated. Use 0 to skip the pressure correction.

Operating Fuel Pressure (optional)

Pressure the injector will run at. Use 0 to skip the pressure correction.

Pressure Unit

Unit for both pressures.

What This Calculator Does

Injectors have to supply the fuel an engine burns at full power, with room to spare so they never run open all the time. This calculator turns a horsepower target into the flow each injector needs, using brake specific fuel consumption (BSFC), the number of injectors, and a maximum duty cycle. It reports flow in lb/hr and cc/min and corrects an injector's rated flow for a different fuel pressure. The BSFC presets are typical starting values to confirm on a dyno, and the cc/min conversion depends on the density of the fuel, so check it if you run something other than gasoline.

It combines Target Crank Power, Power Unit, Number of Injectors, BSFC (lb/hp·hr) to estimate Flow needed per injector, Flow in mass units, Total fuel flow at target power.

Formula & Method

Required flow per injector:

\dot m_{inj} = \frac{\text{HP} \times \text{BSFC}}{N_{cyl} \times DC}

in lb/hr, with BSFC in lb/hp·hr and

DC

the maximum duty cycle as a fraction. Conversion to volume:

\text{cc/min} \approx \text{lb/hr} \times 10.5

for gasoline. The factor is

453.6/(60\,\rho_{fuel})

with

\rho_{fuel}

in g/cc, so it changes with fuel density. Pressure correction:

F_2 = F_1\sqrt{\frac{P_2}{P_1}}

where

F_1

is the flow at the rated pressure

P_1

and

F_2

is the flow at the operating pressure

P_2

, since flow through a fixed orifice scales with the square root of the pressure difference. Use the pressure difference across the injector (fuel pressure minus manifold pressure) for

P_1

and

P_2

when you know it.

Notation used in the formulas: $R$ = Flow needed per injector; $x_{1}$ = Target Crank Power; $x_{2}$ = Power Unit; $x_{3}$ = Number of Injectors; $x_{4}$ = BSFC (lb/hp·hr); $x_{5}$ = Custom BSFC; $x_{6}$ = Maximum Duty Cycle.

Method summary: inputs are normalized to consistent units, core equations are evaluated, then secondary values are derived and rounded for display.

Use it to pick an injector size for an engine build with a power goal, to check whether current injectors can support more power, or to see how raising fuel pressure changes the flow. Always leave margin so the injectors stay below about 80% duty at peak power.

Worked Examples

400 hp turbo V8, gasoline (imperial)

Given: 400 hp at the crank, BSFC 0.60 lb/hp·hr (turbo gasoline), 8 cylinders, maximum duty cycle 0.80 Method: flow per injector = HP × BSFC / (cylinders × duty cycle). Total fuel flow = 400 × 0.60 = 240 lb/hr Per injector = 240 / (8 × 0.80) = 37.5 lb/hr = 393.8 cc/min (× 10.5) An injector rated 37.5 lb/hr at 43.5 psi flows 37.5 × √(58/43.5) = 43.3 lb/hr if run at 58 psi. To get 37.5 lb/hr at 58 psi, buy an injector rated 37.5 × √(43.5/58) = 32.5 lb/hr at 43.5 psi.

300 kW E85 six-cylinder (metric)

Given: 300 kW crank power = 402.3 hp, BSFC 0.80 (E85), 6 cylinders, duty cycle 0.85 Method: convert kW to hp first, then the same equation. Total fuel flow = 402.3 × 0.80 = 321.8 lb/hr Per injector = 321.8 / (6 × 0.85) = 63.1 lb/hr = 28.6 kg/h Volume using 10.5 cc/min per lb/hr gives about 663 cc/min, but E85 is denser than gasoline. With E85 at roughly 0.78 g/cc the factor is closer to 9.7, giving about 612 cc/min. Check the fuel density.

Typical BSFC Starting Values and Fuel Density Factors

BSFC values are typical starting points for sizing, not measurements. Confirm on a dyno with an air-fuel ratio meter, since BSFC rises when the mixture is richened for power or knock protection.

Engine type	Typical BSFC (lb/hp·hr)
Naturally aspirated gasoline	0.50
Turbocharged gasoline	0.60
Supercharged gasoline	0.65
E85	0.80

The cc/min per lb/hr factor is $453.6 / (60\,\rho)$ with $\rho$ in g/cc. Approximate fuel densities give these factors:

Fuel density (g/cc)	cc/min per lb/hr
0.72	10.5
0.74	10.2
0.78 (E85)	9.7
0.79 (ethanol)	9.6

Reference

BSFC and its use in engine fuel flow are defined in Heywood, Internal Combustion Engine Fundamentals. Fuel system sizing is covered in the Bosch Automotive Handbook.

Common Mistakes

Dividing by the wrong number of injectors. Port injection uses one per cylinder, but a second set of injectors, or direct plus port, needs the split counted separately.
Using wheel horsepower where crank horsepower belongs. Convert with the drivetrain loss calculator first.
Sizing to 100% duty cycle. Injectors need headroom, and a maximum duty of 0.80 is a common ceiling.
Taking the rated flow at the wrong pressure. Injector flow ratings are given at a specific fuel pressure, often 43.5 psi (3 bar), and flow changes with the square root of pressure.
Ignoring manifold pressure. Under boost the pressure across the injector falls unless the regulator raises fuel pressure to match.

When Not To Use This Calculator

Direct injection, where pressures reach hundreds of bar and flow scales differently.
Final tuning. Verify flow against measured air-fuel ratio on a dyno.
Diesel, methanol, or other fuels without a BSFC and density suited to them.
Batch-fire or staged setups where the injector count does not equal the cylinder count.

Frequently Asked Questions

How do I calculate the injector size I need for a horsepower target?: Multiply the target horsepower by BSFC to get total fuel flow in lb/hr, then divide by the number of injectors and the maximum duty cycle. For 400 hp at 0.60 BSFC on 8 cylinders at 80% duty, each injector needs 37.5 lb/hr, about 394 cc/min.
What BSFC should I use?: Typical starting values are about 0.50 lb/hp·hr for naturally aspirated gasoline engines, 0.60 for turbocharged, 0.65 for supercharged, and 0.80 for E85. They are starting points. The right value depends on the engine and the air-fuel ratio, and should be confirmed on a dyno.
What duty cycle should I design for?: Leave headroom. Around 80% at peak power is a common design ceiling, which keeps the injectors from running open and leaves room for the engine to need more fuel than predicted.
How does fuel pressure change injector flow?: Flow through a fixed orifice scales with the square root of the pressure difference. An injector rated 37.5 lb/hr at 43.5 psi flows about 43.3 lb/hr at 58 psi. Under boost, the useful pressure is the fuel pressure minus the manifold pressure.
How do I convert lb/hr to cc/min?: Multiply by about 10.5 for gasoline. The exact factor is 453.6 divided by 60 times the fuel density in g/cc, so denser fuels like E85 give a lower factor, near 9.7.

Inputs Used

Target Crank Power: Target power at the crank, not the wheels.
Power Unit: Unit for the value entered above.
Number of Injectors: One per cylinder for port injection.
BSFC (lb/hp·hr): Used directly in the calculation.
Custom BSFC: Used only when BSFC is set to Custom.
Maximum Duty Cycle: A fraction between 0 and 1. 0.80 is a common ceiling.
cc/min per lb/hr: 10.5 for gasoline. It depends on fuel density: 453.6 / (60 × density in g/cc).
Injector Rated Pressure (optional): Pressure at which the injector rating is stated. Use 0 to skip the pressure correction.
Operating Fuel Pressure (optional): Pressure the injector will run at. Use 0 to skip the pressure correction.
Pressure Unit: Unit for both pressures.

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