Home / Engine

Fuel injector size calculator

Enter the power you want to make and how many injectors feed it. See the flow each injector needs in lb/hr and cc/min, then check what a fuel pressure change does to an injector you already have.

Engine and assumptions

Target crank power

Enter a power figure above zero.

Number of injectors

Enter a whole number from 1 to 24.

BSFC (assumption)

lb/hp/hr

Brake specific fuel consumption: pounds of fuel per horsepower per hour. Enter a BSFC from 0.30 to 1.20.

Max duty cycle (assumption)

The share of time the injector is open at peak power. Default 80%. Enter a duty cycle from 10 to 100%.

Fuel density (assumption)

g/ml

Used only to turn lb/hr into cc/min. Default 0.74 g/ml. Enter a density from 0.60 to 0.95 g/ml.

BSFC rules of thumb

These are common rules of thumb, not measured values for your engine. The E85 buttons apply the common rule of about 30% more fuel than gasoline. A dyno or a wideband log of your own engine beats any of them. For E85, also change the fuel density to the figure for your fuel.

Injector flow at a new fuel pressure

Rated flow

Enter a flow above zero.

Unit

Pressure it is rated at

psi

Enter a pressure above zero.

New fuel pressure

psi

Enter a pressure above zero.

Many injectors are rated at 3 bar (43.5 psi), but check the data sheet for yours. The square root rule is an estimate: real injectors drift from it at very high or very low pressure.

lb/hr and cc/min converter

Flow

Enter a flow above zero.

Unit

The conversion depends on the fuel density entered at the top. Change it there to match your fuel.

What the inputs mean

Target crank power: the engine power you plan to make, at the crank, not at the wheels.
Number of injectors: usually one per cylinder. Enter the total if you run a second set.
BSFC (brake specific fuel consumption): how many pounds of fuel the engine burns per horsepower per hour. It depends on the engine, the fuel and the tune, so it is an assumption you set. Common rules of thumb are 0.45 to 0.50 for a naturally aspirated gasoline engine and 0.60 to 0.65 for a boosted one. E85 needs more fuel, and a common rule is about 30% more than gasoline.
Max duty cycle: the share of each engine cycle the injector is held open at peak power. Sizing for 80% leaves headroom. This is also an assumption.
Fuel density: only used to convert pounds per hour into cubic centimetres per minute. Gasoline is roughly 0.72 to 0.77 g/ml; the default is 0.74.

The formula

injector flow (lb/hr) = power × BSFC ÷ (injectors × duty cycle)cc/min = lb/hr × 453.59 ÷ density (g/ml) ÷ 60

453.59 is grams per pound, and dividing by 60 turns hours into minutes.

Worked example

A naturally aspirated V8 aimed at 400 hp, 8 injectors, BSFC 0.50 and an 80% duty cycle:

400 × 0.50 = 200 lb/hr of fuel for the whole engine. 200 ÷ (8 × 0.80) = 31.25 lb/hr per injector. At 0.74 g/ml that is 31.25 × 453.59 ÷ 0.74 ÷ 60 = 319 cc/min. At the target power each injector really delivers 25 lb/hr, which is 80% of its rated flow.

Fuel pressure and injector flow

An injector flows more when the fuel pressure across it rises. The usual estimate is that flow scales with the square root of the pressure ratio:

new flow = rated flow × √(new pressure ÷ rated pressure)

A 36 lb/hr injector rated at 43.5 psi flows about 36 × √(58 ÷ 43.5) = 41.6 lb/hr at 58 psi. That is 15.5% more, from pressure alone.

The result is maths from the numbers you enter. Before buying injectors, confirm the fuel pump and lines can supply the total flow, and have the engine tuned for the new parts.

Questions

What size injectors do I need for 400 hp?

With 8 injectors, a BSFC of 0.50 and an 80% duty cycle, you need at least 31.25 lb/hr (319 cc/min) per injector. A boosted engine at a BSFC of 0.65 needs 40.6 lb/hr for the same power.

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

Multiply by 453.59, divide by the fuel density in g/ml, then divide by 60. At 0.74 g/ml, 1 lb/hr is about 10.2 cc/min, and 550 cc/min is about 53.8 lb/hr.

Does raising fuel pressure increase injector flow?

Yes, by the square root of the pressure ratio. A 36 lb/hr injector rated at 43.5 psi gives about 41.6 lb/hr at 58 psi, 15.5% more.

What duty cycle should I size injectors for?

80% is the default here because it leaves headroom. It is a sizing assumption, not a limit set by a standard, so change it to whatever margin you want.