Fuel injector size calculator

Estimate the per-injector flow implied by explicit target power, BSFC, duty-cycle and fuel-density assumptions.

Formulalb/hr per injector = horsepower × BSFC ÷ cylinders ÷ duty fraction

BSFC connects power to fuel mass

Brake-specific fuel consumption estimates pounds of fuel per horsepower-hour. Divide total fuel mass flow by injector count and the chosen maximum duty-cycle fraction.

Rated flow depends on fuel and pressure

The cc/min conversion uses the fuel density you enter. Actual injector flow also depends on differential fuel pressure, injector characterization and the fuel system.

A worked example

Take 400 hp from eight cylinders, a naturally aspirated BSFC of 0.50 lb per hp-hour, and a maximum duty cycle of 80%. Total fuel is 200 lb/hr, which is 25 lb/hr per injector at full duty and 31.25 lb/hr once the 80% ceiling is applied. At a petrol density of 0.745 kg/l that is about 317 cc/min.

Every one of those inputs is an assumption you are choosing rather than a fact the engine has given you. Raise the BSFC to 0.60 for a boosted build and the same 400 hp asks for about 380 cc/min. That sensitivity is the reason the answer is a shortlist to discuss with a tuner, not a part number.

Frequently asked questions

What BSFC should I enter?

For a naturally aspirated petrol engine, 0.45 to 0.50 lb per hp-hour is the usual working range. Forced induction runs richer and hotter, so 0.55 to 0.65 is more realistic there. E85 needs far more fuel by mass for the same power, commonly around 0.65 to 0.75 naturally aspirated and higher again with boost.

Why not size for 100% duty cycle?

Because an injector held fully open has no margin left, and flow becomes non-linear near the top of its range where the pintle barely closes between pulses. Eighty percent is the conventional ceiling and leaves headroom for a hot day, a richer tune or a power increase later. Some tuners work to 85%, few go beyond.

Are bigger injectors always safer?

No. Oversized injectors idle badly and are hard to tune at low load, because the pulse width needed to deliver a tiny amount of fuel falls into the part of the range where the injector is least consistent. Modern designs cope better than older ones, but sizing well above what the engine needs buys a driveability problem.

How do lb/hr and cc/min relate?

Through the density of the fuel, which is why the field is there. At the 0.745 kg/l assumed for petrol, one lb/hr is about 10.2 cc/min. E85 is denser, near 0.78 kg/l, so the same mass flow is fewer cc/min. Injectors sold with both figures usually quote them for petrol at a stated pressure.

Does fuel pressure change the rating?

Yes, and it is the most common reason a real injector does not match its label. Flow rises with the square root of the differential pressure, so an injector rated at 3 bar flows about 15% more at 4 bar. Check the pressure a rating was measured at before comparing two injectors, and remember that the pump and rail have to supply the extra.