Injector size calculator

What injector size means

An injector's size is the fuel it can pass in a minute or an hour with the valve fully open, at a stated pressure. The engine at full power needs a certain amount of fuel each hour. The injectors, which are open only part of the time, must be able to deliver that amount without being open all the time. That is why the size is bigger than the average flow, and the number that links the two is the duty cycle.

The method, variable by variable

Required flow per injector = (hp × BSFC) ÷ (n × duty). hp is the crank horsepower you want at the engine's peak. BSFC is the pounds of fuel the engine burns per horsepower per hour. n is the number of injectors, one per cylinder on port injection. Duty is the fraction of each engine cycle the injector is allowed to stay open at full power (0.80 means 80 percent). The product hp × BSFC is the total fuel flow in pounds per hour; dividing by n shares it among the injectors, and dividing by the duty cycle makes each injector's rated flow larger than the average flow it must deliver, so it is never fully open.

Worked examples

A turbocharged four-cylinder aiming for 600 hp at BSFC 0.55 with an 85 percent duty cycle needs 97.1 lb/hr (985 cc/min) per injector. This is the HP Academy example, and the page's own functions give the same figure. A naturally aspirated four at 300 hp, BSFC 0.45 and 80 percent duty needs 42.2 lb/hr (428 cc/min). For pressure, an eight-cylinder aiming for 500 hp at BSFC 0.60 and 80 percent duty needs 46.9 lb/hr per injector at your 58 psi rail. If the data sheet rates injectors at 43.5 psi, the rating to look for is 40.6 lb/hr (412 cc/min), since 46.9 × √(43.5 ÷ 58) = 40.6 (our derivation from the square-root law). Going the other way, eight 42 lb/hr injectors at BSFC 0.50 and 80 percent duty feed about 538 hp.

Limits of this method

First, BSFC is not a constant. It changes with load, fuel, mixture and ignition timing, and the 0.50 to 0.60 range Garrett gives for turbocharged gasoline engines reflects exactly that spread. A rich, retarded tune at high boost sits at the top. Second, the duty cycle limit is a convention from tuners, not a figure from a standard, and the numbers vary by source from about 80 to 90 percent. Third, the formula covers steady full-power flow only. It does not account for the injector's opening and closing delay, which matters at idle and small pulse widths, or for rail pressure that falls under load when the pump is too small. A pump that cannot keep the pressure up makes the injectors flow less than their rating, so size the pump and lines for the same power. Finally, a lean mixture at full boost can destroy an engine, so tune with a wideband sensor and have the work checked by a professional.

BSFC figures used in practice

These are the ranges behind the calculator's default of 0.50. Turbocharged gasoline: about 0.50 to 0.60 and higher (Garrett Motion). HP Academy's own example uses 0.55 for a turbo engine. Naturally aspirated: 0.40 to 0.50 in tuning-shop articles, with 0.45 a common pick, but we did not find an engineering text that prints the range. Pick the number that matches your fuel and tune, and round the injector size up.

How do I size fuel injectors for a given horsepower?

Take the crank horsepower you are aiming for, multiply by the engine's brake specific fuel consumption (BSFC, in pounds of fuel per horsepower per hour), and divide by the number of injectors times the highest duty cycle you will allow. The answer is the flow each injector must deliver, in pounds per hour. Our calculator also gives cubic centimeters per minute, the unit most data sheets use outside the U.S.

What BSFC should I use?

Garrett Motion puts turbocharged gasoline engines at about 0.50 to 0.60 lb/hp·hr and higher, and says the low end takes race fuel and aggressive tuning. For naturally aspirated engines the figures we found, 0.40 to 0.50, come from tuning shops and forums rather than from an engineering text, so treat them as a starting point. Use a higher number if you are unsure: it gives larger injectors and safer margins. A measured BSFC from a dyno run on your own engine is better than any table.

What duty cycle is safe?

Most tuning sources size injectors to run no higher than about 80 to 85 percent at full power, leaving headroom for a richer mixture on a hot day or after a fuel change. HP Academy advises staying at or below roughly 85 to 90 percent and not sitting at 95 to 100 percent, because some injectors behave erratically near fully open. These are tuning conventions, not standards: the injector maker's data sheet sets the real limit.

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

Multiply by 453.59 grams per pound, divide by 60 minutes, and divide by the fuel density in grams per milliliter. With the density the calculator uses by default, 0.745 g/mL, 1 lb/hr is 10.15 cc/min. Shops often quote the factor as 10.5 (density 0.72, which gives 10.50) or 10.2 (density 0.74, which gives 10.22). The difference is under 4 percent, but you can change the density field to match your fuel.

Do injectors flow more at higher fuel pressure?

Yes. Flow through a fixed opening rises with the square root of the pressure drop across it, so doubling the pressure raises flow by about 41 percent. Data sheets state flow at a test pressure, so a rating taken at one pressure must be adjusted if your regulator is set to another. With a manifold-referenced regulator the pressure drop stays steady as boost builds; with an unreferenced one the drop shrinks by the boost pressure, and flow falls with it.

Can I use bigger injectors than I need?

You can, but there is a price. Very large injectors flow so much in a short pulse that idle and light-load fueling gets harder to control, and tuning it takes more work. If your goal is more power later, choose a size that suits the plan, not the largest one in the catalog.

Does this work for E85, diesel or direct injection?

No. The calculator assumes gasoline. Ethanol blends need more fuel by mass for the same power, so the BSFC is higher and the density differs, diesel injection has its own sizing, and direct-injection systems run at pressures that change the flow rating. Use the fuel density field and a BSFC from the fuel supplier's data if you adapt it, and expect a rough answer.

Sources: HP Academy, “Fuel injectors & injector sizing” (hpacademy.com, a tuning-course vendor): the formula, the 600 hp example and the 85 to 90 percent duty-cycle advice. Garrett Motion, “How to select a turbo, part 2” (vendor): BSFC for turbocharged gasoline engines. Gasoline density: 2,819 g/gal from the GREET fuel table as reproduced in the University of Illinois NPRE 470 course notes. The square-root pressure correction and the ranges for naturally aspirated BSFC are shop practice or our own derivation, as marked above. Related: horsepower calculator, boost and airflow calculator, engine displacement calculator. How we check our calculators.