Fuel Injector Size Calculator
Find the injector size for your horsepower target from BSFC, injector count and duty cycle, in lb/hr and cc/min, with fuel pressure, E85 and wheel hp corrections.
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About this toolWhat is the Fuel Injector Size Calculator?
An engine can only make as much power as it has fuel to burn, and the injectors are often the limit. This fuel injector size calculator works out the flow each injector needs for your horsepower target with the standard sizing formula: injector flow = horsepower × BSFC ÷ (number of injectors × maximum duty cycle). It gives the answer in lb/hr, cc/min and g/min, then corrects it to the pressure the injector is rated at, so you know which size to buy.
Pick naturally aspirated, blower (belt-driven boost) or turbocharged and gasoline, E85 or methanol to load a typical brake specific fuel consumption (BSFC), or type your own. Enter wheel horsepower with a drivetrain loss if that is what your dyno sheet shows. A second panel answers the reverse question: how much power will the injectors you already have support?
- Injector size from hp or kW, crank or wheel power
- BSFC presets for naturally aspirated, blower and turbo engines on gasoline, E85 and methanol
- lb/hr, cc/min and g/min with fuel specific gravity
- Square-root pressure correction to the injector's rated pressure
- Reverse mode: horsepower supported by a given injector
- Common injector sizes and a shareable link
How to use the Fuel Injector Size Calculator
- Enter the target power in hp or kW and say whether it is at the crank or the wheels. For wheel power, set the drivetrain loss.
- Choose the induction type and fuel to load a typical BSFC, or type your own BSFC from dyno data.
- Set the number of injectors (usually one per cylinder) and the maximum duty cycle you are willing to run.
- Enter the pressure the injector is rated at and the fuel pressure it will see in use.
- Read the required flow per injector and the rating to buy, then use the second panel to check what a given injector size supports.
Worked examples
400 hp V8, naturally aspirated
Input
400 crank hp, BSFC 0.50, 8 injectors, 80% duty
Output
31.25 lb/hr (328 cc/min) per injector
400 × 0.5 = 200 lb/hr total; 200 ÷ (8 × 0.8) = 31.25. A 36 lb/hr (380 cc) injector leaves margin.
Turbo four from wheel horsepower
Input
300 whp, 15% loss, turbo BSFC 0.60, 4 injectors, 80%
Output
352.9 crank hp, 66.2 lb/hr (695 cc/min) per injector
Wheel power is divided by 0.85 first. A 750 cc injector is the next common size.
Same engine on E85
Input
As above, BSFC 0.84
Output
92.6 lb/hr (898 cc/min) per injector
E85 needs about 40% more fuel by mass. The step from 750 cc to 1000 cc injectors is common when converting.
Raising fuel pressure
Input
66.2 lb/hr needed, injector rated at 43.5 psi, run at 58 psi
Output
Buy 57.3 lb/hr (602 cc/min) rated
At 58 psi the injector flows 15.5% more than its rating, so a smaller rated size will do.
What 550 cc injectors support
Input
4 × 550 cc/min, 80% duty, BSFC 0.60, 3 bar
Output
About 279 crank hp (237 whp at 15% loss)
550 cc is 52.4 lb/hr; 52.4 × 4 × 0.8 ÷ 0.6 = 279 hp.
Background and insight
BSFC is the mass of fuel an engine burns per unit of power per hour, in lb/hp·hr. A healthy naturally aspirated gasoline engine at full load uses about 0.45 to 0.50, an engine with a belt-driven blower 0.55 to 0.60 and a turbocharged engine 0.60 to 0.65, because boosted engines run richer and with more charge cooling from the fuel. Multiplying crank horsepower by BSFC gives the total fuel flow; dividing by the number of injectors and by the fraction of time each one may stay open (duty cycle) gives the flow each injector must deliver. Tuners keep peak duty at or below 80 to 85% so the injector has time to close and the fuel system has margin.
Alcohol fuels carry less energy per pound. E85 needs roughly 30 to 40% more fuel than gasoline for the same power (its stoichiometric air-fuel ratio is about 9.8 against 14.7), and methanol about twice as much (stoichiometric about 6.4). The calculator scales the gasoline BSFC by 1.4 for E85 and 2.2 for methanol. Injector ratings are usually given in lb/hr or cc/min at a stated pressure, often 3 bar (43.5 psi). The trade conversion of 10.5 cc/min per lb/hr assumes a fuel specific gravity of about 0.72; the calculator uses 0.72 for gasoline, 0.78 for E85 and 0.79 for methanol.
Injector flow follows the orifice law: it rises with the square root of the pressure difference across the injector. An injector rated at 43.5 psi flows √(58 ÷ 43.5) = 1.155 times as much at 58 psi. With a 1:1 rising-rate regulator the pressure difference stays constant under boost, so the base pressure is the one to enter. Real injectors depart from the square-root law at the extremes and their dead time changes with pressure and battery voltage, which is why tuners use the injector maker's characterisation data in the ECU.
Common use cases
Turbo and blower builds
Size injectors before adding boost so the engine never runs lean at the top of the rev range.
E85 and flex-fuel conversions
See how much bigger the injectors must be to make the same power on ethanol.
Checking an existing setup
Find the power ceiling of stock or used injectors before a tune.
Engine swaps and EFI conversions
Pick injectors for a carburettor-to-EFI kit or a swapped engine with a different cylinder count.
Tips and limitations
- BSFC varies by engine. If you have dyno data with fuel flow, calculate the real BSFC and use it.
- Leave headroom: plan for 80% duty at peak power, not 100%. Injectors that go static (always open) can no longer meter fuel.
- Very large injectors can be hard to control at idle. Choose the smallest injector that meets the target with margin.
- The fuel pump must supply the total flow at the pressure you run, plus a margin; check the pump's flow chart at that pressure and voltage.
- Injector ratings depend on the test fluid and pressure. Compare ratings at the same pressure.
- Unit check: 1 lb/hr ≈ 10.5 cc/min for gasoline. Many injector suppliers quote both.
Frequently asked questions
What is the fuel injector size formula?
Injector flow (lb/hr) = horsepower × BSFC ÷ (number of injectors × maximum duty cycle). For 400 hp, BSFC 0.5, 8 injectors and 80% duty: 400 × 0.5 ÷ 6.4 = 31.25 lb/hr.
What BSFC should I use?
Typical sizing values are about 0.50 lb/hp·hr for naturally aspirated gasoline engines, 0.55 for engines with a belt-driven blower and 0.60 to 0.65 for turbocharged. Multiply by roughly 1.4 for E85.
How do I convert lb/hr to cc/min?
Multiply by about 10.5 for gasoline. The exact factor is 453.6 g ÷ 60 ÷ the fuel density in g/cc, so 7.56 ÷ 0.72 = 10.5.
What duty cycle should injectors run at?
Most tuners size for 80 to 85% at peak power. Above that, the injector has little time to close and flow becomes less predictable.
How much horsepower will 1000cc injectors support?
With six 1000 cc injectors on gasoline (95.2 lb/hr each), 80% duty and a turbo BSFC of 0.60, about 762 crank hp. Fewer injectors or E85 lower that figure.
Does higher fuel pressure increase injector flow?
Yes, by the square root of the pressure ratio. Going from 43.5 psi to 58 psi raises flow by about 15%.
Should I use crank or wheel horsepower?
BSFC is defined at the crank. If you only know wheel horsepower, divide by (1 − drivetrain loss), for example 300 whp ÷ 0.85 = 353 hp.
Is there an in-depth guide to the Fuel Injector Size Calculator?
Yes. “Fuel Injector Sizing: BSFC, Duty Cycle and Fuel Pressure” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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