What the Fuel Injector Calculator Does
This calculator estimates the fuel injector size (in pounds per hour, lb/hr) your engine needs to support a target power level. It is useful for car enthusiasts, tuners and mechanics planning a build, a turbo upgrade or an injector swap. By matching injector flow to horsepower, you avoid running lean at peak power (which can damage an engine) or oversizing injectors so much that idle and low-load fuelling become hard to control.
The Inputs You Enter
- Engine Displacement (cc) – the total swept volume of your engine. This is recorded for reference; the core injector-size math is driven by horsepower, cylinder count and pressure.
- Number of Cylinders – how many injectors share the total fuel demand. More cylinders means each injector handles less flow.
- Max Horsepower – the peak power you intend to make at the crank.
- Fuel Pressure (PSI) – your fuel rail pressure. Injectors are rated at a base pressure (43.5 PSI here, the common 3 bar standard), so the calculator adjusts for your actual pressure.
The Formula Explained
The calculator works in two steps. First it finds the fuel flow each injector must deliver:
- Fuel flow per injector = (Horsepower \(\times\) 0.07 \(\times\) 60) \(\div\) Cylinders
The 0.07 figure is a brake specific fuel consumption (BSFC) assumption typical of a naturally aspirated petrol engine, and \(\times\) 60 converts to an hourly rate. The result is then corrected for fuel pressure and a target duty cycle:
- Injector size = (Fuel flow \(\times \sqrt{43.5 \div \text{Pressure}}\)) \(\div\) 0.8
Combining both steps gives the full formula:
$$\text{Injector Size} = \frac{\left(\dfrac{\text{Max HP} \times 0.07 \times 60}{\text{Cylinders}}\right) \times \sqrt{\dfrac{43.5}{\text{Fuel Pressure (PSI)}}}}{0.8}$$The square-root term rescales the standard 43.5 PSI rating to your pressure, and dividing by 0.8 builds in an 80% maximum duty cycle so injectors are not maxed out at full load.
Worked Example
Suppose you have a 2000 cc, 4-cylinder engine targeting 300 hp at 43.5 PSI:
- Fuel flow per injector = \((300 \times 0.07 \times 60) \div 4 = 315\)
- Pressure factor = \(\sqrt{43.5 \div 43.5} = 1\)
- Injector size = \((315 \times 1) \div 0.8 \approx\) 394 lb/hr
This indicates the flow figure your injector selection should meet or exceed for that power goal at the chosen pressure.
FAQ
Why does fuel pressure matter? Injectors flow more at higher pressure. Running above 43.5 PSI makes the same injector flow more, so the calculator reduces the required rated size; lower pressure increases it.
Does engine displacement change the result? Displacement is captured as part of your build details, but the injector sizing here is governed by horsepower, cylinder count, pressure and duty cycle.
Should I round up the injector size? Yes. Always choose an injector rated at or above the calculated value, leaving headroom for future power increases and fuelling stability.
