Fuel Injector Size Calculator
Calculate the fuel injector size needed for your target horsepower.
Fuel injectors must flow enough fuel to support your target power without running out of headroom. This calculator sizes injectors in lb/hr and cc/min from your horsepower goal, cylinder count, fuel efficiency, and a safe duty cycle.
Injector Size Formula
BSFC (brake-specific fuel consumption) reflects how much fuel the engine burns per horsepower; naturally aspirated gasoline engines are around 0.5, forced-induction higher. Sizing for an 80–85% maximum duty cycle leaves safety headroom so injectors aren't maxed out.
How to Use This Calculator
- Enter target horsepower and cylinder count.
- Set BSFC (0.5 NA, higher for boost) and max duty cycle (≤85%).
- Read injector size in lb/hr and cc/min.
Worked Example
Understanding BSFC and Duty Cycle
BSFC (brake-specific fuel consumption) is how much fuel an engine burns per horsepower per hour — roughly 0.45–0.50 for naturally aspirated gasoline engines and 0.55–0.65 for forced induction, which runs richer to control heat and knock. Duty cycle is the fraction of time an injector is open; sizing for an 80–85% maximum leaves headroom so the injectors never run fully open, which would mean no reserve and unstable fuel delivery.
Static Flow and Fuel Type
Injectors are rated at a reference fuel pressure, so raising rail pressure increases flow and lowering it decreases flow. Fuel choice matters too: E85 needs roughly 30–40% larger injectors than gasoline for the same power because it has lower energy density. Always size with margin, then verify with a tune — undersized injectors max out and lean out the engine under load, a fast route to damage.
Injector Size by Power (V8, NA, 0.5 BSFC, 85% duty)
| Target HP | lb/hr each | cc/min each |
|---|---|---|
| 400 | 29.4 | 309 |
| 500 | 36.8 | 386 |
| 650 | 47.8 | 502 |
| 800 | 58.8 | 617 |
Frequently Asked Questions
Use lb/hr = (HP × BSFC) ÷ (cylinders × max duty cycle), then multiply lb/hr by about 10.5 to get cc/min per injector.
Brake-specific fuel consumption is fuel used per horsepower per hour. Use about 0.5 for naturally aspirated gasoline engines and 0.55–0.65 for forced induction.
Leaving headroom prevents injectors from running at 100% (static), which loses fuel control and risks a lean condition. 80–85% max is a safe target.
Multiply pounds per hour by roughly 10.5 (for gasoline) to get cubic centimetres per minute.
It's wise to leave some margin, but very oversized injectors can hurt idle and low-load fuel control, so don't go drastically larger than needed.