Fuel Injector Size Calculator
Size fuel injectors in cc/min and lb/hr from your horsepower target, injector count, BSFC and duty cycle.
How it works
Injector lb/hr = (hp × BSFC) ÷ (injectors × duty cycle) · cc/min = lb/hr × 10.5
Injector sizing starts from how much fuel the target power demands. Brake specific fuel consumption (BSFC) is the pounds of fuel burned per horsepower per hour — around 0.45–0.55 for a naturally aspirated gasoline engine and higher under boost. Multiplying horsepower by BSFC gives the total fuel flow needed; dividing by the number of injectors and by the maximum duty cycle you want to run gives the flow each injector must deliver, with the duty cycle leaving headroom so the injectors never hit 100% and lose control. The 10.5 factor converts pounds per hour to cc per minute for gasoline’s density.
Worked example
Targeting 400 hp with 8 injectors at a 0.50 BSFC and an 85% duty cycle: total fuel is 400 × 0.50 = 200 lb/hr, so each injector needs 200 ÷ (8 × 0.85) ≈ 29.4 lb/hr, or about 309 cc/min. You would choose an off-the-shelf injector at or just above that — say 320–340 cc/min.
Frequently asked questions
What BSFC should I use?
For naturally aspirated gasoline engines, 0.45–0.55 is typical; forced-induction setups run richer and higher, often 0.55–0.65. Using a slightly conservative (higher) BSFC gives extra injector headroom, which is safer than sizing too small.
Why size for 85% duty cycle instead of 100%?
Above roughly 85–90% duty, an injector is open so much of the time that it loses the ability to meter fuel precisely and can’t add more if demand rises. Sizing to 85% leaves a safety margin so the injectors stay in their controllable range.
Can injectors be too big?
Yes. Very large injectors struggle to deliver tiny, precise amounts at idle and light load, hurting idle quality and drivability. Pick the smallest injector that comfortably supports your power goal with the duty-cycle headroom above.
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Educational estimate. Real-world results vary with conditions, tuning and measurement.