Fuel Injector Size from Horsepower, BSFC, and Duty Cycle
The injector each cylinder needs: lb/h = HP x BSFC / (n_cyl x duty).
Example
You enter
- Target horsepower (hp) 400
- BSFC (lb/hp-h; 0.50 NA, 0.55-0.65 boost) 0.5
- Number of injectors 8
- Maximum duty cycle (0-1) 0.8
You get
- Total fuel demand 200 lb/h
- Per-injector flow 31.25
- Inj ccmin 328
Details, formula, and sources
A 400 hp V8 at BSFC 0.50 and 80% duty needs 31.3 lb/h (328 cc/min) per injector; add boost (BSFC 0.60) and it jumps to 37.5 lb/h - a 20% bigger injector for the same power, why a boosted build steps up injector size before adding power. Port injection, entered BSFC; not rail pressure or DI. A tuning aid; the measured fueling and the tuner govern.
total_lbh = HP x BSFC; inj_lbh = total_lbh / (n_cyl x duty); inj_ccmin = inj_lbh x 10.5.
The fuel-injector-sizing relation lb/h = HP x BSFC / (n_cyl x duty), with the BSFC ranges, the customary 80% duty cycle, and the gasoline cc/min conversion, a standard tuning-reference result, by name.
The injector-sizing relation and the BSFC ranges are public results in the engine-tuning references.
Estimate. AHJ and licensed professional govern.
Field names used by the API: hp, bsfc, n_cyl, duty, total_lbh, inj_lbh, inj_ccmin
- Injector flow HP x BSFC divided across the injectors at the maximum duty cycleengine-tuning practice
- BSFC ~0.50 naturally-aspirated gas, 0.55-0.65 boosted; entered by the usertuning references
- Conversion lb/h x 10.5 = cc/min for gasoline (SG ~0.72)unit conversion