Calculate horsepower supported by fuel injectors or solve injector flow, duty cycle, and injector count using BSFC, fuel density, and pressure correction.
Formula
Supported HP from injectors:
HP = (Flb/hr × N × DC) ÷ BSFC
where Flb/hr = flow per injector (lb/hr), N = number of injectors, DC = max duty cycle (decimal, e.g. 0.80), BSFC = brake specific fuel consumption (lb/hp·hr).
Injector size needed (per injector):
Flb/hr = (HP × BSFC) ÷ (N × DC)
For density ρ in kg/L: cc/min = lb/hr × 0.45359237 / (0.06 × ρ). At the illustrative gasoline density 0.745 kg/L, the factor is approximately 10.1475, not a universal 10.5. Use pressure-corrected flow in the HP formula; the required rated flow divides by the pressure multiplier.
Interpretation
The result estimates injector fuel-flow capacity expressed as crank horsepower. It does not establish engine output or verify the pump, regulator, lines or ECU injection window. The editable 80% duty and 0.50 BSFC defaults are planning assumptions; follow the injector manufacturer and engine calibration data.
- Lower duty: more theoretical flow reserve; this alone does not prove safe tuning.
- 70–80%: a possible planning range, not a universal optimum.
- 80–90%: check the manufacturer’s usable pulse-width and operating limits.
- Above 90%: high commanded duty requires careful evaluation;90% is not automatically static. The model rejects required duty above 100%.
BSFC is engine- and fuel-specific. As a mathematical example, changing an assumed BSFC from 0.50 to 0.75 raises modeled fuel demand by 50% at the same horsepower; those values do not prescribe every gasoline or E85 engine.
BSFC Input Guidance
| Fuel / Application | BSFC (lb/hp·hr) |
|---|---|
| Diesel | Use engine/fuel test data |
| Naturally aspirated gasoline | 0.50 illustrative mild-street example |
| Nitrous / mild boost gasoline | Use engine/fuel test data |
| Turbo / supercharged gasoline | 0.65 illustrative boosted-gasoline example |
| High-boost gasoline (race) | Use engine/fuel test data |
| E85 naturally aspirated | Use engine/fuel test data |
| E85 forced induction | Use engine/fuel test data |
FAQ
Should I enter flywheel (crank) horsepower or wheel horsepower?
Use crank horsepower consistent with the BSFC basis. Wheel-to-crank conversion requires a justified drivetrain-loss estimate; fixed 15% or 20% add-ons are not universal. This calculator does not convert wheel horsepower.
My injectors are rated at a different fuel pressure. Does that matter?
Yes. For unchanged fuel and injector behavior, flow approximately scales with the square root of differential pressure. A 550 cc/min injector rated at 43.5 psi gives about 635.1 cc/min at 58 psi by this model. Enter the pressure difference across the injector, including manifold-pressure effects; rail gauge pressure alone may differ.
Why is E85 BSFC so much higher?
Ethanol blends generally require more fuel for a given power, but the mass-flow increase depends on blend, engine efficiency and calibration. Use appropriate measured BSFC and fuel density; a density preset does not update BSFC or establish a required injector size.
What if I only know cc/min and the calculator asks for lb/hr?
Just pick cc/min in the unit dropdown — the calculator converts internally. For a manual conversion: lb/hr = cc/min × 0.06 × fuel density in kg/L / 0.45359237. Density depends on fuel blend and temperature.
