How to use this calculator
Enter the engine brake power you want to evaluate and a documented or explicitly assumed brake specific fuel consumption (BSFC) for that fuel, speed and load. Enter the number of injectors sharing the fuel demand equally, then the duty percentage you want to investigate. The result is the estimated static mass-flow requirement for each injector, plus the total fuel consumed by the engine. Static flow means continuous-open flow under specified test conditions; it is larger than the average flow per injector when duty is below 100%.
This steady-state estimate assumes one equal-flow injector set and a linear relationship between duty fraction and delivered mass. It does not select a compatible injector or validate a tune. Actual injector characterization, fuel and differential pressure must match the intended use. Short-pulse behavior, opening and closing delays, transient enrichment, staged or unequal injectors, and direct-injection timing windows are outside this model. No fuel-density, pump-capacity or drivetrain-loss assumption is added.
The formula
A worked example
For an illustrative 480 hp engine at 0.50 lb/(hp·h), total fuel demand is 480 × 0.50 = 240 lb/h. With eight equal-flow injectors and an assumed 80% duty, each needs 240 ÷ (8 × 0.80) = 37.50 lb/h static flow, equivalent to 17.01 kg/h. Average flow per injector at that operating point is only 240 ÷ 8 = 30 lb/h. For a separate metric example, 240 kW at 300 g/kWh consumes 72 kg/h; six injectors at 75% require 72 ÷ (6 × 0.75) = 16 kg/h each. These are arithmetic examples, not measured engine specifications.
How the chosen duty changes required static flow
Original hypothetical comparison: 480 mechanical hp, 0.50 lb/(hp·h), eight injectors. Total demand remains 240 lb/h. Percentages illustrate the equation, not recommended operating limits.
| Chosen duty | Required static flow per injector (lb/h) | Average flow per injector (lb/h) |
|---|---|---|
| 60% | 50.00 | 30.00 |
| 75% | 40.00 | 30.00 |
| 80% | 37.50 | 30.00 |
Common questions
Which BSFC should I use?
Use data appropriate to the engine operating point and fuel, or label your estimate as an assumption. BSFC can change with speed and load. The 0.50 example is not a universal gasoline value, and this tool provides no preset for gasoline, ethanol blends or diesel. Try a documented range to see how sensitive the result is.
Is 80% duty a safe limit for every injector?
No. It is only the example input. Permitted duty depends on the injector, driver, application and usable injection window. This calculator accepts a chosen percentage between 0 and 100 but cannot establish that it is suitable. Check the relevant manufacturer and calibration requirements.
Can I use wheel horsepower?
The model pairs fuel consumption with crankshaft brake power. Wheel power includes a different measurement boundary. Do not enter wheel horsepower as though it were crankshaft horsepower or apply an assumed universal drivetrain-loss percentage.
Why are there no cc/min results?
Cubic centimeters per minute describe volume, while the equation gives mass per time. Converting requires the density of the actual fuel at the stated conditions. A universal lb/h-to-cc/min multiplier would hide that assumption, so this tool reports mass flow only.
Does changing injector count change total fuel demand?
No. For fixed power and BSFC, total fuel demand is unchanged. More equal-flow injectors share that demand, reducing the required flow per injector. Use the actual count in the single set being modeled, not cylinder count by default. Staged sets require a different allocation model.
Can I compare this number directly with an injector listing?
Only after checking the listing’s fuel, pressure and measurement basis and the injector’s suitability for the application. A flow rating at different conditions is not automatically comparable. Electrical characteristics, control data and fitment are separate requirements; this tool makes no product recommendation.
Why reject 100% duty?
A continuous-open calculation leaves no off-time allowance. The tool is intended to explore a duty assumption below continuous operation, not certify operation at saturation. Values just below 100% are mathematically accepted but are not a recommendation. Zero duty cannot deliver positive fuel demand.
What happens if my BSFC estimate is too low?
Required flow scales directly with BSFC. For example, changing the illustrative 0.50 to 0.60 at the same 480 hp, eight injectors and 80% raises the estimate from 37.50 to 45.00 lb/h each. This sensitivity calculation does not supply a validated safety margin.
Sources & calculation method
Holley StealthRam manual, Appendix 1 (page 14) gives the static-flow equation using engine power, BSFC, injector count and duty fraction. Its application-specific example settings are not adopted as universal limits here. Holley’s injector selection explanation provides context on flow requirements and application-dependent timing constraints. The tool evaluates the mass-flow balance only.
NIST conversion factors provide the unit references. Mechanical horsepower is distinct from metric horsepower (PS); do not enter a PS figure in the hp field. Worked examples are original calculations.
Derive consumption per brake energy with the BSFC Calculator. For a logged pulse duration and RPM, use the separate Injector Duty Cycle Calculator; electrical commanded duty does not by itself establish physical fuel delivery. Use the Horsepower & Torque Calculator when matched torque and RPM are available.
Primary sources checked October 30–November 2, 2026. Sources do not endorse this site or provide independent professional review.
Formula examples and input validation have automated checks. This page has not received an independent automotive professional review. Read our review standards.