Injector Sizing Results
These results update after pressing the calculate button.
Fuel System Chart
Tuning Notes
Advanced Calculator Inputs
Example Data Table
| Build Type | Power | BSFC | Injectors | Flow | Duty Limit | Common Use |
|---|---|---|---|---|---|---|
| NA four cylinder | 180 hp | 0.48 | 4 | 250 cc/min | 80% | Street tuning |
| Turbo four cylinder | 450 hp | 0.55 | 4 | 550 cc/min | 85% | Boosted equipment or motorsport |
| Heavy load six cylinder | 600 hp | 0.60 | 6 | 750 cc/min | 80% | Long duty construction engine |
Formula Used
Total fuel demand: Fuel lb/hr = target horsepower × BSFC.
Required injector size: Required cc/min = fuel lb/hr ÷ injectors ÷ duty fraction × 453.592 ÷ fuel density ÷ 60.
Pressure correction: Corrected flow = rated flow × √(actual rail pressure ÷ rated pressure).
Supported horsepower: Max hp = corrected injector lb/hr × injectors × duty fraction ÷ BSFC.
Duty cycle needed: Duty % = target fuel lb/hr ÷ total available injector lb/hr × 100.
Pulse width per squirt: Pulse ms = fuel volume per squirt ÷ injector flow per millisecond.
Estimated Required Fuel: Req Fuel ms = cylinder air mass ÷ stoich AFR ÷ fuel density ÷ injector flow per millisecond.
MegaSquirt style pulse estimate: PW = downloaded Req Fuel × VE × MAP factor × enrichments + acceleration enrichment + dead time.
How to Use This Calculator
- Enter the engine power target and a realistic BSFC value.
- Add injector count, rated injector flow, rated pressure, and actual rail pressure.
- Set a safe duty cycle limit for the engine use case.
- Enter fuel density, stoich AFR, target AFR, displacement, and cylinder count.
- Add RPM, VE, MAP, enrichments, dead time, and squirts per cycle.
- Press calculate and review injector size, duty cycle, pulse width, and horsepower margin.
- Download the CSV or PDF report for your build sheet.
- Confirm final settings with logs, wideband feedback, and tuning software.
Injector Planning Article
Why Injector Sizing Matters
Accurate injector sizing protects an engine before serious tuning begins. A MegaSquirt setup needs fuel delivery that matches airflow, power goals, pressure, and safe duty cycle. Small injectors can run static. Large injectors can idle poorly. This calculator helps you compare both problems in one place.
How the Calculator Works
The tool starts with horsepower and brake specific fuel consumption. It then divides total fuel demand across the injector count and chosen duty cycle. It also corrects rated injector flow when rail pressure changes. A higher rail pressure increases flow by the square root of the pressure ratio. A lower pressure reduces the safe horsepower limit.
Construction Load Considerations
Construction engines often work under heavy load for long periods. Generators, pumps, loaders, compressors, and custom equipment may stay near peak torque. That makes duty cycle planning important. A racing value may not be safe for continuous work. The margin section shows how much injector headroom remains after the target power is supplied.
Pulse Width and Setup Value
The pulse width outputs help with base calibration. The calculator estimates fuel per engine cycle, fuel per squirt, and injector open time. It also estimates a Required Fuel style value using displacement, cylinder count, air density, fuel density, stoich ratio, and injector flow. Use it as a planning value. Final numbers should be checked inside your tuning software.
Fuel Type and Boost
Fuel type matters. Gasoline, ethanol blends, methanol, and diesel have different density and stoichiometric ratios. A richer target air fuel ratio also increases fuel demand. Turbocharged engines may need more fuel because boost increases trapped air mass. The MAP and enrichment inputs show how load corrections change the commanded pulse width.
Safe Tuning Habits
Use conservative assumptions first. Enter a duty limit below the absolute maximum. Use the real rail pressure measured at the injector. Include a realistic injector dead time. Compare horsepower demand with airflow based demand. If both methods point toward high duty cycle, the injector is too small. If pulse width is very tiny at idle, the injector may be too large.
Final Review
This calculator does not replace professional calibration. It gives a fast fuel system review. It is useful before buying injectors, changing pressure, or building a first startup map.
Always verify results with logged wideband data.
FAQs
1. What does this calculator size?
It estimates injector flow, duty cycle, horsepower support, pulse width, pressure correction, and Required Fuel style setup values for planning an engine fuel system.
2. What BSFC value should I enter?
Use a value that matches your engine type. Naturally aspirated gasoline engines may use lower values. Turbo engines, rich mixtures, and heavy load engines often need higher values.
3. Why does rail pressure change injector flow?
Injector flow rises with the square root of pressure ratio. Doubling pressure does not double flow. It gives a smaller increase and may stress the fuel system.
4. Is 100% duty cycle safe?
No. A saturated injector loses control and may deliver unstable fuel. Many builders target 80% to 85% for safer operation and better headroom.
5. What is injector dead time?
Dead time is the delay before the injector starts flowing fuel. It changes with voltage and injector design. Use tested data when possible.
6. Can I use this for ethanol?
Yes. Change fuel density, stoich AFR, target AFR, and BSFC. Ethanol usually needs more fuel volume than gasoline for the same air mass.
7. Why include MAP and enrichments?
They show how load and correction factors affect commanded pulse width. This helps compare basic injector size with real tuning conditions.
8. Should I copy these values directly?
No. Use them as planning estimates. Final settings need real injector data, stable fuel pressure, wideband logs, and careful tuning software checks.