Calculator Inputs
Example Data Table
| Scenario | Airflow | RPM | Injector | Target Ratio | Pressure | Use Case |
|---|---|---|---|---|---|---|
| Street cruise | 35 g/s | 2500 | 315 cc/min | 14.7 | 3 bar | Economy check |
| Naturally aspirated pull | 120 g/s | 6000 | 550 cc/min | 12.5 | 3 bar | Power estimate |
| Boosted test | 220 g/s | 7000 | 1000 cc/min | 11.8 | 4 bar | Duty planning |
Formula Used
Air mass flow: all airflow inputs are converted to grams per second.
Target mixture: target AFR equals direct AFR, or stoich AFR multiplied by lambda.
Fuel flow: fuel flow equals air flow divided by target AFR. Trim and enrichment multipliers are then applied.
Pressure correction: corrected injector flow equals rated flow multiplied by the square root of actual pressure divided by rated pressure.
Injector mass flow: injector g/s equals corrected cc/min multiplied by fuel density, then divided by 60.
Fuel per injection: fuel per injection equals total fuel g/s divided by cylinders and injection events per injector second.
Base pulse width: base pulse width equals fuel per injection divided by injector mass flow, then multiplied by 1000.
Commanded pulse width: commanded pulse width equals base pulse width plus injector dead time.
Duty cycle: duty cycle equals commanded pulse width divided by available injection period, multiplied by 100.
How to Use This Calculator
- Enter measured or estimated airflow.
- Select the correct airflow unit.
- Enter cylinder count, engine speed, and engine cycle type.
- Choose direct ratio or lambda mixture targeting.
- Enter injector flow at its rated test pressure.
- Enter actual differential fuel pressure across the injector.
- Add fuel density, dead time, trim, and enrichment values.
- Press calculate and review pulse width, duty cycle, and headroom.
- Use CSV or PDF downloads to save the result.
Advanced Pulse Width Planning
Airflow is the main signal behind fuel demand. When an engine ingests more air, it needs more fuel to keep the chosen mixture. Injector pulse width is the time an injector remains open during one injection event. This calculator links those ideas with practical tuning inputs.
It starts by converting every airflow unit into grams per second. Then it divides air mass by the target air fuel ratio. The result is the total fuel mass needed each second. Fuel density changes the mass delivered by each cubic centimeter of injector flow. Pressure correction changes injector flow again, because liquid flow rises with the square root of pressure ratio.
Why Duty Cycle Matters
Pulse width alone is not enough. Engine speed controls how often each cylinder needs fuel. At higher rpm, each injector has less time before the next cycle. The tool compares commanded pulse width with available cycle time. This gives duty cycle and headroom. High duty can reduce control accuracy. It can also hide fuel pressure problems.
Useful Tuning Checks
Use measured airflow when possible. A mass air sensor, dyno log, or estimated airflow model can work. Enter injector flow at its rated pressure. Then enter actual differential fuel pressure. The calculator corrects flow from rated pressure to real pressure. Add dead time for battery and injector delay. Add enrichment only when warmup, acceleration, or safety margin requires it.
A safe result usually has duty below the chosen limit. Many tuners prefer spare headroom. This helps during heat, voltage drop, or boost changes. A result near the limit suggests larger injectors, higher pressure, lower airflow, or a richer strategy review. This page does not replace calibration software. It gives a physics based estimate that supports better decisions.
Best Practice
Check units before trusting any result. Small unit mistakes can create very large pulse width errors. Use consistent fuel data from the injector supplier. Confirm fuel density for gasoline, ethanol blends, or methanol. Recheck target mixture under cruise, power, and transient conditions. Save CSV or PDF copies with each test. That creates a useful record for future tuning sessions. Repeat calculations after hardware changes. Fresh logs make each next estimate easier to audit and improve later.
FAQs
What is injector pulse width?
Injector pulse width is the time an injector stays open during one injection event. It is usually measured in milliseconds. More fuel demand requires a longer pulse width.
Why does airflow affect pulse width?
Airflow shows how much air enters the engine. The chosen mixture ratio decides how much fuel must match that air. Higher airflow usually needs more fuel and longer injector open time.
What is injector dead time?
Dead time is the delay needed before the injector reaches useful flow. It changes with battery voltage, injector design, and fuel pressure. Add it to the base fuel pulse.
Why is pressure correction included?
Injector flow changes when differential fuel pressure changes. The calculator corrects rated flow using the square root of the pressure ratio. This gives a better real flow estimate.
Should I use AFR or lambda?
Use AFR when you already know the target air fuel ratio. Use lambda when fuel type changes. Lambda multiplied by stoichiometric ratio gives the target AFR.
What duty cycle is safe?
Many tuners prefer staying below 80 to 90 percent duty. More headroom helps during heat, voltage drop, boost changes, and sensor error. Your safe limit depends on hardware.
Can this replace tuning software?
No. This tool gives a physics based estimate. Final calibration should use real logs, oxygen sensor data, fuel pressure data, and proper engine management tools.
Why does fuel density matter?
Injector ratings are often volumetric. Engines need fuel mass. Fuel density converts volume flow into mass flow, which improves estimates for gasoline, ethanol blends, and other fuels.