How to use this calculator
Read engine speed and injector pulse width from the same operating point. Check the ECU documentation for the pulse-width channel definition and injection schedule. Select the interval between repeated pulses for the same injector. Do not select it from the engine cylinder count or assume every four-stroke strategy uses one pulse per 720 degrees.
This is a timing calculation, not an injector sizing recommendation or a fuel-flow estimate. It does not predict mixture, fuel pressure, injector temperature or a safe operating limit. Direct injection may have a much shorter usable crank-angle window than a full cycle. Multiple unequal pulses, staged injectors and driver current-control PWM require their own measurement definitions and are outside this single-pulse model.
The formula
A worked example
At 6,000 RPM with one pulse every two revolutions, the repeat period is 120,000 ÷ 6,000 = 20 ms. A 12 ms electrical pulse gives 12 ÷ 20 × 100 = 60% duty, leaving 8 ms not commanded on. The injector receives 50 pulses per second. At the same RPM, a 6 ms pulse every revolution also gives 60% duty: 6 ms out of a 10 ms period.
A fixed 10 ms pulse at different speeds
Hypothetical one-pulse-per-720° examples, not operating targets.
| RPM | Repeat period (ms) | Duty | Off-time (ms) |
|---|---|---|---|
| 3,000 | 40 | 25% | 30 |
| 6,000 | 20 | 50% | 10 |
| 9,000 | 13.33 | 75% | 3.33 |
| 12,000 | 10 | 100% | 0 |
Common questions
Should I choose 360 or 720 degrees for a four-stroke engine?
Choose the actual repetition interval for the same injector. One pulse per 720 degrees fits a simple full-sequential schedule; another strategy may pulse that injector every 360 degrees. Confirm the ECU configuration rather than inferring the answer from four-stroke alone.
Does pulse width include injector dead time?
Use total electrical commanded on-time to calculate electrical duty. Some logged channels describe an effective fuel-delivery duration or exclude compensation. Check the channel definition; this tool does not add a guessed dead time or infer physical opening time.
Why is a value above 100% rejected?
A single non-overlapping pulse cannot be longer than its repeat period. A calculated request above 100% indicates incompatible timing inputs in this model. The tool reports that conflict instead of silently clipping the result.
What does exactly 100% mean?
The pulse duration equals the repeat period, leaving zero off-time in this arithmetic model. It is not a recommended operating target or proof that an injector can sustain that condition.
Do I multiply by the number of cylinders?
No. This calculation follows one injector. A six-cylinder engine does not multiply the duty of each injector by six. Use that injector’s pulse duration and repetition interval.
Can I use a total pulse duration from multiple injections?
Not as the width of one repeated pulse here. A separate full-cycle occupancy calculation can sum non-overlapping electrical durations within a known cycle, but it does not establish whether each pulse fits its allowed injection window.
Is this the same as the PWM percentage of a peak-and-hold driver?
No. Current-control switching within a commanded injection event is a different duty measure. This calculator describes the injection envelope, not the high-frequency switching used to regulate coil current.
How does RPM change duty?
If pulse width and the selected interval stay fixed, doubling RPM halves the repeat period and doubles duty until the request reaches the model boundary. Real ECU pulse widths may also change with operating conditions.
Sources & calculation method
Spitronics Fuel Settings manual describes full-sequential and split-sequential pulse schedules. The selected interval here is explicit rather than inferred from a system name. Haltech short pulse width adder documentation illustrates why commanded pulse duration and fuel-delivery behavior need distinct definitions. The calculator applies basic time arithmetic; examples and table are original. It does not import either manufacturer’s tuning recommendations.
For other operating-point calculations, see the Horsepower and Torque Calculator and Engine Airflow Calculator. Neither converts this duty result into a verified fuel-flow requirement.
References accessed September 25, 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.