Workshop calculator
Injector duty cycle calculator (pulse width and rpm)
On a four-stroke engine each injector fires once every two revolutions. The time available is 120,000 ÷ rpm milliseconds; the pulse width as a share of that time is the duty cycle.
ms
rpm
- Time per cycle
- 150.0 ms
- Duty cycle
- 2.0 %
Within the injector's working range.
How to use it in a diagnosis
- Read injector pulse width (ms) and rpm from live data or a scope at the condition you want to check.
- Enter both values.
- Above about 80–85 % the injectors are close to their limit; at idle petrol engines usually sit at 1.5–4 %.
Questions mechanics ask
How do you calculate injector duty cycle?
Duty cycle (%) = pulse width (ms) × rpm ÷ 1,200 for a four-stroke engine with one injection per cycle. For example 3 ms at 800 rpm is 2 %.
What is a safe maximum injector duty cycle?
Most tuners keep it below about 80–85 %. Above that the injector barely has time to close between injections and fuelling becomes unpredictable.