Every fuel injector is essentially a solenoid-controlled valve that opens for a precisely measured slice of time. That slice is called pulse width, measured in milliseconds, and the percentage of the engine cycle the injector spends open is called duty cycle. Understanding the relationship between these two numbers is the difference between a tune that makes power and one that leans out and melts a piston at 6,500 RPM.

Here's the mechanical reality: an injector needs time to physically open, stabilize flow, and close. That opening and closing transient — typically 0.5 to 1.5 ms — flows less fuel than the fully-open period and isn't perfectly linear. ECUs compensate using an injector latency table (sometimes called offset or dead time), which adds extra pulse width based on battery voltage. Drop from 14V to 10V during cranking and dead time can nearly double, which is why a weak battery causes hard starts even with a perfect fuel system.

The 80% rule: never size injectors to exceed 80% duty cycle at peak power. Above that, you're running out of cycle time and the injector enters static flow — it can't close fully between pulses, fuel delivery becomes nonlinear, and lean spikes appear right where cylinder pressure is highest.

Quick sizing calculation: Required injector flow (lb/hr per cylinder) = (Target HP × BSFC) ÷ (Number of cylinders × 0.80)

Real example: a turbo 2.0L making 400 hp on a 4-cylinder with a BSFC of 0.55 (typical for boosted gasoline):

• (400 × 0.55) ÷ (4 × 0.80) = 68.75 lb/hr injectors minimum
• You'd pick 72 or 80 lb/hr off the shelf for headroom

But there's a catch on the low end. A massive 120 lb/hr injector pulsing at idle might only need 1.2 ms pulse width — barely longer than its dead time. The injector spends most of its open event in the nonlinear transient region, causing rough idle, poor cold start, and lousy fuel economy. This is the dynamic range problem.

Modern injectors like Bosch EV14s and Injector Dynamics ID1050x are prized because they have excellent short pulse width linearity — well-characterized behavior down to ~0.4 ms — letting one injector cover both idle and 700 hp without drivability tradeoffs.

Diagnostic tip: scan tool showing 95%+ duty cycle at WOT means you're already out of fuel before you know it. Datalog injector duty cycle alongside AFR on every dyno pull.