How injector sizing works
Fuel demand scales with power and how thirsty the engine is (BSFC). Spread that across the injectors and their duty cycle headroom:
lb/hr per injector = (HP × BSFC) ÷ (injectors × duty cycle)
then cc/min ≈ lb/hr × 10.5 for petrol. BSFC is roughly 0.50 for a naturally-aspirated petrol engine, 0.55–0.65 for boost, and higher again on E85 (which needs ~30–40% more fuel by volume). Keeping max duty cycle at ~85% leaves the injectors headroom so they don't run static (fully open) and lose fuelling control.
FAQ
Why not run injectors at 100% duty?
At 100% (static) an injector is just held open — it can no longer meter fuel, so you lose control right when you need it most. Sizing to ~80–85% max keeps a safety margin and steadier idle/part-throttle behaviour.
Do bigger injectors hurt idle?
Very large injectors can idle poorly because the tiny pulse widths needed at idle get inconsistent. Modern high-impedance injectors with good data handle big sizes far better than old ones, but there's always a trade-off — size for your target, not way beyond it.
Is this crank or wheel power?
Use crank (flywheel) power to match typical BSFC figures. If you only have wheel hp, add roughly 15% for drivetrain loss before sizing, or bump BSFC slightly to stay safe.