Injector sizing explained

Injectors are the taps your engine drinks through. Size them too small and you run out of fuel at full power — the classic cause of a lean, melted piston. Size them absurdly large and idle and part-throttle control get coarse. The goal is enough flow, with headroom, at a sane duty cycle.

Start from horsepower, not injector charts

Fuel need scales with power. Two numbers translate hp into fuel flow:

• BSFC (brake-specific fuel consumption) — how much fuel the engine burns per horsepower per hour. Roughly 0.45–0.50 for efficient NA petrol, 0.55–0.65 for boosted, and higher again for E85 because you burn ~30% more of it.
• Duty cycle — the share of time the injector is open. Tuners cap sustained duty at 80% so the injector never runs flat-out (static), where control gets sloppy and heat builds.

flow per injector = (hp × BSFC) ÷ (cylinder count × duty)

cc/min vs lb/hr

The same flow rating is quoted two ways: lb/hr (pounds of fuel per hour, the US convention) and cc/min (the metric convention). The bridge is fuel density — for gasoline, cc/min ≈ lb/hr × 10.5. A "1000cc" injector is a 95 lb/hr injector. Mind the test pressure too: flow ratings assume a standard fuel pressure (usually 3 bar / 43.5 psi); raising base pressure increases flow roughly with the square root of pressure.

Worked example

A 400 hp boosted four-cylinder on petrol: 400 × 0.55 = 220 lb/hr of fuel total. Across 4 injectors at 80% duty: 220 ÷ (4 × 0.8) = 68.8 lb/hr ≈ 720 cc/min each. You'd shop for 750–1000cc units to keep headroom for future power. On E85, bump BSFC to ~0.70 and the same engine wants ~925 cc/min — why E85 conversions so often need bigger injectors and a bigger pump.

Why not to oversize wildly

At idle the engine sips fuel, so pulse widths get tiny. An injector too big for the job spends its life in the sub-1 ms range where flow is non-linear and cylinder-to-cylinder spread grows — rough idle, hunting, poor emissions. Modern high-impedance injectors with good low-pulse linearity (and ECUs that model it) have widened the usable range a lot, but "biggest that fits" is still not a strategy.

Don't forget the pump

Injectors meter fuel; the pump delivers it. Total fuel flow at your power target (hp × BSFC, converted to litres per hour) is the pump's job — with margin for voltage drop and wear. A pump that can't keep up shows up as falling fuel pressure at high load, exactly when you can least afford it.