Injector Sizing Formula
Injector flow per injector (lb/hr) = Target crank HP × BSFC ÷ (number of injectors × max duty cycle)
Example: 600 hp × 0.55 lb/hp-hr ÷ (8 × 0.85) ≈ 48.5 lb/hr per injector. The BSFC value is an assumption, so use a value appropriate to fuel, induction and tune rather than treating 0.55 as universal.
Rated Pressure Is Part Of The Injector Rating
Always record the pressure at which an injector was characterized. Holley’s published data illustrates why: the same injector family can have different stated flow when tested at different pressure. If you change differential pressure, use manufacturer data when available rather than treating a generic square-root conversion as complete calibration data.
Duty Cycle Is Headroom
DeatschWerks’ injector calculator explicitly asks for target crank power, injector count, induction type, fuel type, BSFC and maximum duty cycle. That is the correct way to think about sizing: injector capacity is not just a horsepower sticker.
Pressure Changes Flow
Injector flow changes with the square root of pressure ratio. For the same injector, a useful planning relation is new flow = rated flow × √(new differential pressure ÷ rated differential pressure). Do not confuse rail pressure with differential pressure across the injector.
Naturally Aspirated
If manifold pressure is near atmospheric at WOT, rail pressure and injector differential pressure are relatively close (depending on the system design).
Boosted
A boost-referenced return system raises rail pressure with manifold pressure so differential pressure across the injector stays near the intended base value. A non-referenced system requires the calibration/injector model to account for the changing differential.
Offset / Dead-Time Is Not Optional Detail
Injector opening behavior changes with voltage. Holley publishes off-time curves from low to high supply voltage for its injectors. On controllers that model this behavior, correct characterization is especially important at idle and short pulse widths; “60 lb/hr” by itself is not enough information to build a high-quality injector calibration.
Pump Flow Must Be Checked At Pressure
Aeromotive specifically warns against judging a pump by free-flow GPH alone. Pump selection depends on fuel type, engine power, BSFC, maximum system pressure and voltage at the pump under load. A pump that looks huge at zero pressure can lose substantial flow at EFI pressure.
- Size from target crank power with realistic BSFC and duty-cycle assumptions.
- Verify injector characterization data matches your ECM strategy.
- Verify pump flow at the actual operating pressure and voltage.
- Account for E85/ethanol’s greater fuel-volume requirement.
- Log commanded vs actual lambda and fuel pressure under load.