fueltech injector calculator

How this FuelTech injector calculator works

Correct injector sizing is one of the most important decisions in any EFI build. If injectors are too small, duty cycle spikes, fuel control gets unstable at high load, and your engine can run dangerously lean. If injectors are oversized without good calibration, idle and transient behavior can become harder to tune. This calculator gives you a practical starting point for selecting the right injector for your FuelTech ECU project.

At the core, the calculator uses a standard performance tuning formula: required fuel flow = horsepower × BSFC. It then divides that total flow across your injector count and maximum safe duty cycle to estimate the injector flow needed per cylinder.

The formula behind the estimate

1) Total fuel flow required

Total Fuel Flow (lb/hr) = Target Horsepower × BSFC

BSFC (brake specific fuel consumption) is the key variable. A well-optimized naturally aspirated gasoline engine may run around 0.45 to 0.55. Forced-induction gasoline combinations are often around 0.55 to 0.70. E85 setups generally require higher BSFC values.

2) Injector flow per injector at your chosen duty cycle

Injector Flow Needed (lb/hr) = Total Fuel Flow ÷ (Number of Injectors × Duty Cycle)

Example: if you cap duty cycle at 85% instead of pushing 100%, you leave important headroom for tuning stability and safety.

3) Pressure correction

Injector flow changes with pressure according to the square-root relationship: Flow₂ = Flow₁ × √(Pressure₂ ÷ Pressure₁)

Since many injectors are rated at 43.5 psi (3 bar), this calculator back-calculates the required rated size if your operating differential pressure differs from the rating pressure.

Recommended setup assumptions for real-world FuelTech tuning

• Stay conservative on duty cycle: 80–90% is a common practical ceiling.
• Use realistic BSFC values: optimistic BSFC leads to undersized injectors.
• Account for future power goals: add at least 10–20% headroom if upgrades are planned.
• Use differential pressure: not just static rail pressure, especially on boosted engines.
• Validate with logs: injector duty, lambda correction, and fuel pressure tracking should all match expectations in FuelTech datalogs.

Quick example

Suppose you want 700 crank hp on turbo gasoline, with BSFC 0.62, eight injectors, and 85% duty cycle:

• Total fuel flow = 700 × 0.62 = 434 lb/hr
• Per injector at 85% IDC = 434 ÷ (8 × 0.85) = 63.8 lb/hr
• If your injector is rated at 43.5 psi and you run 58 psi differential, required rated size drops due to pressure gain

In practice, you would choose the next available injector size above the calculated minimum and keep some reserve.

Common mistakes this calculator helps avoid

Using wheel horsepower without correction

Injector math is usually done from crank horsepower. If you only have wheel horsepower, estimate drivetrain losses before sizing.

Ignoring the fuel type

E85 and gasoline do not require the same mass fuel flow for the same power target. Always update BSFC when changing fuels.

Running injectors at the ragged edge

High duty cycle can reduce tuning authority and response, especially in transient conditions. Give yourself margin.

Final note

This calculator is intended as a sizing guide, not a substitute for dyno testing or professional calibration. After selecting injectors, confirm deadtimes, short-pulse behavior, and pressure consistency in your FuelTech tune. Proper injector characterization is what turns a decent setup into a reliable, repeatable one.