Fuel Injector Calculator

Calculate the required fuel injector size to support your horsepower goals. Properly sized injectors ensure adequate fuel delivery while maintaining safe duty cycles and good drivability.

Desired engine horsepower output

Base fuel pressure at the injector

Total number of engine cylinders

Brake Specific Fuel Consumption (0.45-0.55 typical)

Maximum injector on-time percentage (80% recommended)

Injector Size (lb/hr per cylinder) = (Target HP × BSFC) ÷ (Number of Cylinders × Max Duty Cycle). Flow Rate = Base Flow × sqrt(New Pressure ÷ Base Pressure)
For a 400 HP turbocharged 4-cylinder with 0.55 BSFC at 80% duty cycle: Per cylinder = 68.75 lb/hr or 722 cc/min. Recommended: 750 cc/min injectors

What size fuel injectors do I need for my horsepower goal?

Injector size depends on your target horsepower, number of cylinders, and fuel pressure. A general formula is: (HP × BSFC) ÷ (Number of Cylinders × Duty Cycle). For example, a 400 HP four-cylinder engine with 0.5 BSFC at 80% duty cycle needs approximately 625 cc/min (60 lb/hr) injectors per cylinder. Always size slightly larger to avoid running injectors at maximum capacity.

What is BSFC and what value should I use?

BSFC (Brake Specific Fuel Consumption) measures fuel efficiency in pounds of fuel per horsepower per hour. Naturally aspirated engines typically use 0.45-0.50 lb/hp/hr. Turbocharged or supercharged engines use 0.50-0.60 lb/hp/hr. Less efficient or racing engines may use 0.60-0.70 lb/hp/hr. Use 0.5 as a safe middle ground for most applications.

What is injector duty cycle and why does it matter?

Duty cycle is the percentage of time an injector is open (spraying fuel) versus closed. Running injectors above 80-85% duty cycle reduces their lifespan, causes inconsistent fuel delivery, and limits power. At 100% duty cycle, the injector cannot deliver more fuel regardless of tuning. Always size injectors so they operate at 80% duty cycle or less at maximum power.

How does fuel pressure affect injector flow?

Injector flow increases with the square root of the pressure ratio. If you increase fuel pressure from 43.5 PSI to 58 PSI, flow increases by about 16%. The formula is: New Flow = Old Flow × sqrt(New Pressure ÷ Old Pressure). This allows you to increase flow without changing injectors, but higher pressure requires a higher-capacity fuel pump.

Can I use larger injectors than needed?

Yes, but with limitations. Oversized injectors make it harder to tune at idle and light throttle because they deliver too much fuel in short pulses. Going 20-30% larger than needed is usually acceptable with proper tuning. Going too large (50%+) can cause poor drivability, rough idle, and difficulty achieving proper air-fuel ratios at low RPM.