Calculate fuel injector size for horsepower goals and estimate airflow requirements. Essential tools for EFI conversions, turbo builds, and performance fuel system upgrades.
Calculate the minimum injector size needed to support your horsepower goal. Accounts for BSFC (fuel consumption rate) and safe duty cycle limits.
BSFC Guidelines:
Duty Cycle: 80% is safe for street use, 85-90% acceptable for racing. Higher duty cycles risk fuel starvation.
Estimate required airflow for intake manifolds, throttle bodies, and carburetors. Based on the rule of thumb: 0.5 CFM per horsepower for naturally aspirated engines.
Formula: CFM = HP × 0.5 (for naturally aspirated engines)
Note: Forced induction applications require larger airflow capacity. This calculator provides baseline estimates for naturally aspirated engines.
Proper fuel system sizing is critical for reliability and performance. Undersized injectors run at excessive duty cycles and can't deliver enough fuel, causing lean conditions and engine damage. Oversized injectors make tuning difficult at idle and light throttle. These calculators help you select correctly-sized components.
Fuel injectors are rated in pounds per hour (lb/hr) or cubic centimeters per minute (cc/min) at a specific fuel pressure, typically 43.5 PSI (3 bar). Common conversions:
Brake Specific Fuel Consumption (BSFC) measures how efficiently an engine converts fuel into power. Lower numbers indicate better efficiency. Naturally aspirated engines typically achieve 0.45-0.50 lb/hp-hr under ideal conditions. Forced induction engines run richer for safety and cooling, resulting in higher BSFC values around 0.55-0.65. E85 fuel requires even more volume due to lower energy density, pushing BSFC to 0.65-0.75.
Duty cycle represents the percentage of time an injector is open (flowing fuel) during each engine cycle. An 80% duty cycle means the injector is open 80% of the available time and closed 20%. Running injectors above 90% duty cycle is dangerous because there's insufficient time to deliver fuel during high-load conditions, potentially causing catastrophic engine damage from detonation.
The 0.5 CFM per horsepower rule provides a starting point for sizing intake systems. A 400 HP engine needs approximately 200 CFM of airflow. However, this is conservative—many high-performance engines achieve better volumetric efficiency and may require 0.6-0.7 CFM per HP. Forced induction applications need even more airflow capacity to account for boost pressure.
500 HP Naturally Aspirated LS3: Using BSFC of 0.50 at 80% duty cycle requires 47 lb/hr injectors per cylinder (376 lb/hr total for 8 cylinders).
600 HP Turbocharged 2JZ: Using BSFC of 0.60 at 85% duty cycle requires 70 lb/hr injectors per cylinder (420 lb/hr total for 6 cylinders).
800 HP E85 Big Block: Using BSFC of 0.70 at 80% duty cycle requires 88 lb/hr injectors per cylinder (704 lb/hr total for 8 cylinders).
When increasing horsepower significantly, you may need to upgrade multiple fuel system components:
Approximate injector sizes for V8 engines at 80% duty cycle with gasoline (BSFC 0.50):
| Target HP | Injector Size | Application |
|---|---|---|
| 300 HP | 30 lb/hr | Stock/mild performance |
| 400 HP | 40 lb/hr | Bolt-on performance |
| 500 HP | 48 lb/hr | Cam, heads, intake |
| 600 HP | 58 lb/hr | Stroker, forced induction |
| 750 HP | 72 lb/hr | Serious boost/nitrous |
| 1000 HP | 95 lb/hr | Race/extreme builds |
Note: These are estimates for naturally aspirated or mild boost applications. E85 and high-boost applications require larger injectors.