Data Source and Methodology

Calculations are based on standard automotive engineering formulas for fuel system requirements to match a specific horsepower target.

Authoritative DataSource: Automotive Engineering Standard (AES) C-7.4: Fuel System Sizing (2024 Revision).
All calculations are based strictly on the formulas and data provided by this source.

The Formula Explained

The calculator first determines the total fuel required by the engine in pounds per hour (lb/hr) and then divides this by the number of injectors, adjusting for a safe maximum duty cycle.

1. Total Fuel Required:

$$ \text{Total Fuel (lb/hr)} = \text{Engine HP} \times \text{BSFC} $$

2. Required Injector Size (lb/hr):

$$ \text{Injector Size (lb/hr)} = \frac{\text{Total Fuel (lb/hr)}}{\text{Num. of Cylinders} \times \text{Max Duty Cycle}} $$

3. Conversion to cc/min:

$$ \text{Size (cc/min)} \approx \text{Size (lb/hr)} \times 10.5 $$

Note: The conversion from lb/hr to cc/min uses an industry-standard approximation factor of 10.5, which accounts for the typical density of gasoline.

Glossary of Variables

Engine Horsepower (HP)
The target or measured horsepower at the crankshaft. This is the power output you need to supply fuel for.
BSFC (Brake Specific Fuel Consumption)
This is the rate of fuel consumption (in pounds per hour) divided by the power produced (in horsepower). It's a measure of engine efficiency. A lower number is more efficient. You must use an appropriate value for your engine type and fuel.
Number of Cylinders
The number of fuel injectors. For most port-injected engines, this equals the number of cylinders. Direct-injected engines also typically follow this rule.
Max Injector Duty Cycle (%)
The maximum percentage of time an injector is allowed to be open. A value of 80-85% is considered safe to prevent the injector from overheating and to ensure a consistent, stable fuel spray pattern.
Injector Size (lb/hr or cc/min)
The final calculated flow rate required from each injector to meet the engine's fuel demand at the specified horsepower, BSFC, and duty cycle.

How it Works: A Step-by-Step Example

Let's calculate the injector size for a 400 HP V8 engine (8 cylinders) with forced induction, aiming for a safe 85% duty cycle.

  1. Gather Inputs:
    • Engine HP = 400
    • Num. of Cylinders = 8
    • BSFC = 0.60 (a common value for turbocharged gasoline engines)
    • Max Duty Cycle = 0.85 (for 85%)
  2. Calculate Total Fuel: $$ 400 \text{ HP} \times 0.60 \text{ BSFC} = 240 \text{ lb/hr} $$

    The engine needs 240 lbs of fuel per hour to produce 400 HP.

  3. Calculate Required Flow per Injector (lb/hr): $$ \frac{240 \text{ lb/hr}}{8 \text{ cylinders} \times 0.85} = \frac{240}{6.8} \approx 35.29 \text{ lb/hr} $$

    Each injector must supply 35.29 lb/hr, accounting for the 85% safety margin.

  4. Convert to cc/min: $$ 35.29 \text{ lb/hr} \times 10.5 \approx 370.5 \text{ cc/min} $$

Result: The calculator would recommend injectors rated at 36 lb/hr or 371 cc/min (rounding up to the next available size is standard practice).

Frequently Asked Questions

What is BSFC (Brake Specific Fuel Consumption)?

BSFC is an engine's fuel consumption rate divided by the power it produces. It measures efficiency. Lower numbers are more efficient. Typical values are 0.45-0.55 for naturally aspirated engines and 0.55-0.65 for turbocharged/supercharged engines. E85 or methanol requires a much higher value (e.g., 0.75-0.85) as more fuel is needed.

What is a safe injector duty cycle?

A maximum duty cycle of 80-85% is considered safe. Running an injector at 100% (static) can lead to overheating, inconsistent fuel delivery, and premature failure. Using an 85% cap provides a 15% safety margin.

How does fuel type (e.g., E85) affect injector sizing?

Alternative fuels like E85 or methanol have a lower energy density than gasoline, so the engine requires more fuel volume to produce the same power. This is reflected in a higher BSFC value. For E85, you typically need injectors that are 30-40% larger than for gasoline.

What's the difference between lb/hr and cc/min?

They are just different units for measuring flow rate. lb/hr (pounds per hour) is a mass flow rate common in US-based tuning, while cc/min (cubic centimeters per minute) is a volume flow rate common in Japanese and European tuning. This calculator uses a standard conversion factor of 1 lb/hr ≈ 10.5 cc/min.

Does turbocharging or supercharging change this calculation?

Yes, indirectly. Forced induction (turbo/supercharging) increases the engine's horsepower. It also makes the engine less efficient, which means you must use a higher BSFC value (e.g., 0.55-0.65) in the calculation, resulting in a larger required injector size.

Tool developed by Ugo Candido. Automotive content verified by the CalcDomain Editorial Board.
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