Fuel Injector Size Calculator

Size fuel injectors for a naturally aspirated, supercharged or turbocharged gasoline engine.

hp
lb/hp·hr
%

How to use the Fuel Injector Size Calculator

  1. Enter the target crank horsepower.
  2. Choose the engine type (bsfc).
  3. Enter the bsfc.
  4. Enter the number of injectors.
  5. Enter the maximum duty cycle.
  6. Press Calculate. The result updates automatically as you type.

About the Fuel Injector Size Calculator

Injectors that are too small run out of capacity at high RPM and lean out the engine, which can destroy pistons. Injectors that are far too large make idle and part-throttle fueling harder to tune. The goal is enough flow to support the target power while keeping duty cycle (the fraction of time the injector is open) at or below about 80–85%.

Forced-induction engines use more fuel per horsepower, partly because they are tuned richer for detonation protection, so their BSFC is higher. Ethanol blends such as E85 need roughly 30–40% more flow than gasoline. Injector flow ratings are measured at a specific fuel pressure (often 43.5 psi or 3 bar); raising fuel pressure increases flow in proportion to the square root of the pressure ratio.

Formula

Injector size (lb/hr) = (HP × BSFC) ÷ (Number of injectors × Max duty cycle)
cc/min ≈ lb/hr × 10.5

BSFC (brake specific fuel consumption) is pounds of fuel per horsepower per hour. The cc/min conversion assumes gasoline at about 6.1 lb per gallon. Example: 400 hp × 0.50 ÷ (8 × 0.80) = 31.25 lb/hr ≈ 328 cc/min.

Example calculation

Using the default values:

  • Target crank horsepower = 400 hp
  • Engine type (BSFC) = Naturally aspirated, typical (0.50)
  • Number of injectors = 8
  • Maximum duty cycle = 80%

the calculator returns:

  • Injector flow needed: 31.3 lb/hr
  • Injector flow needed: 328 cc/min
  • Total fuel flow at full power: 200 lb/hr
  • Total fuel flow (gallons per hour): 32.8 gal/hr

Frequently asked questions

What size injectors do I need for 500 hp?

On a naturally aspirated V8 at 0.50 BSFC and 80% duty: 500 × 0.5 ÷ (8 × 0.8) ≈ 39 lb/hr. A turbo V8 at 0.65 BSFC needs about 51 lb/hr.

How do I convert lb/hr to cc/min?

Multiply by about 10.5 for gasoline. 42 lb/hr ≈ 441 cc/min.

What duty cycle is safe?

Most tuners plan for a maximum of 80–85% to leave headroom. Above about 90% many injectors stop responding linearly.

Last updated: · Category: Automotive calculators

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