Fuel injector size calculator
Injector size for your horsepower on gasoline or E85, naturally aspirated or boosted. It also corrects for fuel pressure and checks duty cycle from a datalog.
Injector size needed
- Buy injectors rated at
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- Rated flow in cc/min
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- Total fuel at target
- –
- Measured duty cycle
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How injector size is calculated
cc/min ≈ lb/hr × 10.5
BSFC (brake-specific fuel consumption) is how many pounds of fuel the engine burns per horsepower per hour. The defaults are common rules of thumb: about 0.50 for naturally aspirated gasoline, 0.60 boosted, 0.70 on E85 and 0.80 on boosted E85. A tuner with real data from your combination will have a better figure.
Fuel pressure changes flow
Injectors are rated at a specific pressure, often 43.5 psi (3 bar). Running more pressure raises flow, but only with the square root, so doubling pressure gives about 41% more flow, not twice as much.
Checking duty cycle
On a four-stroke engine each injector gets 720° of crank rotation to fire, which takes 120,000 ÷ rpm milliseconds. Pulse width divided by that time is the duty cycle.
Frequently asked questions
Why does switching to E85 change the injector size I need?
E85 requires roughly 30% more fuel volume than gasoline for the same power level because it needs a richer mixture and has a different BSFC, so an injector sized correctly for gasoline will run short of fuel and risk a dangerously lean condition on E85.
What duty cycle should I target when sizing injectors?
Most tuners recommend staying at or under about 80% max duty cycle at your target power level, since running injectors much past that leaves little margin and can lead to fuel delivery problems as they approach fully open with no rest time between pulses.
How does checking duty cycle from a datalog help?
If you already have a logged duty cycle at a known horsepower level, that's real-world data that's more trustworthy than a theoretical estimate — this calculator lets you check that actual figure against the safe threshold rather than only working from a forward calculation.
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