How to use it
- Enter the Target power and choose its Power unit.
- Enter the Fuel consumption, BSFC with its BSFC unit, then the Number of injectors and the Maximum duty cycle, %.
- Read Flow per injector, cc/min and Flow per injector, lb/hr: the size each injector must deliver.
- To check an injector you have, enter its flow in Injector you have, cc/min (optional) and read Duty cycle with the injector you have.
How the flow is worked out
The fuel an engine burns is its power times its BSFC. The flow of one injector is the total fuel divided by the number of injectors and by the share of each cycle they are open. An engine making 300 hp at 0.53 lb/(hp·h) burns 159 lb of petrol an hour.
Fuel per hour (g) = power (kW) × BSFC (g/(kW·h))
BSFC (g/(kW·h)) = BSFC (lb/(hp·h)) × 608.277
Total (cc/min) = fuel per hour ÷ 60 ÷ density (kg/L)
Per injector = total ÷ (injectors × maximum duty cycle)
Duty cycle of an injector = (total ÷ injectors) ÷ its flow × 100Which BSFC to use
- BSFC is the fuel burned per unit of power and hour. The petrol cycle average in the BSFC article is 322 g/(kW·h), which is 0.53 lb/(hp·h).
- BSFC is at its lowest where the air is unthrottled and the engine runs near its peak torque, so the cycle average is above the best figure. A higher BSFC gives a safer, larger injector.
- The real figure depends on the tune, the ignition timing and the fuel. Injector Dynamics says flow figures from an estimated BSFC are very error prone.
What this calculator does not include
- The fuel pressure: injector flow ratings are given at a stated pressure, so check the rating at the pressure your system runs.
- The air-fuel ratio and the extra fuel for full power, which change the real fuel burned.
- The shortest time an injector can stay open, which limits the smallest duty cycle it can run at.
Worked example: 300 hp, 0.53 lb/(hp·h), four injectors, 80 % duty
Fuel: 300 hp is 223.71 kW, and 223.71 × 322.4 = 72,121 g/h, which is 159.0 lb/h. In volume that is 72,121 ÷ 60 ÷ 0.755 = 1,592 cc/min.
Per injector at 80 %: 1,592 ÷ (4 × 0.80) = 498 cc/min, or 49.7 lb/h. A 550 cc/min injector runs at 398 ÷ 550 = 72.4 % duty, below the 80 % warning level.
| Result | |
|---|---|
| Flow per injector | 498 cc/min (49.7 lb/h) |
| Total fuel | 1,592 cc/min (159.0 lb/h) |
| Duty with a 550 cc/min injector | 72.4 % |
Frequently asked questions
What units is injector flow measured in?
Flow is measured in cubic centimetres per minute (cc/min) or pounds per hour (lb/hr). At the 0.755 kg/L density used here, one lb/hr of petrol is about 10.0 cc/min.
Which duty cycle should I plan for?
Injector Dynamics marks a duty cycle above 80 % as hot and above 90 % as over the limit. Planning at 80 % or less keeps the margin those warnings describe. The figure you choose is yours to set.
Where do I find the BSFC of my engine?
It is rarely published, and a figure measured on a dyno is the best. For a petrol engine, 0.53 lb/(hp·h), which is 322 g/(kW·h), is a starting point; a higher figure gives a safer, larger injector.
Why is the flow per injector only part of the total?
Each injector supplies a share of the fuel, so the total is divided by the number of injectors. The duty cycle sets how long each one stays open: at 80 % it supplies 80 % of its rated flow.
Is the result exact?
No. It is an estimate, because the real fuel use depends on the tune, the timing and the fuel, and BSFC is itself an estimate. Confirm the size on a dyno or with a tuner.
Does the calculator keep the numbers I type?
No. The calculation runs in your browser, and nothing you type is sent to a server or saved for later.
Sources
- Wikipedia: Brake-specific fuel consumption (BSFC, the 608.277 conversion, the petrol cycle average of 322 g/(kW·h))
- Wikipedia: Gasoline (the 0.755 kg/L European reference density)
- Wikipedia: Duty cycle (the share of time a signal is active)
- Injector Dynamics: injector selector (duty cycle flags above 80 % and 90 %, and the BSFC caution)
Figures and text last checked on Tuesday, November 10, 2026.