Fuel Injector Sizing Calculator
Injector size from target power, brake specific fuel consumption and the duty cycle you are willing to run.
| Injector size needed | 39.1 | lb/hr |
| Injector size needed | 410 | cc/min |
| Total fuel flow | 312.5 | lb/hr |
- injector lb/hr = (horsepower × BSFC) ÷ (injector count × duty cycle)
- cc/min ≈ lb/hr × 10.5 for petrol at the usual rail pressure
- Injector flow is quoted at a set pressure — raising fuel pressure raises flow by √(new ÷ old).
When you need it
When planning an EFI fuel system, and when checking whether the injectors you already own will support a power target.
BSFC is the variable that moves the answer
Naturally aspirated gasoline sits around 0.45 to 0.50 pounds of fuel per horsepower per hour. Boosted gasoline runs 0.55 to 0.65, because you deliberately run richer and pull timing to control detonation.
E85 needs roughly thirty percent more fuel by mass for the same air, so budget 0.65 to 0.75. Methanol is roughly double gasoline. Picking the BSFC for the wrong fuel is the single most common way to end up short.
Duty cycle and headroom
Eighty percent is the sane ceiling for sizing. Beyond that you lose resolution at the top of the scale, the injector runs hotter, and you have nothing left for a cold morning or a bad tank of fuel.
Do not just buy the biggest
An oversized injector spends idle and cruise at very short pulse widths, where its flow is least linear and hardest to characterize. That makes a car that is difficult to idle smoothly and worse at part throttle. Modern injectors handle this far better than older designs, but the principle stands: size for the target you have, not the one you daydream about.
The injector is one part of the system
Flow is quoted at a specific fuel pressure and scales with the square root of the pressure ratio, so raising base pressure buys less than people expect. The pump has to deliver that flow at that pressure, and the lines, filter, regulator and return have to keep up. Sizing injectors in isolation is how fuel systems run out of pressure at the top of third gear.