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Injector size

Injectors that are too small put a ceiling on power and push the duty cycle up to dangerous levels. Enter the target power, the number of cylinders and the conditions, and you get the minimum injector size per cylinder.

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Units
Power at the crankshaft.
Fuel consumption per power. Petrol 0,45-0,65, methanol 1,2-2,1.
Max 80-85% on the street, up to 90% for racing.
The pressure the injectors are rated at, often 3,0 bar.
The rail pressure while running.

Injector size per injector

679 cc/min

Total fuel demand: 220,0 lb/h
Per injector: 55,0 lb/h

The size is per injector at the given duty cycle. Choose the next larger standard size. The factor 10,5 cc/min per lb/h applies to petrol.

How the calculation works

The total fuel demand is target power times BSFC, that is how many pounds of fuel the engine uses per hour at full power. It is divided by the number of cylinders and converted to cc/min with the factor 10,5 that applies to petrol. Then we divide by the duty cycle, because you never run the injectors fully open.

Fuel pressure matters. An injector flows more at higher pressure, roughly by the square root of the pressure ratio. If you run higher rail pressure than the injector is rated for, a smaller injector will do, and vice versa. Leave a margin and pick the next larger standard size.

cc/min = (power × BSFC / cylinders × 10,5) / (duty cycle × √(actual pressure / base pressure))

Example

400 hp on a four-cylinder turbo engine with a BSFC of 0,55 and 85% duty cycle needs around 680 cc/min per injector at 3,0 bar.

Guiding injector sizes

ApplicationInjector size
Weak naturally aspirated, street220-310 cc/min
Mid-range NA or turbo440-550 cc/min
High-power turbo750-1000 cc/min
Race fuel or high boost1000 cc/min and up
Methanol2-3 times larger than petrol

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Common questions about injector size

BSFC is fuel consumption per power per hour. For petrol, 0,5 is often used for a naturally aspirated engine and 0,55-0,60 for turbo. Methanol and E85 need considerably more fuel and therefore a higher BSFC.

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