Fuel Injector Size Calculator

Enter your target horsepower, number of cylinders and fuel to get the injector flow rate you need in pounds per hour and cc per minute, based on an 80% duty cycle.

How to calculate fuel injector size

Injector size (lb/hr) = horsepower × BSFC ÷ (number of injectors × duty cycle)

This is the formula Summit Racing gives for injector sizing, and the calculator uses an 80% duty cycle, which Summit recommends so the injector is closed 20% of the time even at wide-open throttle. Horsepower times BSFC is the pounds of fuel the engine burns per hour. Dividing by the number of injectors splits it between them, and dividing by the duty cycle sizes each injector so it only has to be open 80% of the time.

The calculator converts pounds per hour to cc per minute by multiplying by 10.5, an approximation based on the density of gasoline. Use the pounds-per-hour figure when comparing injectors.

BSFC by fuel and induction

BSFC, or brake specific fuel consumption, is how much fuel the engine uses per hour for each horsepower it makes. A lower number means a more efficient engine. Summit Racing lists these ranges:

Engine	Gasoline	E85	Methanol
Naturally aspirated	0.45 to 0.50	0.63 to 0.70	0.90 to 1.00
With nitrous oxide	0.60 to 0.65	0.84 to 0.91	1.20 to 1.30
Forced induction	0.60 to 0.75	0.84 to 1.05	1.20 to 1.50

From Summit Racing's What is BSFC?

The calculator's fuel choices use a BSFC inside each range: 0.5 for naturally aspirated gasoline, 0.7 for boosted gasoline or naturally aspirated ethanol, 0.9 for boosted ethanol, 1.0 for naturally aspirated methanol and 1.5 for boosted methanol.

Example: a 300 hp V8

A naturally aspirated 300 hp engine on gasoline with eight injectors:

300 × 0.5 ÷ (8 × 0.8) = 23.44 lb/hr, or about 246 cc/min

Injector size for a V8

Pounds per hour per injector for an eight-cylinder engine at an 80% duty cycle, using the calculator's BSFC values:

Horsepower	Gasoline (0.5)	Boosted gasoline or E85 (0.7)	Boosted E85 (0.9)	Methanol (1.0)
300	23.44	32.81	42.19	46.88
400	31.25	43.75	56.25	62.50
500	39.06	54.69	70.31	78.13
600	46.88	65.63	84.38	93.75
700	54.69	76.56	98.44	109.38
800	62.50	87.50	112.50	125.00

Calculated with the formula above.

Picking an injector

Injectors come in set sizes, so you'll rarely find one that matches the calculation exactly. Holley's injector selection guide says to round up to the next size: if you calculate 26 lb/hr and can choose between 24 and 30 lb/hr injectors, pick the 30. The same guide says to enter the horsepower you plan to make later, not just today's number, so the injectors are already big enough when the engine is built up.

The injectors also need a fuel pump that can supply them. The fuel pump flow calculator sizes the pump for the same horsepower and fuel. If you're staying with a carburetor, use the CFM calculator instead.

Frequently Asked Questions

How do you calculate fuel injector size?

Multiply horsepower by BSFC, then divide by the number of injectors times the duty cycle. The result is the flow each injector needs in pounds per hour.

What size injectors do I need for 400 hp?

For a naturally aspirated V8 on gasoline at 0.5 BSFC and an 80% duty cycle, about 31.25 lb/hr per injector. A boosted gasoline or naturally aspirated E85 engine at 0.7 BSFC needs about 43.75 lb/hr.

What BSFC should I use?

Use 0.45 to 0.50 for a naturally aspirated gasoline engine, 0.60 to 0.75 for gasoline with forced induction, 0.63 to 0.70 for naturally aspirated E85 and 0.90 to 1.00 for naturally aspirated methanol.