Fuel Injector Calculator

The fuel injector calculator helps you find the recommended fuel injector flow rate for different engine specifications. Enter your desired parameters, and our calculator will do the hard work for you in the blink of an eye.

In this article, you’ll learn about:

• What is a fuel injector​?
• Brake-specific fuel consumption​;
• What is the duty cycle​?
• How can I find the fuel injector flow rate for a V8 engine?
• And much more.

So, prepare your engine, and let’s compute the fuel injector flow rate.

First things first. Let us start with the definition of a fuel injector. In general terms, a fuel injector is an electronically controlled valve that delivers a precise and controlled amount of fuel into the engine’s intake manifold or directly into the combustion chamber, replacing the carburetors used in older vehicles.

In modern gasoline-injection​ systems, the engine control unit (ECU) opens and closes each injector thousands of times per minute, adjusting the pulse width to match the engine’s load, speed, and air intake.

The injector is also characterized by the fuel injector flow rate​, which is typically expressed in pounds per hour (lb/hr), cubic centimeters per minute (cc/min), or grams per second (g/s). So, to choose the right injector size, it is important to know some features of your engine, such as the target engine horsepower, the number of cylinders, and the fuel pressure. You can learn more about horsepower by accessing our engine horsepower calculator.

The aspiration type of the engine is also relevant. For instance, a naturally aspirated engine draws air in at atmospheric pressure, requiring a lower flow rate. However, in turbocharged or supercharged engines, you need to increase the flow rate, since more air is forced into each cylinder and, consequently, more fuel is required to maintain the correct air-fuel mixture.

Another important parameter when sizing a fuel injector is the brake-specific fuel consumption, or simply BSFC. This parameter measures how much fuel an engine burns to produce a given amount of power over a certain time.

BSFC is usually expressed in pounds per horsepower-hour (lb/hp·hr) or grams per kilowatt-hour (g/kWh), and it is a direct indicator of engine efficiency. In general, the lower the BSFC, the less fuel the engine needs to deliver the same power output.

The typical values for gasoline engines range from about $0.40$ to $0.50 \,\mathrm{lb/hp \cdot hr}$ for naturally aspirated engines. For turbocharged and supercharged engines, the BSFC tends to fall between 0.55 and 0.65 lb/hp·hr, since these engines usually run richer mixtures to control combustion temperatures and prevent knock.

The table below lists the BSFC values used by this fuel injector calculator. These values are estimates: real BSFC depends on the engine, tune, fuel system, air-fuel ratio, and operating conditions.

How do we actually find the right injector flow rate for an engine? Don’t worry, you don’t need to be a mechanical engineer or a specialist. To calculate injector flow rate, you only need four numbers: the target horsepower, the brake specific fuel consumption (BSFC), the number of cylinders, and the maximum duty cycle.

First, be precise about how much power you expect. For example, if you have modified your engine with a new camshaft, a better exhaust, or a turbo, take the stock horsepower and add the gains from each upgrade. Next, pick a BSFC that matches your setup. You can check our previous table to find the proper specifications. Finally, choose the duty cycle, which is how long the injector is allowed to stay open during each engine cycle. Around $80\%$ is a safe bet for stock engines, while for high-performance applications you may push it to $90\%$.

Now, you can substitute these parameters into the following formula for injector flow rate $\rm IFR$:

where $k$ is the horsepower correction factor. The standard values for this parameter are: $k = 1$ if the horsepower is measured at the crankshaft, and $k = 1.15$ if it's measured at the wheels.

It is interesting to point out that an injector flow rating is usually quoted at a reference pressure of $43.5\,\mathrm{psi}$ (3 bar). So, if your fuel system runs at a different pressure, the injector will flow more or less than its rated value suggests. Thus, you need to account for a correction factor in the determination of the injector flow rate, as we can see in the following equation:

where $\mathrm{IFR}_N$ is the normalized injector flow rate for a $43.5\,\mathrm{psi}$ fuel injector.

Here is an example showing how to use the fuel injector calculator. Suppose that you have a naturally aspirated four-cylinder engine running on gasoline, and you’re aiming for $200\,\mathrm{hp}$ at the crankshaft. Select the options gasoline for fuel type and naturally aspirated for aspiration to get $\mathrm{BSFC} = 0.50 \,\mathrm{lb/hp \cdot hr}$. Moreover, take $80\%$ as the maximum duty cycle for this vehicle.

Now, you can enter these values into the calculator and find that the injector flow rate is

As you can see, our calculator automatically converts the injector flow rate to cc/min or g/s. Feel free to explore our tool and create your own examples instantly.