Guide to Sizing Fuel Injectors Properly

Welcome to the fuel injector calculator. This calculator is designed to allow anyone to properly size their injectors.

Naturally Aspirated or Forced Induction?
Select Your Fuel?
How Many Cylinders Do You Have?
What is Your Flywheel Horsepower?

Fuel Injector Facts

First of all, adding larger fuel injectors WILL NOT CREATE EXTRA HORSE POWER!  There is a common misconception that the larger the fuel injector, the more power they create. Fuel injectors do not create horsepower, they support horsepower. The larger the flow rate, the more horsepower they are able to support.

Larger fuel injectors are only required when you have exceeded the horsepower capacity of the existing fuel injectors. Once again, the addition of a larger fuel injector will do nothing to add horsepower; a larger injector will merely SUPPORT more horsepower.

You can use this page to properly determine what size injectors are needed for various applications. Keep in mind that we are referring to flywheel HP, not wheel HP.

Engines require a certain rate of fuel (generally measured in “LB/HR”) which is determined from its Brake Specific Fuel Consumption (BSFC). By definition, ‘BSFC’ represents how much fuel is required for each horsepower being made. Most gasoline engines operate on a .42 to .52 BSFC.   Higher compression engines are more efficient and depending on the compression ratio can have a BSFC as low as .38 to .42.  The average street engine usually runs about a .5 BSFC, so that is the figure we use for naturally aspirated engines. We use a BSFC of .55 for forced induction.

So, to calculate an engine’s fuel requirement, we use the following formula:         HORSEPOWER x BSFC

EXAMPLE:    A 300 HP naturally aspirated engine running a BSFC of .5 requires what size fuel injector?
300 x 0.5 = 150 LB/HR total fuel requirement

Now we need to divide the total fuel requirement by the number of injectors being used. If this were an 8 cylinder engine, using 1 injector per cylinder, figure injector size as follows:

150 LBS/HR / 8 injectors = 18.75 LB/HR per cylinder

Using this formula, this engine requires a 19 # fuel injector to support 300 HP, but this will require that the injector is at nearly a 100% duty cycle in order to achieve this horsepower level. A duty cycle refers to how much time is available to supply fuel vs. how much time you actually need to use. When you have used the entire allotted time window available, this is referred to as a 100% duty cycle.  In other words, the engine computer does not hold the injector on steady. Rather, it “pulses” the injector on and off. The longer the “pulse”, the more fuel the injector sprays. At 100% duty cycle, the injector would be on constantly. At 50% duty cycle, the injector is on 50% of the time. 

There seems to be some common confusion that running an injector at a duty cycle in excess of 80% has a tendency to be bad for the injector, and that it can possibly ‘overheat’ the injector. There is no truth to this. First, the injector has fuel constantly passing through it, which acts as a coolant for on the injector. You could actually run an injector at 100% duty cycle for days at a time with no adverse affects whatsoever to the injector. Second, keep in mind that it is virtually impossible to run the injector at 100% duty cycle for any significant length of time. The injector will only be at 100% duty cycle for as long as the engine is making peak horsepower, which can only be for a short period of time (a matter of seconds), before you either run out of race track, or road, or hit something!  

That said, you should always figure in a “buffer” to account for some margin of error. Changes in weather conditions can increase horsepower and there is always the chance that your horsepower calculations were off a little in the first place. A 10% buffer should be sufficient, so this is what we use in our calculations. So to figure out what size fuel injector will result in a 90% duty cycle, multiply your result by 1.1 as follows:

18.75 LB/HR x 1.1 = 20.625 BSFC per cylinder

You would then take this number and round up to the next available injector size. Since the next injector size available is 24 LB/HR, this is the correct size injector that you should choose for this application.


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