Fuel Injector Flow Rate Calculator
Enter your desired engine horsepower and the specs of your motor to calculate the recommended fuel injector flow rate.
Recommended Injector Flow Rate:
Gasoline
Race Gas
E85
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How to Find the Right Fuel Injector Flow Rate
Getting the right amount of fuel into your engine is crucial to maximize the performance of the motor and optimize the benefits from other performance modifications you plan to make. These modifications can include upgrades to the pistons, camshafts, and exhaust system, as well as the addition of a turbocharger or supercharger.
Having the right fuel injectors with a flow rate suitable for the engine’s higher power output is essential for achieving the performance level you expect from your build.
Using fuel injectors that are too large will result in sluggish acceleration. Using fuel injectors that are too small will starve the engine of fuel.
Follow these easy steps to choose the right size fuel injectors for your motor.
Step One: Determine the Target Horsepower
How much power do you realistically expect your modified engine to put out? You can create a realistic estimate by adding the expected horsepower gains from your upgrades to the stock engine horsepower.
Generally speaking, you will need fuel injectors with a higher flow rate that matches the higher output of your modified engine.
You might also want to try our engine horsepower calculator.
Step Two: Calculate the Brake-Specific Fuel Consumption
Brake-Specific Fuel Consumption, or BSFC, is the amount of fuel (in pounds) consumed for each horsepower made per hour. BSFC is expressed in pounds per hour (lb/hr).
BSFC values will vary based on the type of fuel being used and how the engine is aspirated. You can calculate the BSFC for your engine, or estimate it using these factors.[1]
Refer to the following chart for average BSFC values:
| Fuel Type | Aspiration | BSFC |
|---|---|---|
| Gasoline | Naturally Aspirated | 0.5 |
| Supercharged | 0.55 | |
| Turbocharged | 0.6 | |
| Race Gas | Naturally Aspirated | 0.545 |
| Supercharged | 0.5995 | |
| Turbocharged | 0.654 | |
| E85 | Naturally Aspirated | 0.667 |
| Supercharged | 0.733 | |
| Turbocharged | 0.8 |
Step Three: Find the Motor’s Safe Duty Cycle
The safe duty cycle is the percentage of time that the fuel injector’s valve is open during each combustion cycle, which includes intake, compression, power, and exhaust. Most stock engines use a maximum safe duty cycle of 80%, which means that each fuel injector’s valve is open for 80% of the time and is closed for 20% of the time.
Engines with performance modifications can use a maximum Safe Duty Cycle of 90%.
Step Four: Calculate the Ideal Injector Size
Once you have determined the target horsepower, BSFC, and safe duty cycle, it becomes possible to calculate the ideal fuel injector flow rate. Use the following formula to calculate the proper flow rate for your injectors.
injector flow rate = horsepower × BSFC / # cylinders × safe duty cycle
To find the ideal flow rate, simply plug in the values found in steps one, two, and three into the formula and solve.
The result is the flow rate in lb/hr. To get the result in cc/min use a lb/hr to cc/min conversion calculator.
For example, let’s find the correct fuel injector flow rate for a turbocharged V8 motor with a desired output of 450 horsepower, which will run on normal gasoline.
The BSFC for this setup will be 0.6 according to the chart above. Let’s enter the values into the formula.
injector flow rate = 450 × 0.6 / 8 × 85%
Now, let’s solve it.
injector flow rate = 270 / 6.8
injector flow rate = 39.706 lbs/hr
You should also check out our compression ratio calculator.
References
- x-engineer, Brake Specific Fuel Consumption (BSFC), https://x-engineer.org/brake-specific-fuel-consumption-bsfc/