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DIY: How to Calculate Fuel Pump and Injector Size

If you enjoy building your own custom engine designs or customizing existing engines, you know you have to take more than just pistons and valves into consideration. The best engine design will never run correctly without the right capacity fuel pump and, in fuel injected engines, the correct injector size.

Liters, gallons, and brake specific fuel consumption

Fuel pumps are rated in either liters or gallons per hour. One U.S. gallon is equivalent to 3.785 liters or 231 cubic inches. You can round up to 3.8 liters for most calculations. One liter is then equal to approximately 0.26 U.S. gallons. Brake Specific Fuel Consumption (BSFC) is a ratio of how much power an engine produces compared to the amount of fuel used. The rating is often measured in kilowatts per hour.

Calculating the size of fuel pumps

An electric fuel pump used in regular engines is smaller because the engine requires less fuel than nitrous, supercharged, or turbo engines. To determine the fuel pump size for a naturally aspirated engine, take the projected horsepower and multiply it by 0.38 to determine liters per hour. If you want to convert that number to gallons, divide the answer by 3.785.

Turbo and nitrous considerations

Electric fuel pumps must be de-rated for forced induction engines. More horsepower is needed to turn or drive compressors. To determine the required fuel pump size for these engine types, take the projected horsepower and multiply it by 0.47 to determine liters per hour.

Calculating injector size

In carbureted engines, it is possible to have way too much carburetor. This results in flooding and wasted fuel. When it comes to injectors, over-sized does not create major issues but still results in waste. Injectors are normally rated in cubic centimeters (cc’s) per minute. Take the calculated fuel pump size in liters and divide the number by 16.67. If you want this number in pounds per minute, divide the cc’s per minute by 10.5. Fitment and drive must also be considered. Injector ECUs may be low or high impedance. Some ECUs will drive either type, but you need to obtain this information before changing your injectors. The fitment is the injector inlet and outlet size and the length of the injector.

Running a fuel pump that is larger than you need should not cause any problems. Excess fuel should be returned to the gas tank. The return lines do need to be large enough to handle the returning fuel. However, you do want to avoid the potential for fuel waste and excess work on the part of the fuel pump if you are using one that is too large.

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