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Here is the formula for calculating injector size in pounds per hour

blog Monday, February 13, 2006 @ 1:11 PM

Calculating Fuel Injector Size
Here is the formula for calculating injector size in pounds per hour:

Injector lbs./hr. = horsepower x BFSC/number of cylinders

BFSC is short for Brake Specific Fuel Consumption, which is the amount of fuel required to produce one horsepower. According to Accel, a BFSC of .45 is a good general figure for normally aspirated engines, and .55 is good for supercharged engines.

Let’s do a calculation for a 400 horsepower, unblown V8 engine:

Injector lbs./hr. = 400 x .45/8
Injector lbs./hr. = 400 x .05625
Injector lbs./hr. = 22.5

While nobody makes a 22.5 lbs./hr. injector, our aftermarket injector size chart shows several 22 lbs/hr. injectors, which would work fine on this engine.

Another thing to keep in mind is fuel pressure, which can affect injector flow. You can compensate for a smaller injector by boosting the fuel pressure. An adjustable fuel pressure regulator is ideal for doing this. With all things being equal, though, it’s best to use the right size injector.

Here are the injector sizes for the most popular Ford performance and truck engines. These engines are the most popular for swapping into early musclecars, street rods, and trucks.
•5.0L passenger (302), port EFI: 14 lbs./hr.
•5.0L HO and truck (302), port EFI: 19 lbs./hr.
•4.6L SOHC (Mustang GT), port EFI: 19 lbs./hr.
•4.6L DOHC (Mustang Cobra), port EFI: 24 lbs./hr.
•5.7L truck (351W), port EFI: 19 lbs./hr.
•7.4L truck (460), port EFI: 28 lbs./hr.

Fuel Pumps
Calculating fuel pump size also involves horsepower and BFSC. The formula is divided into two parts. First, you must calculate the pounds per hour (pph) of fuel your engine will require, as follows:

horsepower x BFSC = pph

Then, you divide the pounds per hour figure by six (one gallon of fuel equals approximately six pounds) to determine gallons per hour:

pph / 6 = gph

Let’s calculate the fuel pump size for our hypothetical 400 horsepower motor using a BFSC of .5:

400 x .5 = 200 pph
200/6 = 33.33 gph

To feed that hungry 400 horsepower engine, you will need a fuel pump that delivers 33 gallons per hour. This formula works for any type of engine, carbureted or fuel injected.

Stock EFI Fuel Pump Ratings
If you are swapping a complete factory EFI engine into an early car or truck, the factory fuel pump used with that engine (or an aftermarket pump of the same rating) will be more than enough to feed it. This chart shows the pump ratings for Ford EFI engines in the vehicles equipped with them.
•Mustang 5.0L HO and 2.3L Turbo: 23 GPH @ 50 psi
•Thunderbird 5.0L and 3.8L Super Coupe: 29 GPH @ 45 psi
•F-series trucks, 4.9L, 5.0L, 5.8L, 7.5L (in-tank pump): 50 GPH @ 70 psi
•F-series trucks, 4.9L, 5.0L, 5.8L, 7.5L (external pump): 36-40 GPH @ 70-95 psi

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