Two ways into the same mass balance
Fuel sizing is a mass balance, and there are only two doors into it. Through the first, you name a power figure and a brake specific fuel consumption, and the fuel mass falls out as a multiplication: 450 hp at 0.50 lb per hp-hour is 225 lb of fuel an hour. Through the second, you name an airflow and an air/fuel ratio, and the fuel mass falls out as a division. The page runs whichever door you pick and then shows the other side anyway, because the two have to agree — 225 lb/hr of fuel at 12.5 to 1 is 2,813 lb/hr of air, which at 0.0765 lb per cubic foot is 613 CFM.
If those two numbers do not sit near each other for your combination, one of the inputs is wrong, and it is almost always the BSFC.
The duty ceiling is a design choice, not a limit
225 lb/hr across eight holes is 28.13 lb/hr each with the injectors held open permanently, which is not a thing anybody runs. Dividing by a duty ceiling of 80 percent gives 35.2 lb/hr static, or 357 cc/min at a specific gravity of 0.745. Move that ceiling to 85 percent and the requirement drops to 33.1; move it to 75 and it climbs to 37.5. The ceiling is yours to set and it comes from what your injector supplier says about their part, not from this page.
Why lb/hr and cc/min never quite line up
They are the same flow measured as mass and as volume, and the bridge between them is the density of the fuel. At 0.745 specific gravity one lb/hr is 10.15 cc/min. Change the fuel and the bridge changes: the same injector quoted on a denser fuel reads as fewer cc/min for the same lb/hr. That is why two suppliers can list what is physically the same part at different numbers and both be telling the truth.
Pressure buys less than people expect
Flow through a fixed orifice goes as the square root of the pressure across it. An injector rated 42 lb/hr at 43.5 psi passes 48.5 at 58 psi — the ratio is 1.333 and its square root is 1.155, so 15.5 percent more fuel for a third more pressure. Doubling the pressure gets 41 percent. The correction on the page uses the differential across the injector, which on a manifold-referenced regulator is the gauge reading and on a returnless system is not, once the manifold goes under boost.
The carburettor line is a comparison, not a match
If you put a CFM rating in the last field, the page prints it beside the airflow the mass balance implies and subtracts one from the other. That subtraction is honest arithmetic on two numbers you supplied and nothing more. A carburettor rating is measured at a stated pressure drop across the venturi on air at the test conditions; the mass balance figure is real air at the density you entered. They are different measurements of different things that happen to share a unit, and the page will not tell you whether one suits the other.
What this cannot know
It does not know your engine, the fuel in the tank, whether the injectors will spray a usable pattern at the pulse width the duty cycle implies, or whether the pump and the lines can supply the rail at that pressure. Short pulse widths are where injectors stop being linear, and a very large injector on a small engine idles badly for exactly that reason — the page has nothing to say about it because the arithmetic here is only about how much fuel goes through in an hour.
Questions people ask
What BSFC should I use?
Yours. It is fuel mass per unit of work and it varies with the fuel, the compression, the induction and how the tune is set up, so a figure that fits one engine can be a third out on another. If you have a dyno sheet with fuel flow recorded, divide the flow by the power at the same point and you have measured it rather than guessed. The 0.50 in the field is a placeholder to replace, not a recommendation.
How do I convert lb/hr to cc/min?
Multiply by 453.59 to get grams per hour, divide by the specific gravity to get cubic centimetres per hour, divide by 60. At a specific gravity of 0.745 that works out at 10.15 cc/min for every lb/hr. The specific gravity is the part people leave out, which is why the same injector gets quoted at different cc/min numbers by suppliers working on different fuels.
Does raising fuel pressure make an injector flow more?
Yes, as the square root of the pressure ratio. Going from 43.5 to 58 psi is a ratio of 1.333, and the square root of that is 1.155, so about 15 percent more flow. It is a small return for a large change and it asks more of the pump, the regulator and the injector driver everywhere else in the map. The page prints the corrected flow so you can see what the change is actually worth.
Why does the page show an airflow when I only asked about injectors?
Because the fuel figure and the air figure are two ends of the same mass balance, and printing both catches bad inputs. If you enter a power target and a BSFC and the implied airflow comes out at a number your induction could not pass, or far below it, then one of the two figures you typed is wrong. It is a cross-check rather than an extra answer.
Will these injectors work on my engine?
This page will not say. It divides fuel mass by a hole count and a duty ceiling and corrects for pressure, and that is the whole of it. Spray pattern, minimum usable pulse width, driver current, fuel compatibility with the seals and whether the rail and pump can hold the pressure are all outside the arithmetic, and they are the questions that decide whether a particular injector suits a particular engine.