The formula for calculating the correct size fuel injector for your Mustang is calculated by: Injector Flow Rate (lb/hr) = (Engine HP) x (BSFC) / (Number of Injectors) x (Injector Duty Cycle)
If you are trying to compare injector flow rates and you have flow data at one delta pressure, you can easily calculate the flow rate at a different delta pressure as follows:
Flow rate at new delta pressure = (flow rate at old pressure) x v(new pressure/old pressure)
Engines require a certain fuel flow rate that is generally measured in lb/hr (pounds per hour) and can be calculated via knowledge of its Brake Specific Fuel Consumption (BSFC). BSFC represents how much fuel (in lb) is required per hour per each brake horsepower the engine produces. More clearly stated, this means if you have a gasoline engine that makes 300 brake horsepower, its total maximum fuel requirement in lb/hr can be calculated as follows:
Fuel flow requirement = (brake horsepower) x (BSFC)
This calculation can also be reversed to give the maximum safe hp a set of injectors can support, which gives:
Max safe hp = [ (injector size) x (total # of injectors) x (max duty cycle) ]/BSFC
Example: The following guide is a general rule of thumb for sizing fuel injectors on an 8-cylinder engine using a BSFC of 0.50. Forced-induction engines typically range from a BSFC of 0.55 to 0.65, with the latter value arising from the fuel enrichment necessary to keep exhaust temperatures below 1650 deg F and catalyst temperatures below 1750 deg F.
Naturally Aspirated: (19 lb x 8 x .85)/.50 = 258.4 or approx 258 hp @ 85% duty cycle Forced-Induction @ 0.55: (19 lb x 8 x .85)/.55 = 234.9 or approx 235 hp @ 85% duty cycle Forced-Induction @ 0.65: (19 lb x 8 x .85)/.65 = 198.8 or approx 199 hp @ 85% duty cycle
The below calculations assume a fuel pressure of 39.15 psi. If you can raise fuel pressure and still be sure your fuel pump can supply the desired flow rate, then these maximum horsepower numbers will increase.