As stated above, fuel injectors are under pressure, which is known as flow rate. A fuel injector’s flow rates are measured in “pounds per hour” and are available in different sizes depending on application. AmericanMuscle carries fuel injectors ranging from 19 lb/hr up to 120 lb/hr, but you never want to upgrade your Mustang’s fuel injectors to a larger size unless it is absolutely necessary. Too large a fuel injector will cause your engine to run rich, which will actually cause you to lose power. Bigger is not always better.
The basic formula for calculating the correct size fuel injector for your Mustang is listed below:
Injector Flow Rate (lb/hr) = (Engine HP) x (BSFC) / (Number of Injectors) x (Injector Duty Cycle)
Engine Horsepower (a.k.a. brake horsepower) is measured at the flywheel, NOT at the rear wheels.
BSFC (Brake Specific Fuel Consumption) represents how much fuel (in lb) that’s required per hour per each brake horsepower the engine produces. BSFC is usually a number between 0.4 and 0.7 for gasoline engines. BSFC can be estimated but for best results, the actual number measured on a Dyno should be used.
- For naturally aspirated engines, estimate the BSFC to be 0.4 to 0.5
- For nitrous engines, estimate the BSFC to be 0.5 to 0.6
- For forced induction, estimate the BSFC to be 0.6 to 0.7
Injector Duty Cycle refers to how long the injector needs to be open (flowing fuel) in order to supply the required amount of fuel. If the injector has a 100% duty cycle, it means it is open all the time injecting fuel whether the valves are open or closed. This will cause unburned fuel to be blown past the exhaust valve or to be poorly atomized, which results in poor combustion, loss of fuel economy and power. Because of this, it’s generally recommended to select an injector with a high flow rate that does not exceed an 85% (0.85) duty cycle.
Example: A 300HP naturally aspirated V8 Mustang requires what size fuel injector?
Injector Flow Rate = 300 (HP) x 0.5 (BSFC) / 8 (Number of injectors) x 0.85 (duty cycle)
The Injector Flow Rate would equal 22.05 lb/hr or 24 lb/hr, rounding up to the next popular injector size available.