To size fuel injectors, estimate how much fuel the engine needs at peak power, divide it among the injectors, and allow headroom so they are not held fully open. The standard formula is injector size (lb/hr) = horsepower x BSFC / (number of injectors x maximum duty cycle). BSFC (brake specific fuel consumption) is about 0.45 to 0.50 for naturally aspirated gasoline engines and about 0.55 to 0.65 for supercharged and turbocharged engines, and a maximum duty cycle of about 80 percent is a common target.
Injector sizing formula and example
Example: a turbocharged V8 targeting 600 hp with a BSFC of 0.60 and eight injectors at 80 percent duty: 600 x 0.60 / (8 x 0.80) = about 56 lb/hr per injector. To convert to cc/min, multiply by roughly 10.5, giving about 590 cc/min. Round up to the next available size and let the engine calibration take care of the rest.
E85 needs bigger injectors
E85 needs considerably more fuel than gasoline for the same power, roughly 30 to 50 percent more depending on the ethanol content and calibration, so injectors and pumps sized for gasoline are often too small on E85; see what is E85?
Injector duty cycle
Duty cycle is the percentage of time an injector is open. On a four-stroke engine each injector fires once every two revolutions, so duty cycle (%) = pulse width (ms) x rpm / 1,200. For example, 16 ms at 6,000 rpm is 80 percent. Above about 85 to 90 percent, injectors lose control and the engine can run lean, so a larger injector is needed.
Fuel pressure changes flow
Injector flow rises with the square root of the pressure ratio: running a 1,000 cc/min injector rated at 43.5 psi at 58 psi gives about 1,000 x the square root of (58 / 43.5), roughly 1,155 cc/min. That is why injector ratings always state a pressure; our FIC 1200cc injector set for LS1 engines ($1,077.00) is listed at 130 lb/hr at the factory 58 psi.
High impedance vs low impedance injectors
High-impedance (high-Z, roughly 12 ohm) injectors work with the saturated drivers in most modern engine computers. Low-impedance (low-Z, roughly 2 to 3 ohm) injectors respond faster but need peak-and-hold drivers or resistor packs. Use the type your engine computer supports; most factory and plug-and-play replacement injectors are high impedance.
Do not forget the pump
Bigger injectors need a fuel pump that can supply them at the pressure you run; see fuel pump sizing, fuel pressure regulator and fuel injector symptoms. For supercharged builds, see how much horsepower does a supercharger add?
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