How To Properly Size Fuel Injectors

[8695Beaters]

As promised from the turbo article, here is the formula to size a fuel injector for any 4-stroke Otto engine:

(Horsepower x B.S.F.C.) / (# of Injectors x Duty Cycle) = Injector Flow

-B.S.F.C. = Brake Specific Fuel Consumption. Basically, this tells how efficient your engine is. It’s the ratio of fuel burned per horsepower made(flywheel). For natural aspiration, use 0.5. Use 06. or 0.65 for turbo engines. Or use the magic of the internet to find exactly what your car can do. OR, you can figure it out for yourself, but you will need a dyno. BSFC is just fuel consumption / horsepower x time. This is measured in units of lbs/hp-hr. A gallon of gas averages to about 6.15 pounds. So fill your gas tank, hop on the dyno, run for about a minute, and then fill again without driving anywhere. So it would be best to just use the numbers given.

-Use whatever horsepower number you are shooting for. However, horsepower must be measured at the flywheel for this to work. Otherwise your injector will be too small. Most cars have about 20% power loss from drivetrain losses. Shoot for 25% and you will have an injector with enough headway.

-Injector Flow is in lb/hr. Since most injectors (at least for imports) are measured in metric, you must convert. Use lb/hr*10.5= cc/min.

-Duty Cycle is how much time the injector is open. Over 80% and the injector can not physically keep up with the engine. This is also where injectors are rated. So use 0.8.

My sources for this article are Sport Compact Car Magazine and HOTROD Magazine. You think I'm smart enough to figure this out on my own?

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