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?

DISCLAIMER: The contents of this article are not necessarily the official views of, nor endorsed by, the owner of HondaClub(com).

[jimbofury]

hey! awesome advice. however, i have another question. is there a way to estimate your HP instead of taking it to a dyno? i'm not sure how much horsepower my car is going to have and would like to install the new injectors before i make the trip to the dyno tuner?

[8695Beaters]

Sorry I missed this before. It's pretty simple, have a horsepower goal and size your injectors accordingly. You can adjust the boost on your turbo if you start to outrun your fuel injectors. Using a margin of safety never hurts either, but fuel injectors that are excessively large will not idle well and will run very rich at low RPM.

[jimbofury]

ok so i'm going to estimate around 300HP with all the upgrades and the turbo....maybe it's more i dunno. tell me if this is right for 300HP.

300X.6 divided by 4X.8 X10.5 for cc

so about 590CC??? that doesn't sound right?

[8695Beaters]

That's about right. NISMO makes 555 cc injectors for the SR20 which can handle 350 hp. Playing around with the number gave me a BSFC of 0.48. That seems odd, so I'm going to contact NISMO and see what the deal is. I'm also going to look up my BSFC numbers again, but the .60 will keep your engine alive. You can downsize to the closest injector if need be.

[95_Del_Sol]

Very cool write up. I'm looking into doing a mini-me setup (Head procured with all the port and polish work done - cam installed etc) I'll punch this formula in and if all is well (I'm sure it is...) then I'll be up and running in no time! Can't wait to see how the 5 gas analyzer reads

[mikeross]

I have installed performance chips on my car and it seems that i haven't taken into consideration the fuel injectors yet. Gotta size em up though.