was looking at new fuel injectors and for all the same price, i can get either 24 26 or 30# fuel injectors.
What would your reccomendations be. (.060 over bore)

 

I have seen Injuneer post this formula many times to get in the ballpark on the injector size you need. This is based on keeping your injector duty cycle no greater than 85%.

Flywheel HP x 0.07 = Injector size

Example:

I'm targeting around 450 hp at the flywheel for the future....so....

450 x 0.07 = 31.5#

I'm going to get 36# just to provide me with more if I want it....however 32# would get me there....

The Fuel and Ignition forum does not display many posts, but if you search it...pages of topics appear. They must archive older posts or something.

 

thanks!
350 x .07 = 24.5
looks like im just going to go fr the 26#

 

That formula assumes a better-than-stock brake specific fuel consumption (BSFC = 0.476 #/HR/HP), and keeps your duty cycle at no more than 85%. It is only for NA applications. I developed it a couple years ago, based on my experience. I recently downloaded an MSD injector sizing program, and it gives virtually identical results. It is way less conservative than the online "injector sizing" formulas you can find, which assume a sloppy 0.50 BSFC and an 80% DC.

These are the practical limits for injectors - yes, you can push the duty cycle higher, and/or you can have an extremely efficient combustion chamber and tune, and you can make a lot more power. But if you are buying new injectors, this prevents you from making a mistake. It gives you a size you will not have problems with.

Working backwards, the practical limits are:

24 #/HR - 345 HP at the flywheel
26 #/HR - 375 HP at the flywheel
30 #/HR - 430 HP at the flywheel
36 #/HR - 515 HP at the flywheel
42 #/HR - 600 HP at the flywheel

There is no problem with using an injector larger than you calculate, as long as the PCM is correctly programmed for it. The risk with an oversize injector is the ability of the PCM to "turn down" the pulse widths short enough to allow the engine to idle efficiently. The stock LT1 PCM has demonstrated the ability to drive 42 #/HR injectors with no idle problems.