Ok, I do not have a data logging program or anything, so im trying to figure out if there is a chance I will need larger injectors using the set-up in my sig. I am on the stock injectors and wondering if i'll need to upgrade them. not sure of my horsepower yet because I havent had time to get to a dyno since I got it running. Lloyd recommends on his site to use 30# for best results, so I was thinking of getting a set of these 32# racetronix injectors or even these 37# injectors for the same price and have the PCM tuned for them(even the 38#'s are the same $)
am I being too ambitious in thinking I should upgrade the stock injectors, or am I probably not even close to maxing these things out?

 

Definitely replace them.

 

If you expect to be over 400 flywheel HP, and you should expect that, you will be shooting craps with the stock injectors. You will be driving them close to 100% duty cycle, and that is not a good idea. To get in the ballpark, multiply your flywheel HP X 0.07. An example.....

450 flywheel HP X 0.07 = 31.5 #/HR.

That assumes a BSFC of 0.476 #/HR/HP and an 85% duty cycle, so it gives you the minimum size injector you should consider. There is no problem with going to a larger injector, as long as the PCM is programmed for it. The stock PCM has demonstrated an ability to drive 42 #/HR injectors and still turn the pulse widths down low enough to get a decent idle.

 

sounds like a plan then...i'm getting some new injectors then. I guess i'll get some 37#'s from racetronix. i'll put them in while the PCM is being sent off for the re-tune. thanks so much!!

 

Pardon the hi-jack but it looks like you have everything. Injuneer, I know this seems like a silly question but does that same formula apply to FI as well. According to that formula, I should be okay with 50# injectors. Even at 585hp ( I had to add methanol to get that much) the formula works out to be 585*18% (est. drivetrain loss)= 690 FWHP. 690*.07= 48.3. I ended up having to bump the pressure up, run a second in tank pump to keep the pressure up at WOT and add alkycontrol to get enough fuel. Is there another part of the formula Im missing? Thanks

 

No, ONLY for normally aspirated engine. That formula is based on an NA engine, running an 0.476 #/HR/HP brake specific fuel consumption (BSFC). For forced induction, you would typically be using about 1/3 more fuel than an NA application.

Your math is also a bit off. If you have 585 rwHP, and know you have an 18% drivetrain loss:

585 / (1 - .18) = 585 / 0.82 = 713 flywheel HP

At ann assumed BSFC of 0.600 and shooting for an 80% DC:

713 HP x 0.600 #/HR/HP = 428 #/HR

428 #/HR total / 8 cyls = 53.5 #/HR per injector, at 100% DC

53.5 #/HR / 0.80 DC = 66.9 #/HR

You would have to verify the BSFC for your engine. I've got dyno BSFC results for my own engine and some other combinations, but I don't have any for FI applications.