injectors

[chevy283]

should i buy 4.3 or 305 injector for a 292 (283 bore 0.60).

[Russell]

I'd go with the 305 injectors myself. More fuel is better than not enough

[chevy283]

thats what i thought thanks

[jamal]

depends, both will probably be 19lb injectors. The opening and closing depends on the computer

[chevy283]

i got a 350 computer.

[Longhorn Man]

Plus, injectors are normally put in an 80% aplication.

[Russell]

I have a 305 computer, if you want to trade eproms. The 350 prom probally wouldn't work too well on the TBI 292... 305 is the closest to the 292 (as compared to the 4.3) but I am not sure how different a 292 runs from a V6 or a V8...

[chevy283]

what do u mean 80% aplication?

[Longhorn Man]

when working as hard as the car which owns the eng/comp/inj, set up...or any comp eng controll manngmt, the computer will only demand 80% of the total available work load

[68LSS1]

I think what he's referring to is not to exceed 80% duty cycle (some say 85%). This could get complicated, follow closely. There will be a test. In order to know what size of injectors you need, you need to know how much fuel it will take to make the horsepower you want. This is called BSFC (Brake Specific Fuel Consumption). This is how many pounds of fuel it will take to make one horsepower for one hour. For you N/A guys this will be 0.45. For the blower/turbo guys use a BSFC of 0.50. Now if we know our BSFC and can estimate/calculate our peak horsepower then we can can do some math: BSFC x Peak HP=lbs/hr (fuel). (Example .45 X 400=180lbs/hr.) Your injectors will need to flow this amount (from the formula) to support that hp. So each injector will have to flow 22.5 lbs/hr at 100% duty cycle (180lbs/hr divided by number of injectors) which we don't want.
So on to injector sizing. To find your injector sizing the formula is lbs/hr=(BHP X BSFC)/(number of injectors X DC). Got it? That is fuel in pounds per hour equals Peak Brake Horsepower times Brake Specific Fuel Consumption divided by number of injectors times duty cycle. (Here's that duty cycle again, basically don't exceed 80-85% because bad things can happen, meaning going lean). So for example lets say we have a 400hp (crank) engine. (400 X .45)/(8 X .8)=28.125 lbs/hr. Since a 24 lb injector would not flow enough (without cranking the pressure) we'd use a 30 lb injector here.
And on a side note most injectors are rated at 43.5psi (3 bar). If you are short on your flow rate you can increase the pressure to get more flow if you are just a little short as opposed to stepping up to a larger injector. And for you LS1 guys remember they operate on 58psi so a 24 lb injector will be flowing close to 28 lbs/hr at 58psi. And lastly to figure out how much horspower your injectors can support the formula is HP=(lbs/hr X #of injectors X duty cycle)/BSFC.

[68LSS1]

Originally Posted by Longhorn Man

when working as hard as the car which owns the eng/comp/inj, set up...or any comp eng controll manngmt, the computer will only demand 80% of the total available work load

No, you can max out the duty cycle. The ECM will put the deamnd out and the injector will open all the way to being continually open. Usually our speed limiters and rev limiters prevent this from happening unless you "tune" around them.

[chevy283]

so can i use 350 injectors?

[68LSS1]

Like jamal posted earlier, they are all probably 19lb injectors at 43.5 psi. The computer controls when and how long the injector is open. A larger injector is of course going to flow more. Without knowing for sure what size the injectors are I can't say but again, they are probably the same injectors. If they aren't I would guess the 305 injectors should flow enough for a stock to mildly built 292. If you want to go with the 350 injectors that should be fine and if they do happen to be larger than the 305 ones, it will give you some extra insurance on the duty cycle.

[chevy283]

so i just get a 305 ecm right?

[68LSS1]

Originally Posted by chevy283

so i just get a 305 ecm right?

My guess is the injectors are the same (305/350) so it shouldn't matter which ECM you use. The 305 injectors will work with the 350 ECM and vice versa. If you can get thesame injector/ECM I would, just to ensure there are no problems and then if I modified the engine later I definately do some math to ensure I wasn't going to go lean. There are (physically) different injectors out there and some take different electrical connectors but I doubt there are any differences with what we are talking about here.

[chevy283]

thanks

[chevy283]

im going to try to get it running saturday. thanks for the help

[Russell]

With TBI, a 305 injector is 40 lb/hr, and a 350 injector is 55 lb/hr

The injector rating is programmed directly into the eprom on the computer, so it will not run its best, however, the 305 that was in my 89 had a 305 ECM, but 350 injectors, and it ran just fine

[jamal]

[QUOTE=Russell]With TBI, a 305 injector is 40 lb/hr, and a 350 injector is 55 lb/hr

Ok we are talking about tbi, I was thinking 8 injectors, sorry I was lost. I would still go with the 350 tbi setup though. The computer knows when to pull back or richen fuel and advance/retard timing. Can't go wrong with 350 tbi, better to have bigger injectors than too small. If it does run rich you may want to look at o2 sensors and other electronics b/c it wont be your injectors