I am getting coflicting stories about 43# injectors being the correct size with the mods I have. Any input would be appreciated. Read mods in signature.Thanks :crazy:


[Modified by Allblownup, 11:27 PM 2/15/2002]

 

Fuel injector sizing depends alot on your programming. Are you still running the FMU? My combo states that 50#/hr injectors are not adequate, but I run it with 36#/hr injectors and the fuel pressure jumps to just over 80 psi at full boost (11 psi - so far). I actually bult a test stand so that I could determine the relationship between boost reference and fuel rail pressure on my combo. I plotted them based on the various adjustments of the FMU, and developed a very nice chart to use as reference. From that, my tuner was able to get a stab at the baseline set-up, and then minor tweaks were made. And yes I am running the factory ECM. That makes a difference.

Also, is that the boost that ATI has claimed for the system that you were sold, or is that verified prior? If you have not run it, you will probably be quite suprised at the 3-4# of boost that people typically get from ATI's system upon first install.

Aaron


[Modified by AKS Racing, 12:15 AM 2/16/2002]

 

the #43 ford is rated at around 39psi where as gm rates theirs at 43.5psi. so #43 injectors should flow around 46 to 48 lbs/hr at 43.5 and even higher flows at higher fuel pressure. at 48 lbs/hr the injectors should support up to around 700hp. hopes this helps and you are going to need a good fuel system.. :cheers:

 

I got 10# of boost with no problem through twinn intercoolers. I am not and I do not want to run FMU I have been told to not use and in all books they rate the injectors to run up to 750hp. I know I will not even be close to that.Thanks :crazy:

 

I am glad to hear that you were able to get the 10# of boost, as advertised, from the ATI system. What RPM are you achieving this at. I will assume that this is an LT-1? To determine the correct injector sizing, estimate the flywheel hp, and the expected fuel pressure and you can plug into one of the programs, to get the sizing. You will also need to know what psi the injectors were originally rated (i.e. were the FMS injectors originally sized at 39 psi).

Aaron

 

As a point of reference, I have a P600B on my 383 and measure an actual 8psi boost at the manifold. I'm using FMS 36lb injectors @ 52psi fuel pressure, which yields a new flow rate of 42 lb/hr. At an 85% duty cycle, and assuming BSFC of .55 for a supercharged engine, this should support ~517hp.

Coincidentally, this is almost exactly the crank horsepower I see (assuming 15% drivetrain losses and 433rwhp / 506rwtq). So I'm pretty much maxxed out on fuel and could probably use those bigger injectors. Not sure what horsepower you're shooting for, but those FMS 42lb injectors can support some pretty high #s; by the time they're maxxed out your stock fuel lines / rails are probably also.

One thing- replace the stock in-tank pump with a good 255lph one or you'll be sorry.

 

George,
Are you utilizing the FMU? Or is the aftermarket ECM adding all the fuel required? What is the idle fuel psi, what about with the factory vacuum reference removed? What pulley diameter are you running on the P-600B? Is it still the one that was supplied with the unit from ATI? What RPM was the boost value of 8 psi noted? Do you have a cockpit mounted fuel and boost gauge? Hope you don't mind all the questions, just very interested.

When my tuner did the final on my set-up, she commented that with the factory ECM, she will program nothing larger than a 36# injector in the C-4 cars. States that the larger (say 42# or 50#) injectors that are available on the market, have no idle quality and that part throttle driveablity is very poor. I have heard so many that run the larger injectors, but wonder if they do that with the factory ECM.

BYW, my tuner is Eileen Formato of Faster Proms, who has won nearly every chip shootout that has been run. Seems she does things a little different, just trying to understand more.

Any insight is appreciated.

Also, I have already upgraded the entire fuel system, rails, and the intank pumps. Fuel psi is rock steady wherever it is set.

Thanks,
Aaron

 

I am not using FMU. I am running Haltech DFI setup And I cannot answer some of the others since I have not started it yet. I have intak pump feeding pump outside. I have pillar mount boost gauge,fuel pressure and air to fuel gauge. I am leaning to put in 65# injectors and I am running a D-1 Procharger :seeya

 

No FMU here; fuel pressure @ 52psi with no vacuum; 47psi with. Ordered my blower with ATI's smallest (12psi) pulley and see 8-8.5psi boost about 5000rpm. I think it's only 2.55" and it slips at high rpm cause there's not enough belt wrap. I have boost/vacuum and digital fuel pressure gauges in my dash.

I don't think you'll find many running big injectors with the stock computer. Eileen's right- once you program for full throttle fuel requirements, you can't get the pulse widths short enough for decent idle and don't have fine enough control (stepsize) for part throttle response. Faster Proms has a great reputation; I mean that's an accomplishment getting a supercharged motor with big injectors working with a stock ECM!

I run a Delco MEFI-3 computer, but had to make new injector harnesses and redo most engine-ECM wiring. AZ Speed & Marine supplied the hardware and programming; they love this thing for custom apps- more range and greater resolution (#cells, stepsize) than Accel / Haltech. It also has additional spark retard and fuel enrichment maps for boost/rpm, nitrous, etc. I believe my MEFI saturates the injectors to 100% under WFO throttle + boost to keep it from going lean; kind of what your FMU does by spiking the fuel pressure and forcing the injectors into static operation. Also that 80psi at full throttle makes a 36lb injector flow 51.5lb/hr, which is about what you said is needed.

It's nicer having it all computer controlled, but with the stock ECM you're really need to stick with reasonable injector size so you need the FMU. What kind of power/torque are you making anyhow?

 

From what I am reading I am going in right direction. Does anyone have DFI with big injectors and tell me how they are running?Thanks :crazy: