24 Lb Injectors

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by John Dirks Jr, Dec 22, 2013.

  1. PLX here!!
  2. I'm still learning about injectors and how they are controlled by the ECM and regulator. In getting things right, I think its important to understand how they work. So please bare with me as I try to make sense of this and correct me if needed. Also, if anyone wants to get in depth technical about how fuel mixture is controlled, please do so.

    Does this sound correct? >>

    1) You can raise the FP and get more fuel from an injector. However, that can make an injector run at max continually and thereby not really allow the ECM and regulator to control the mixture properly.

    2) On the other hand, if you have a injector that is bigger than actually needed, you can reduce the FP and thereby still allow the ECM and regulator to do their job and keep correct mixture for different circumstances such as WOT or cruise or idle.

    I know the perfect sized injector would optimal but of the above two options #2 is surely the way to go. Wouldn't you say?
  3. yes, you can raise fuel pressue to squeeze more volume out of an injector. You must also consider your fuel pump rating and capacity

    I wouldn't run less than the factory setting of 39 psi. The optimum way is to have a custom tune done, which can adjust the injector slope among other things to fine tune a larger injector

    I'm running 60#'ers son my car, and it idles like a champ
  4. I have 60 lb injectors waiting to go in my car also
  5. An injector is just an electronic valve. The computer just determines when it opens, and how long it stays open. Like any flow system, if you increase upstream pressure, flow rate going downstream increases (within limits of efficiency). So for the same pulsewidth (time the injector is open), a higher fuel rail pressure will move more fuel through the injector and into your intake port.

    Something that is often misunderstood, is the fuel pressure regulator does not maintain a constant static pressure, but instead a constant pressure differential between the fuel rail and the intake port. In other words, if you have your fuel pressure set to 40 psi, it is 40 psi HIGHER than the pressure in the manifold. So, if your throttle is closed and the manifold is pulling a vacuum, the regulator LOWERS the rail pressure to maintain 40 psi over whatever the manifold pressure is. In this condition, it will appear to have less than 40 PSI on the gauge, by however much your manifold vacuum is. If you're at WOT and you are pulling zero vacuum, then you should have 40 psi gauge. That's why you set fuel pressure with the vacuum line disconnected, so the fuel pressure regulator "thinks" your manifold is at zero vacuum.

    Fuel injectors need some minimum amount of pressure to produce an ideal spray pattern. Because of that, I would never lower fuel pressure under stock settings.

    There are ONLY two ways to use a non-stock sized injector:

    1. Retune the computer to recognize the change in injector flow (ideal way).

    2. Use a "calibrated" MAF meter (less ideal). A "calibrated" meters returns a falsified voltage to the computer to indicate that less air (if using a larger injector) is coming into the engine, which will cause the computer to trim the amount of time the injector is open, thus reducing the fuel flow into the intake. At higher rates of airflow the larger injector is held open the same amount of time the stock injectors would have been, and the modified engine gets the fuel it needs. While good "calibrated" meters can perform well at WOT, the downside to them is that startup and low-rpm operation can sometimes be negatively affected.

    Changing injector size without taking one of the above steps will never result in 100% ideal drivability.

    Cirling back around to pulsewidth- just like a cylinder head intake port, if the engine is demanding more than the injector can supply in the time that the (injector) valve is open, you've hit a wall. The computer can (and does) increase the pulsewidth, but at some point the engine will close the intake valve and you've run out of time. Increasing fuel pressure can only get you so far here, and at some point you have to raise your game on injector size. This concept is known as "duty cycle". Duty cycle is the amount of time that the injector is open, relative to the time it takes for one engine power cycle at a given RPM, expressed in terms of percentage. A duty cycle of 100% means it is open constantly, and 50% means it's open half of the time. Generally speaking, it's not advisable to exceed 80-85% duty cycle.
    A5literMan and John Dirks Jr like this.
  6. Is there a way for back yard tuners to tell what the duty cycle % of the injectors they're using?

    I assume one might be able to guess based on A/F ratio compared with whatever amount of (over stock) FP they might be running.

    In simpler terms, when and how do you know you are maxing out an injector size?
  7. Without the ability to datalog, you don't know DC. The only way to know without datalogging if you're out of injector is if you go lean and you know for SURE everything is working right, the rest of your fuel system is up to par and your tune is spot on.

    Really, without tuning/data logging, you're just making an educated guess. This is why it's a good idea to overshoot the injector size a little. E.G. if an injector calculator tells you that yon need 21.7 lb/hr injectors, get 24s.
  8. I haven't started the heads and intake swap yet. Before I do, I think I'll put a wideband O2 kit on, drive it for a while and get some readings. Then, after the parts swap, see what kind of differences it might show before I move to fine tune.
    A5literMan likes this.