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Thread: - we're working on it :)

  1. #16
    Electronic Ignition!
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    It's still snowing here Dave.

    BUT I've been looking at the Indy problem. I think I need smaller injectors. But I don't know what size - it's so confusing!

    The FAST throttle body has 2 x 88lb injectors (rated at 58psi) so that's like 4 x 44lb if I had port injection.
    But I've been told to run them at 43psi so that would make it closer to flowing like 2 x 74lb or 4 x 37lb?

    The original Fiero 2.5L had a single 60lb injector. I'm thinking 2 x 42lb injectors may be more suitable.
    How do I figure it out with this larger displacement 3.4L SD4 engine and higher RPM and airflow capability?

    The other problem I think is the huge open plenum under the TBI on this Edelbrock intake.
    Since it's a 4-bbl , that open plenum is equal to what I'd see in a V8 intake manifold.
    I'm thinking I need a turtle in there to reduce plenum volume to maintain flow velocity. Any ideas?

  2. #17
    Super Moderator dave w's Avatar
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    Looking back at the data from last summer and fall, there is a challenge understanding what is happening at WOT. Going lean then rich suggests the fuel is puddling in the intake manifold. I agree, keeping the wet flow atomized requires good velocity inside the intake manifold.

    One thought would be to try something like a 42 lb injector (with 4 holes) with 56 psi, to create an ultra atomized mixture? The idea is create a fog like mixture? Have you ever watched a fighter jet take off on a foggy morning? The fog condenses at the wing tips (but it's still a dense fog), which is what needs to happen to the fuel mixture @ WOT. Oh ... wait a minute, isn't that what an injector over an intake valve will do @ WOT?

    dave w
    Last edited by dave w; 02-07-2015 at 10:02 PM.

  3. #18
    Electronic Ignition!
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    Only have a 2-injector TBI - 4 would require a whole new TBI.
    I really don't want to give FAST any more of my money.

    I have one of these to sell as it is (offers anyone?)
    http://www.fuelairspark.com/fas/xfi-...ol-systemhtml/

    SD4 engine is capable of 200-250hp in current configuration.
    How many lbs are the injectors on the 2-injector GM TBIs for the 3.4, 4.3, or 5.0 engines?
    That may give me an idea on where to start.

    Edit: found something here (looks like I should use 2 x 40-45lb instead of 2 x74lb that I have)
    http://www.dynamicefi.com/TBI_Fueling.php
    Last edited by IndyPaceCar; 02-08-2015 at 09:57 AM.

  4. #19
    Super Moderator dave w's Avatar
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    TBI Injector link:

    http://www.gearhead-efi.com/Fuel-Injection/showthread.php?81-GM-TBI-Injector-size-color-and-part-numbers&highlight=injector+flow

    I favor keeping factory TBI fuel injectors pressures between 15 psi to 30 psi.

  5. #20
    Electronic Ignition!
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    Quote Originally Posted by dave w View Post
    TBI Injector link:
    I favor keeping factory TBI fuel injectors pressures between 15 psi to 30 psi.
    Sounds good, but I'm not talking about factory injectors - they won't fit a FAST TBI anyway.
    Besides they are not low pressure injectors, they are MPI type running 43psi (capable of 60psi)

    I didn't intend to confuse the issue but was looking at whether 2 x 45lbs FAST injectors as being sufficient for this engine.
    Last edited by IndyPaceCar; 02-08-2015 at 11:41 AM.

  6. #21
    Super Moderator dave w's Avatar
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    I'm on the same page now, about the injector.

    I'm thinking 90 lbs / hr would be enough fuel for 3.4 liters.

  7. #22
    Fuel Injected!
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    are you running a fuel pressure reg hooked to vacuam if not you will have issues when vacum changes the ecu is expecting a jump in fuel pressure when you crack the throttle

  8. #23
    Electronic Ignition!
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    Quote Originally Posted by delcowizzid View Post
    are you running a fuel pressure reg hooked to vacuam if not you will have issues when vacum changes the ecu is expecting a jump in fuel pressure when you crack the throttle
    We sure are. 43psi engine not running, less with engine vacuum.

  9. #24
    Fuel Injected!
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    Bumping this up to get the thread filled in with the resolution. (FYI My SD4, 2 barrel TBI , runs injectors from a 5.0 liter V8 engine, those these are the GM injectors, and not the EFI/Holley style injectors)
    In terms of sizing, a good ball park is to use 2 times the stock 60lb injectors. If you are making 200hp that is roughly 2x the stock Fiero engine. For some reason, I can only get about 170hp out of mine, so 2 60 lber's is a bit too big for me.

  10. #25
    Electronic Ignition!
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    Presently working on building a MFPI system.
    Here is a mockup (have to build fuel rail).

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