70s EFI Intake Manifold
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500, 472, 425, 368 Discussion, 70s EFI Intake Manifold in Cadillac Engine Discussion; I'm trying to obtain an EFI manifold for my 500, and saw that in '75 and '76 some 500s came ...
  1. #1
    emailer33 is offline Cadillac Owners Member
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    70s EFI Intake Manifold

    I'm trying to obtain an EFI manifold for my 500, and saw that in '75 and '76 some 500s came with an EFI manifold. Is this the same manifold I see on the 425's all the time? I'm curious if they're swappable or if I should keep looking for a specific 500 one.

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  3. #2
    BRUCE ROE is offline Cadillac Owners Fanatic
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    70s EFI Intake Manifold

    Yes the 425 use the same EFI intake and throttle body as a 500. So grab any one you can salvage, they are valuable. The injectors are about 49 lb/hr (blue) on the 500, 39 lb/hr (green) on the 425. The original EFI design was limited to a 50% duty cycle on the injectors, by the alternate bank injecting. So if your setup allows an 85% duty cycle (such as sequential port), the smaller 425 injectors are more than big enough for a 500. Be aware these are low impedance (peak & hold) injectors, not compatible with every EFI. I converted my Cad to sequential port injection, seen on PHOTOBUCKET. Lots of people have used this intake to upgrade 500s, esp the GMC motorhome people. Bruce Roe
    70eldo likes this.

  4. #3
    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: 70s EFI Intake Manifold

    Bruce Roe is right. Also keep in mind that the EFI on Sevilles was different from the EFI on the other cars.

  5. #4
    emailer33 is offline Cadillac Owners Member
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    Re: 70s EFI Intake Manifold

    Thank you. I was considering grabbing a manifold and throttle body, and using more modern parts with everything else with a Megasquirt. So out of curiosity, what is different about a Seville?

  6. #5
    drmenard is offline Cadillac Owners Connoisseur
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    Re: 70s EFI Intake Manifold

    The seville has a Olds motor... If you are going to use The 425 intake and throttlebody you will need a 1981 368 EFI distrubtor.. this is the motor with the high odd shaped rockerarm covers .. this will work with modern computers for the control of timing... I have a 472 with a F/I intake with a 368 distrubtor run by a Haltech... I have done it , be glad to help if you have any questions...

  7. #6
    dpayton79 is offline Cadillac Owners Member
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    Re: 70s EFI Intake Manifold

    I have a 66 coupe deville that I'm putting a 500 in. Found the complete fuel injection from a 1977 eldorado barritz with a 425 in it. I took everything except the dizzy now I realize i need that as mine is a little different upon further review. My question is Im not gonna us the stock computer but should I still get that distributor and will it work with my 500 and a after market system like megasquirt or as you suggested haltech

  8. #7
    drmenard is offline Cadillac Owners Connoisseur
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    Re: 70s EFI Intake Manifold

    If you are not going to use the stock ECM you don't want that distributor.. If you are going to use a aftermarket computer that will control the spark you will need a distributor out of a 1981 4-6-8 368cid motor .. also called a 6.0L... These motors have the big odd looking rocker arm covers...This is a bolt in part and will not need any converting...

  9. #8
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 70s EFI Intake Manifold

    If you wanted to use the 425 EFI on a 500, you need the HEI, bigger (blue) 49 lb/hour
    injectors, and a 76 ECU for a 500 car. If you started with that you would have a running
    system, which could then be upgraded to something more modern.

    The original 76 EFI used 2 batch injector cycles which couldn't overlap. This required
    the bigger injector for the 500. However if you use a newer sequential injection, the
    duty cycle may be increased to 85%, and your 1977 (green) 39 lb/hr injectors will be
    all you need. Bruce Roe
    70eldo likes this.

  10. #9
    dpayton79 is offline Cadillac Owners Member
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    Re: 70s EFI Intake Manifold

    So did it use the bigger injector to spray more fuel so it could sit there until the valve opened in the batch fire setup.Is this why the batch fire required bigger injectors? Am I understanding that basically the greens are to small and if used with the the 500 it would be fuel starved unless I cranked up the duty cycle on an after market sequential fire computer. Whats the difference between the 425 ecu and the 500 ecu? Thanks for all you help and p.s. I got the 77 dizzy before I saw this post and walked right by a 4-6-8 motor and was wondering what was up with the strange valve covers and the wires coming in them(had never seen one in person before) now I gotta run back out to the boneyard and buy it.

  11. #10
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 70s EFI Intake Manifold

    The 425 and 500 ECUs are pretty similar, same pin out. But they are calibrated differently
    to match the displacement and injector size. A 39 lb/hr injector needs to exceed 50% duty
    cycle at maximum rpm, to fuel a 500. Bruce Roe

  12. #11
    Zachary Smith is offline Cadillac Owners Member
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    Re: 70s EFI Intake Manifold

    I have one of those with blue injectors, fuel rail throttle body, and spare fuel pressure regulator. the TPS is not there. It appears this has never been installed. I'm not sure what it is worth.

  13. #12
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 70s EFI Intake Manifold

    Yes the 500 and 425 used the same intake manifold, different injectors. Do you want one,
    are you building up an EFI system from scratch? I fix 76 EFI. Bruce Roe

  14. #13
    ferrisworld is offline Cadillac Owners Fanatic
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    Re: 70s EFI Intake Manifold

    I've been toying with the idea of building a 425 with one, and seeing if combining it with overdrive would get my 25 mpg highway, but I don't think it's worth the time or money when I could probably swap in a used 5.3 for less time, less money, and more power especially with a few bolt ons. Or if I wanted an adventure I could get a used LH2 northstar for $1,500, but then I don't get any aftermarket support or computer mods. Right?

  15. #14
    BRUCE ROE is offline Cadillac Owners Fanatic
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    Re: 70s EFI Intake Manifold

    Quote Originally Posted by ferrisworld View Post
    I've been toying with the idea of building a 425 with one, and seeing if combining it with overdrive would get my 25 mpg highway, but I don't think it's worth the time or money when I could probably swap in a used 5.3 for less time, less money, and more power especially with a few bolt ons. Or if I wanted an adventure I could get a used LH2 northstar for $1,500, but then I don't get any aftermarket support or computer mods. Right?
    Your best shot at economy is a properly geared overdrive lockup transmission. Fuel injection
    isn't going to do it, and the people claiming 25 mpg on a big engine/big car are keeping poor
    records from incorrect gauges. My diesel SOMETIMES did it. Bruce Roe

  16. #15
    the APEMAN is offline Cadillac Owners Fanatic
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    Re: 70s EFI Intake Manifold

    I had an injected high compression 500 +.030 in an '82 TransAm using 67% overdrive and a 2.8?? or so gear.

    It got 25 MPG on a steady highway roll between 55 and 60MPH on 93 octane.

    I agree with Bruce. Not going to happen in a "full size" 4000lb car. The main problem is probably the intake design. The low RPM efficiency is probably not up to more modern standards.

    Tuned intakes would probably do much better.

    8/6/4 RWD cars never got over 21.5 MPG at 60 MPH back in the day.

    I don't know anything about the 5.3 because I'm a dinosaur. It would be great to see someone retrofit something modern into one of these old cars.

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