Carb (?) newbie needs help
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500, 472, 425, 368 Discussion, Carb (?) newbie needs help in Cadillac Engine Technical Discussion; New to carbs other than there basic function. I recently purchased a 75 500 with 61K miles and want to ...
  1. #1
    lead sled is offline Cadillac Owners Member
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    Carb (?) newbie needs help

    New to carbs other than there basic function. I recently purchased a 75 500 with 61K miles and want to do some upgrades. I'm more familar with EFI but think that using a carb will be cheaper. Doesn't seem like there are to many of the shelf EFI for this generation of Cad.

    Is a Q-jet carb a type of carb or an abbrivation for something.
    Is Quadrajet a type of carb or the manufacturer name.
    What kind of carb did Cadillac put on from the factory.
    Is there a better type of carb than what the factory used?
    How do you pick out a new carb?

    Any input would be helpful

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  3. #2
    davesdeville's Avatar
    davesdeville is offline Banned
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    Re: Carb (?) newbie needs help

    Q-Jet is an abbreviation for Quadrajet. Quadrajet is the name for 4 barrel carbs made by Rochester. It's the factory carb on all 472-500-425-368s that weren't FI equipped. There is no better carb in my opinion than the Q-Jet. I can tell you a Holley is a hell of a lot easier to tune. Some say a Holley will make more power, but according to dyno tests by Flash the Holley only started to make more power at around 500hp - which hardly any street engine runs.

    If you have a dirty old clogged up Q-Jet and you replace it with a shiny new Holley, OF COURSE it's going to seem like the Holley is a lot better. But if you'd sent the Q-Jet to be rebuilt and had it setup and tuned properly it would be a differant story.

    Another factor is fuel economy. Look at the small primaries on a Q-Jet as compared with a symmetrical 4 barrel... when cruising at lower RPM the engine is only drawing through the primaries, and smaller primaries = better mileage. The huge secondaries make up for the small primaries in the power department.

    EFI is another option. You can sometimes find factory MPFI (multi port fuel injection) injection manifolds. You'd need to buy a throttle body, injectors, fuel rails (well you can make these,) fuel pressure regulator, and a computer like a megasquirt to control it all. MPFI is much better than TBI (Throttle body injection) which is just marginally better than a carb. There are TBI units available, and CadCo/Flash can adapt a 454 TBI unit for use on a Caddy, but they're not worth the price.

  4. #3
    mark48 is offline Cadillac Owners Member
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    Re: Carb (?) newbie needs help

    I recently rebuilt my 1970 472 and opted to stay with the Q-jet for the reasons stated above, but I put a 1" phenolic spacer between the carb and the manifold. I fabricated a dual snorkle air cleaner in order to feed it more air. Final tuning continues!

  5. #4
    terrible one's Avatar
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    Re: Carb (?) newbie needs help

    Quadrajet is a brand of carb, made by a company called Rochester, I do believe. I'm thinking the Quadra part is telling you it is a four barrell. It isn't a type. That's all the input I can give, considerin ghte fact taht I am a total n00b

    EDIT: sorry guys, comp trouble...didn't see the previous posts.

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    JTraik is offline Cadillac Owners Enthusiast
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    Re: Carb (?) newbie needs help

    Actually Rochester is the city where Delco made the Quadrejets... i live in Rochester :coolgleam and the Delco factory is now abandoned... quite a interesting place to visit.

    As far as FI vs Carb... Carb is probably the most sensible thing to do but does not stack up to TBI or SPFI SPFI in my opinion is only marginally better than TBI and TBI is much better than carberation.

  7. #6
    davesdeville's Avatar
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    Re: Carb (?) newbie needs help

    TBI is only marginally better than carb. Multi Port Fuel Injection is a lot better than TBI. SPFI is usually just called SFI, Sequential Fuel Injection, which is where the injector sprays fuel right before the intake valve opens. SFI is a bit better than MPFI, but it requires cam sensors and a computer that supports it so it's not really an option.

  8. #7
    BluEyes's Avatar
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    Re: Carb (?) newbie needs help

    TBI isn't all that bad. It can be tuned more precisely than a carb, and is alot easier to swap between vehicles than MPFI because all you need is an adapter plate to mount the TBI unit to your current manifold. Plus it only needs around 15psi fuel pressure compared to 45-60 for MPFI.
    The real drawback to TBI is that there are precious few TBI units that flow enough air for a big motor like ours. Q-jets flow 750 or 800 cfm depending on the model, and most Caddies got the 800cfm unit.
    If you go with TBI, you'll most likely lose some topend power unless you hunt around for the big TBI units out there. I think the 500 caddies got pretty good sized ones, and the 454SS trucks flowed pretty well.

    Really, from behind the wheel you won't be able to tell much difference from a well-tuned Q and EFI. I've got two Q-jets running in my collection of cars, and they never stall, bog, or anything other than what my right foot tells them. They really are excellent carbs if they haven't been neglected for 30 years.

  9. #8
    terrible one's Avatar
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    Re: Carb (?) newbie needs help

    So are Quadrajets not made anymore?

    I lov evisiting abaondoned buildings, especially factories. Somethign really interesting about that for some reason.

  10. #9
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    Re: Carb (?) newbie needs help

    Edelbrock makes them nowadays - I believe they bought the rights from GM or something.
    You can also get remanufactured Q-jets from most any parts shop in town, and performance-tuned rebuilt units from several aftermarket places.
    GM put Q-jets on V8's from '65 untill they switched to EFI, so there are ALOT of cores out there!

  11. #10
    terrible one's Avatar
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    Re: Carb (?) newbie needs help

    I'll bet. I see them selling on ebay for $20 or less all the time when I'm looking for stuff for my Caddy. I've been at it as a clean up/ semi-restoration project for quite a while now. It's getting there.

  12. #11
    BluEyes's Avatar
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    Re: Carb (?) newbie needs help

    If you get another Q-jet, I'd keep the primary/secondary metering rods, the power piston spring and the jets from yours. Those set the fuel curves, and are really well dialed in for your engine by GM.
    Personally, unless you really want it all freshly dipped in whatever they used to give Q-jets their color, I'd rebuild it myself. It is really easy. Just buy the book "Rochester Carburetors" by Doug Roe. It is basically the Q-jet bible, and explains throughly how these things work. Then search the internet for Quadrajet articles by Damon Nickles, and by Lars Grismund. You'll be set.

    Also, if you do, before getting one - measure the diameter of the primary throttle butterflies. If they are 1 7/32" then you have an 800cfm unit 1 3/32" means a 750cfm unit.
    90% of Q-jets are 750cfm units, so no trouble if you have one of those. Only the BIG engines got the 800cfm units though, and if you have one, you want to make sure you replace it with one.

  13. #12
    terrible one's Avatar
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    Re: Carb (?) newbie needs help

    Yeah, not planning on getting another one anytime soon at least, just noticing. I was, however, planning to rebuild it. I have plenty of experience rebuilding carbs, so that's no problem.

    I'll measure the butterflies fo rhte hell of it jsut to see, but im thinking it's an 800cfm unit, just on account of the 500 ci big block in there.

  14. #13
    BluEyes's Avatar
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    Re: Carb (?) newbie needs help

    99.9% chance it is then, unless someone replaced it with the wrong unit sometime in the past.

  15. #14
    terrible one's Avatar
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    Re: Carb (?) newbie needs help

    Nerp. Never been replaced. Has been rebuilt once, about 10 years ago. Entire engine was rebuilt around 13 years ago.

    Either way, it doesn't need to be replaced, but if I ever need parts or something, I'll look for an 800cfm model.

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