Old Nailhead Manifold or Quadrajet Manifold

Discussion in ''Da Nailhead' started by suntreemcanic, Nov 24, 2021.

  1. suntreemcanic

    suntreemcanic Well-Known Member

    I have a 425 nailhead that was built for torque. It has Tom Telesco pistons, 1959 401 heads, a torque producing cam, a switch pitch turbo 400 transmission and 3.o8 rear end gears. I am using a Edelbrock AVS2 800 CFM carburetor with a 1959 401 4barrel intake manifold. It runs well and I can spin one tire with ease and gets about 12 MPG.

    Now the question. Going through some stuff I found a Quadrajet manifold for a nailhead. My Edelbrock carb has the same bolt pattern as the Quadrajet intake manifold. I have read about the Quadrajet manifolds making good power. Installing my Edelbrock AVS2 on the Quadrajet manifold would require a spacer. Would I gain anything with this conversion. I have at least 12 inches between carburetor and bottom of hood.
  2. TrunkMonkey

    TrunkMonkey Well-Known Member

    Putting a square bore on a spread bore intake with a spacer/adapter is not going to net you much.

    You would be better putting a spread bore carb on the intake to maximize volumetric efficiency as well as getting the power from higher velocity and impulse.

    Spread bore (and specifically Q-Jets) are great at high velocity through the small primaries and that is realized in low end throttle response and greater power when the secondary's are opened by the volume of air demanded at WOT.

    Using a square bore carb and adapter will defeat those two things, as the airflow will be less than efficient from the design of the intake to the carb, and the adapter cannot overcome/compensate for the differences of the physics each bring to the mismatch.

    The Q-jet manifolds make good power with the correct Q-jet carb. But expecting the same results using a non-spread bore carb will likely leave you disappointed.

    FWIW, I ran both the 2x4 (oem and correct carbs) on my nailhead and the Q-jet with correct Q-jet and the 2x4 was much better power throughout and overall than the Q-jet setup. (manual transmission).

    I think you would benefit from 2x4 setup, either OEM or Eelco manifold and proper CFM carbs and be happier than trying to make the square bore carb and spread bore intake work.
  3. gsgtx

    gsgtx Silver Level contributor

    where are the numbers or facts that a Q-jet intake manifold makes more HP over a square bore intake. I posted flow numbers before Q-jet vs square bore and the Q-jet intake didn't have any higher cfm flow then the square bore intake.
  4. Bigpig455

    Bigpig455 Fastest of the slow....

    Good question, I always felt the gain was in the use of a larger OEM carb .. I never considered the manifold itself to be the advantage unless it was ported like the one Bob did for me way back when...
    gsgtx likes this.
  5. wkillgs

    wkillgs Gold Level Contributor

    Agree with what's been said already.... Use a Q-jet on the Q-jet intake, and power gains are likely from simply using a larger carb (Q-jet or AFB/AVS) than the stock 625cfm AFB.
    Tough call on how the 800 cfm AVS will compare to the Q-jet.
    Buick did offer an optional package for the 66 GS 401, it used the Q-jet, different cam and distributor. It advertised an extra 15 hp. Power gains were in the higher rpm range.

    And I'll throw this up here since I found them in my files.... it's the flow data from Greg Gessler of the 2x4 intake and Q-jet intake he did mods on:

    Pic of his modded intakes:

    Note how he removed the dividers between the front and rear barrels. That may improve the performance of your '59 intake, we think it's due to the larger plenum volume. Another thing you can try is to install a carb spacer which is another method increase the plenum volume.
    TrunkMonkey, gsgtx and PGSS like this.
  6. PGSS

    PGSS Well-Known Member

    I'm surprised to just how much flow difference there is between cylinders with the stock 4 barrel..
  7. suntreemcanic

    suntreemcanic Well-Known Member

    I drove this car with a straight eight for so many years, I have so much more power with the V-8, I will just leave as is. My Edelbrock carburetor reminds me of a modern fuel injected engine, it is very responsive. I will have to find a new home for my Quadrajet intake manifold. Thank you everyone for expert advise.
  8. gsgtx

    gsgtx Silver Level contributor

    i think his flow bench that day on the 2x4 intake was feeling good. what i have written down from him on a stock iron 2x4 was 234 cfm avg. my eelco 2x4 he did for me stock or ported was less than the iron one Walt is showing. no way a stock iron could out flow the eeclo .
    Last edited: Nov 25, 2021
    PGSS likes this.
  9. gsgtx

    gsgtx Silver Level contributor

    flow numbers

    Attached Files:

  10. Bigpig455

    Bigpig455 Fastest of the slow....

    Hey hold on.. !!!! just cause the intake doesnt make the specific difference, the quadrajet is (in my opinion) about the best carb you can run on a Nailhead. Doc might argue for the Holley, but for each range of use (idle, off idle, part throttle and WOT) nothing beats a Q-Jet. And that spreadbore manifold is the first step.. hang on to it, and start making inquiries about acquiring a proper Q-Jet. you'll be very happy with the result.
    PGSS, wkillgs and TrunkMonkey like this.
  11. suntreemcanic

    suntreemcanic Well-Known Member

    Well where do you find a Quadrajet that is not worn out, they are close to sixty years old. I have a 66 (not a Buick) that is all original so I had the Holley rebuilt. The man that rebuilt it was highly recommended by Corvette people. Cost me $450 and I can not really say it runs any better. The Edelbrock on the 425 really runs good. I have been told my Quadrajet manifold is of the old style and newer Quadrajet carburetors linkage will hang up on this intake manifold. Have you heard of that before?
  12. Bigpig455

    Bigpig455 Fastest of the slow....

    I have heard of that, but the answer is to use a 76-79 B4B Quadrajet from a LeSabre/Electra and have it recalibrated to suit your combination. any good rebuilder can replace all the wear components of a quadrajet, I personally like Ken at Everyday Performance, who is featured in the vendor section of this board. I'm pretty sure he has core carbs avaiable if you dont have one,
  13. Super Bald Menace

    Super Bald Menace Well-Known Member

    The solution to the ill fitting later Q-jet on an early manifold is a 67 Buick Q-jet. The added bonus is they are setup to use factory style kickdown and switch pitch linkage.
    A good rebuider can make that carburetor work phenomenal. I have one for my 500 hp stock appearing engine.

  14. Dan Hach

    Dan Hach Well-Known Member

    When I was a kid, we used to call them "Quadra-toilets". There are some very good resources out there for these things. I think this is one of the areas that the internet has improved. Cliff Ruggles is also a fantastic resource. He wrote a pretty good book on the subject and his website has a forum where you can pretty much get any question answered.

    I ended up using a later Q-jet. It runs great, starts easy and has overall been a great carb. The learning curve is a little steeper, but after that, it's all good.

    If you want to keep your older manifold, maybe consider an Edelbrock AVS2? They seem like they share some of the design benefits of the Q-Jet but have the square bore base.


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