Thermoquad or Quadrajet Which one do I want?

Engines, Exhaust and Fuel Systems

  1. ChryslerCruiser

    ChryslerCruiser Well-Known Member

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    I'm leaning towards a spread bore carb to replace the Holley 600 that is present on the imperial... Hoping to get some boost in fuel economy, and off idle crispness in the swap.

    What are your experiences with the two different type of carb?

    At present I do have some quadrajet's on the shelf, but have always been interested in playing with a Thermoquad, so I am open to exploring based upon other's experiences.
     
  2. Woodruff Carbs

    Woodruff Carbs Vendor FFJBO Vendor

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    What should have been on it factory?
     
  3. ChryslerCruiser

    ChryslerCruiser Well-Known Member

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    fuel injection...
     
  4. Aspen500

    Aspen500 Well-Known Member

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    Either carb would work. QJ has easier parts availability and "tuning" parts (jets, metering rods, metering rod pistons, etc) are easy to come by and it's a lot less complicated a carb. The TQ works excellent when it's set up properly, but it has 21 external adjustments which all need to be precise and done in order. The plastic main body can also warp and make it unusable.
     
  5. ChryslerCruiser

    ChryslerCruiser Well-Known Member

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    Q-jet sounds like the easy path.. Thanks!!
     
  6. Woodruff Carbs

    Woodruff Carbs Vendor FFJBO Vendor

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    Thermo-Quads are not quite that scary, and I’m not aware of 21 settings. 14 tops. For every T.Q. I get that needs the main body remachined I get 3 Quadra jets that have the top plate out of kilter. That is why they sell the thick top plate gaskets. Again, not real scary. Yes, tuning parts are more available for the Rochester, but how many jet and rod changes are you really going to make? Both are fantastic carburetors. If you are rebuilding them yourself, I’d do both. Get pre 1975 units and go for it. See which one you like. Rochester are more prone to the primary shaft getting loose and sucking air, (they already idle lean) so wiggle the shaft when choosing a core. 2 cents
     
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  7. Aspen500

    Aspen500 Well-Known Member

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    I was going off memory about the number of adjustments, and could very well be mistaken. It's been over 30 years since I personally dealt with a TQ (or QJ for that matter). I believe you would know more than I do about the good, the bad, and the ugly about the various carbs.:D
     
  8. AJ/FormS

    AJ/FormS Well-Known Member

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    A good running 600 will rival the others.
    Yours has issues obviously, as defined by it's poor economy and not crisp.
    If you buy used, you'll likely need to kit whatever you buy, same as yours.
    As for economy, any carb can make economy; you just lean it out until it won't run no more. The Holley 600 has gone over 30 mpg for me, in point to point driving. Jus saying.
    Timing plays probably an equal role when it comes to economy.
    And crispness comes from cylinder pressure, which is not cheap to change..
    I don't think you will ever find an old 8/1 SBM to display crispness. And with factory retarded timing and a rich low-speed circuit , it will never happen. No matter what carb you put on it.
    And don't even think about running a spreadbore carb, on an adapter spacer, to fit it onto a square-bore intake. It can be made to fit, but usually requires a die-grinder and proper gaskets. But just cuz you get it on there, doesn't mean it's gonna run any better than the fixed-up 600.
    I'm no fan of that 600, I just want you to be aware of your options. I am a big fan of spreadbore carbs on spreadbore intakes for low-compression engines. You bet.
    But if your Imperial has super terrible hiway gears, then crispness should not be in your vocabulary.
    Wiki has Morrisville listed as 732ft above sealevel, so that ain't too terrible.
    If you decide to kit the 600, install a 10.5 PV or something real close to it. And check that the floats are not heavy with gum, and that they actually float.
    and then there's this;
    HOLLEY REBUILT Rochester 4 Bbl. Carburetor 85R-8961 / 64-70107 | eBay
     
  9. Woodruff Carbs

    Woodruff Carbs Vendor FFJBO Vendor

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    Don’t buy that eBay carb for your imperial! That is for an 85 and up with computer controlled metering rods. They called the 3C feedback carburetors. The very last of the 4bbl Mohicans.
     
  10. ChryslerCruiser

    ChryslerCruiser Well-Known Member

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    I'm not buying ANY rebuilt carb, as I can screw one up myself, and have some fun while doing so.

    Thanks for the advice guys. I will proceed with a q-jet, b/c I have one... and keep an eye out for a T-Q that is cheap and looks like it could be rebuilt. .

    As for rehabbing the Holley, I may play with it some, as I already bought a gasket set, but not in love with the design... and parts are kinda spendy... I will not be using an adaptor to get a spread bore on there, but may open up the intake with a grinder... Which reminds me, I will have to check and see if the Q-jet will work with the present air cleaner.. That could be the show stopper if there is one.
     
  11. kkritsilas

    kkritsilas Well-Known Member

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    You will need to specify WHICH Holley carb. Is this a regular 600 CFM Holley (non spreadbore), or the Grant Street Demon (now owned by Holley, and is a spreadbore (mostly), and the spiritual descendant of the TQ)? The Street Demon is really based on a lot of TQ principles, and is available brand new, in a number of finishes. I think, but I am not sure without looking it up, that it is OK with ethanol containing gasoline.

    I call it mostly a spreadbore because the secondaries are a little odd, they are sort of joined together so that it makes more of a figure 8 instead of two separate round barrels. They have taken to calling it a "goggle" (as in eyeglasses) secondary. There are a few Youtube videos on it. It also comes in two sizes, a 600 CFM and a 750 CFM. From all indications, its pretty well calibrated out of the box.
     
    Last edited: Jan 16, 2020
  12. Camtron

    Camtron Well-Known Member

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    I just tuned an old Q jet last night, nothing terrible at all; I’ve had harder times getting single barrel, Ford straight sixes tuned, lol
    I’ve never worked on a legit TQ but, I do have the 625CFM Demon, 1901 Street Demon carb by Holley (mentioned in above post) on my car now and it is more or less a TQ. Aside from internal jets that require the whole carb be opened up for swaps (and that’s not hard at all) it’s real user friendly, super quick install and easy to change out metering rods and springs without taking much of anything apart; Just two screws and disconnecting the choke linkage.
    I’d go with, in this order: a Qjet for part availability and already having some on hand or, a known good TQ for Mopar/heritage sake or, a new Street Demon from Holley...which can cost about as much as a known good TQ in a lot of instances
     
  13. M_Body_Coupe

    M_Body_Coupe Well-Known Member

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    If you are in rush to go OEM TQ route spend some time looking for the aftermarket 9800 series carbs. These have the attached electric choke on the pass side and are a true blessing when you start getting to non-stock installs, things like different intake manifolds, etc.

    That's all I've been using for years now and I swear by them!
     
  14. ChryslerCruiser

    ChryslerCruiser Well-Known Member

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    Good to know about the 9800 model carbs, I will keep that in mind. For the moment I will go with a Q-jet as I already have one.. but it would be nice to look at a TQ to compare the differences..
     
  15. prauk

    prauk Member

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    i love the q-jets. i've rebuilt both and the like the qjet the best. its easy to setup, and good to tune. any questions, you can ask me. i have loads of them and parts.
     
  16. BudW

    BudW Moderator Staff Member

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    I’m a TQ fan but all of the TQ’s used on FMJ’s were, um, not the best version(s) of TQ‘s made (compared to the early ones). Chrysler used QJ’s after Carter went out of business in ’84.

    The Factory installed QJ’s were all computer controlled so not the best ones to use. I don’t know enough about the QJ to know but if a person could adapt a Chrysler QJ baseplate to an older QJ – that might allow for better connectability to your Chrysler.

    I happen to have a Chrysler QJ (Rochester) I would sell real cheap if you wanted to try. It is missing the linkage to the accelerator pump and is not anything I would ever use - but otherwise it works great. I’ve used it many times to test start engines (works better than all of the other 4-bbls in my garage for that purpose) and it worked great for that – but its been a long while sense it has been in use on the road.
    BudW
     
  17. ChryslerCruiser

    ChryslerCruiser Well-Known Member

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    Yep I am interested. That would be a good step forward in terms of getting the transmission to shift properly.. as the previous owner swapped out the Fuel Injection, but I am guessing the linkage is not correct, as the Holley seems to be farther forward on the intake manifold. All of which will be an investigation and correction opportunity.
     
  18. BudW

    BudW Moderator Staff Member

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    PM (conversation) sent.