Blow through vs draw through Quadrajet

Discussion in 'The Venerable Q-Jet' started by Brandon Cocola, Aug 2, 2020.

  1. Brandon Cocola

    Brandon Cocola Well-Known Member

    Which is better, easier, what mods need to be done to make each work. I am thinking about a turbo setup and want to run a Quadrajet. I'm still not sure what engine I will end up doing this with, it may be the Buick 350 currently in my Skylark or a 5.3 LS I have.
  2. wkillgs

    wkillgs Gold Level Contributor

    Draw thru is easier. But there are downsides due to running an air/fuel mixture thru the will need turbo shaft seals to keep the gas from getting into the oil, and intercoolers are obviously not a good usable option. The carb will need a PEVR setup to maintain Power Enrichment (adequate fuel) for cases such as when you let off the gas but engine is still under boost. The old 78-83 turbo Buicks used a draw through setup with a Q-jet.
    Blow through will need the carb set up to handle the pressure, and a fuel system to maintain adequate fuel pressure over and above the boost pressure.

    I'm not up to date on the latest carb setups for blow-thru, Quick Fuel was the goto source in the past.
  3. sailbrd

    sailbrd Well-Known Member

    Do not even think about blow through Qjet. They are vac carbs and some of the best Qjet experts have tried and failed. If you must do blow through carb get a CSU built for your application. If you are going with any centrifugal style blower (turbo or Procharger, etc.) and do not want to do it all at once here is the order in which to do it.
    1. Put in an MSD 6530 box. It will work great now and even better when you do boost.
    2. Make sure the charging system is top notch with big battery. I went with 140 amp Powermaster and wished that I had done a 170 amp Mechman.
    3. New fuel tank, 400 lph Walbro, good return style fuel regulator that can be set up for carb or FI.
    At this point you have overkill for what you have now but lots of options for going to boost.
    If you only care about full throttle performance then a carb is great. Want a good driving street car look at a Fitech or Sniper. With the above stuff you are ready for the FI as well as boost.

    Preferred manifold is a single plane. Also better for FI. y
    For a good driving street car I would never do a blow through carb again. I have a Fitech and it has worked great. Also set up Bobb Makely's Sun Coupe with a Fitech and that has also worked well. If I was starting over would probably try a Sniper now and maybe their new distributor (do they make one for a Buick?)
  4. techg8

    techg8 The BS GS

    I built a blow thru qjet and used it with an Ebay gt45 turbo on a 72 buick 350. I put the turbo in the trunk, ran exhaust pipe to it and air pipe back to the carb. I O-ringed the throttle shafts, boost referenced the fuel pressure. The previous post was correct in that it would be optimum to boost reference the power piston in some manner...or you could just run jets w no rods , appropriately sized. It built boost but I never did get the fuel right in the secondaries and it became a lot like work so I moved on. Maybe someone more motivated could figure it out. I should also mention that I used stock low compression pistons, and that's not the best idea - forged might be the right way to go. Unless like me you just dont care if it breaks cause you're playing around.

    If I had to do it again I would use a trans am turbo 301 draw thru setup, and adapt a 2 barrel intake for it. In fact I have this setup in the basement now, mocked up. This would require a custom exhaust manifold or some creative exhaust plumbing.

    Awhile ago I built a 231 turbo v6 from 1979. Draw thru qjet. It is a good system for what it is. with tweaks I got the boost up to 11 or 12 before experiencing diminishing returns. Under boost the 231 felt like a mild built 350.

    I say for street Performance and general hotrodding fun this type of draw thru system would be fine. If max Power/performance is your goal then maybe not so much.

    Go for it!

    Edit: best bang for the buck is a 125 or less shot of nitrous. I had a LOT more fun juicing my 350 than I did turboing it.
    Last edited: Sep 15, 2020 at 7:21 AM
  5. sailbrd

    sailbrd Well-Known Member

    Just working off of memory here but I think all the early (60's) GM turbo work used draw through systems. Did not do blow through until they had FI.
  6. Brandon Cocola

    Brandon Cocola Well-Known Member

    Right now I am picturing twin turbos with a q-jet hanging off each turbo.

    I have also thought about eliminating the choke and routing the vent tube out the choke shaft hole then it would not see boost pressure.

    I am also thinking of boost and nitrous since in low levels you see the most gains. So no more than 125 shot on mild boost until I want more.

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