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Mechanical secondaries with automatic transmission???

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by k30gambi, Aug 21, 2002.

  1. k30gambi

    k30gambi 1/2 ton status

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    First off we're putting a rebuilt 350 in my little brothers 1/2 ton. It's got a cam, headwork and we have an aluminum intake and edelbrock 600 performer laying around collecting dust. I want the edelbrock on cuz I have a 750 on mine and it rips compared to a q-jet. I was reading in summit one day and it says they strongly recommend against putting a mechanical carb on a automatic vehicle. hmmmmmmm /forums/images/icons/laugh.gif
     
  2. bigmack

    bigmack 1/2 ton status

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    It doesn't make any sense to me why they would recommend not to. I have a Holley 750 double pumper w/mech secondaries on my 2wd truck and it's an automatic. /forums/images/icons/grin.gif
     
  3. Blue85

    Blue85 Troll Premium Member

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    I fail to see how the two are related.
     
  4. nvrenuf

    nvrenuf NONE shall pass! Premium Member

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    They probably don't recommend it for 2 reasons;

    1) Worse fuel mileage (when compared to a similar vac secondary carb)

    2) People get caught up in the theory that "if a little is good, then bigger has to be better!", because of this some people end up over carbureted. Stick shifts or auto's with bigger converters are very forgiving where an auto with a stock converter wont be. It's probably a CYA thing for them (no refunds because it was too big).

    This is not to say it wont work for you, simply guessing at their reason for making the statement.
     
  5. nvrenuf

    nvrenuf NONE shall pass! Premium Member

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    A mech secondary carb will flow more fuel faster than a vac secondary will (because the secondaries are activated by the mechanical linkage rather than vac demand). Depending on the motor, it is possible for a mech sec carb to flow more than the motor can take/burn at low rpm. This can cause all sorts of problems like stumbling, fouling, etc. This same situation can be amplified by an auto with a stock torque converter because the motor now has an additional load to deal with. Stick shifts or bigger stall converters can let the motor get up to an rpm level where is can effectively burn the "excess" fuel.
     
  6. RedDwarf

    RedDwarf 1/2 ton status

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    TH 350 and TH 400 actually do use engine vacuum to operate the modulator.

    Perhaps they are thinking the mech. secondaries might not allow enough off-idle vacuum to work the modulator...shouldn't really be a problem though as the trans doesn't usually shift until the throttles have been open for a few seconds....
     
  7. Beast388

    Beast388 1/2 ton status

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    I used to have a Holley 650 double pumper on my 350 before I stroked it. It was a great set up an ran just fine. I would still be using it but the 388 likes a 750 vac sec. better. I tried a buddy's mech sec. 750 on my truck once and there was no real improvement. I wouldn't hesitate to run a mechanical secondary carb on your brother's truck.
     
  8. twenty_below0

    twenty_below0 1/2 ton status

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    I think that the mechanical secondaries would be a good thing on a well built sm. block that can handle it
    On the other hand if it boggs then it's to much, But if throttle position is considered when dumping the secondaries
    you can make it work fine! Poor gas mileage yes, but more balls instantly is a good thing (at higher rpm's)
    I like cruiseing around in low at about 3000 rpm's then stomp it and watch it come to life /forums/images/icons/smile.gif
    I also think this is a better selection for a 2wd then 4 unless it's really built well.....
    in either selection I feel it's a personal preferance that only you can decide. good luck!
     
  9. k30gambi

    k30gambi 1/2 ton status

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    Cool, I was going to use it anyway, just seeing what you guys think, thanks.
     
  10. Blazer_Boy

    Blazer_Boy 1/2 ton status

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    What'll flow 800cfm stock, and yet has the fuel economy and response of a 500cfm carb with double booster venturis. Its unfairly bashed and misunderstood Quadrajet. With a little nerve and Doug Roe's Rochester Carburetors book, you can setup a killer carb.

    Many people think of carburetors as mystery boxes with personalities and if it doesn't do what they want, then its bad. Replacing with another carb that works better with that motor doesn't make the old one junk. It means that the old one wasn't set properly to the motor and the replacement is closer. Little time, $, and patience and you can make just about any carb run good. Some designs just happen to do the job better.
     
  11. Blue85

    Blue85 Troll Premium Member

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    OK, let me get this straight. A Q-Jet is considered a vacuum secondary carb because the top flaps on the secondaries set the rod position in the jets based on vacuum? A mechanical secondary carb (like some Holley's) actually sets this mechanically?

    I guess I always assumed that the mechanical/vacuum difference was in the opening of the secondary throttle blades. Even the primaries jets of a Q-jet are activated by vacuum, via the power piston.
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    Yes. Primary and secondary *throttle plates* of the Q-jet are mechanical. The secondary *air valve* (which is what needs to open to pull the secondary rods out of their orifices) is held *shut* by vacuum, and opens as vacuum decreases, engine demand increases airflow, pulling the air valve open, and holding it open as long as the engine demands.
     
  13. Blue85

    Blue85 Troll Premium Member

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    So Q-jet is mechanical or vacuum secondary?
     
  14. dyeager535

    dyeager535 1 ton status Premium Member

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    Vacuum. It's a misnomer, as you don't need vacuum for them to work, but they are vacuum. There is no mechanical linkage that forces them open.
     

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