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Do you know how many cfm my carbs are?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by bigerik, Apr 2, 2001.

  1. bigerik

    bigerik 1/2 ton status

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    I thought I knew how many cfms my carbs are, but then I found some conflicting data. The 1976 Jimmy came with a 4 barrel Q-jet on a small block 400/TH350/NP203. It has a divorced choke and 1 3/8" primaries and 2" secondaries. What is cfm? The 1981 Chevy Shortbox started as a 1/2 ton with a 4 barrel Q-jet on a 350/TH400/NP208. It has an electric choke and 1 3/8" primaries and 2" secondaries. What is cfm? I should know this! Trucks are at <A target="_blank" HREF=http://www.bigerik.coloradoK5.com>http://www.bigerik.coloradoK5.com</A>
     
  2. wayne

    wayne 3/4 ton status

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    Either they will be 750 or 795 cfm. I don't think GM had any other Q-jet's other then those cfm ratings.

    <A target="_blank" HREF=http://jamonline.homestead.com/fromthegroundupK54x4/89Jimmy.html>http://jamonline.homestead.com/fromthegroundupK54x4/89Jimmy.html</A>
    <font color=blue>Happy Trails[​IMG]</font color=blue>
    <font color=blue>Wayne</font color=blue>
     
  3. bigerik

    bigerik 1/2 ton status

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    That's what I thought! My mechanic said, though that they were 550 or 600 (I can't remember because I was so shocked). He proceeded to say that "I had the ones with the small primaries, but then I read that "all Q-jets except for the 850 cfm ones had 1 3/8" primaries." The mechanic has owned a 1972 Chevy 1/2 ton for decades and has always been a mechanic (even when he was an infant (JK)). He also said that the stock coil for GM HEI is 60,000 volts which I know is not true because they sell those 40,000+ volt induction coils that say "more voltage than OEM." I think stock is 32,000 volts. It is weird that he would give me bad data, but I have always read that the Jimmy would have a 750 cfm and the Silverado would have a 795 cfm.
     
  4. POWERMAD

    POWERMAD 1/2 ton status

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    Hey all
    I read a post on this at<A target="_blank" HREF=http://www.chevytalk.com>http://www.chevytalk.com</A>
    I seem to remember reading that the 795 has a ridge or ring part way down the venture(sp).
    I don't claim know everthing on Q-jets, but I have never heard of a 550 or 600 Q-jet.
    Hey Ric Nice rigs
    [​IMG]
    See ya

    my truck's not dirty, it's earth tone paint
     
  5. Blazer79

    Blazer79 1/2 ton status

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    I also don't think there were any 500-600 cfm Q-jets. According to my Doug Roe's Book on Q-jets, 1-3/32" primary venturi (not bore) are 750cfm carbs, and 1-7/32 venturi are 795 cfm Q-jets.

    Hope this helps.

    <font color=blue>//////
    <A target="_blank" HREF=http://blazer79.coloradok5.com>http://blazer79.coloradok5.com</A>
    </font color=blue>[​IMG]
     
  6. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    All are 750 or better. The design of a quad is that it adjust by vacume the amount of air it will flow. that's what the doors are for above the secondaries. If the engine can only handle 450cfm the the carb becomes a 450 CFM.
    Fact of the matter is most SB Chevy's that are mostly stock only need about 500-550 CFM. Big Block more. By making the carb self adjusting GM used one carb for Big and Small and saved stocking fee's.
    So yes of your motor can pull 650 CFM your quad will become a 650 CFM.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  7. bigerik

    bigerik 1/2 ton status

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    So just to clarify: you are saying that the carb on the Jimmy appears to be a 750 cfm and the carb on the Silverado appears to be a 795 cfm? I understand that the airflow is subject to vacuum/engine demand. By the way, what is a moderator on your name there, Grim-Reaper?
    It means I can come in and mess with your post [​IMG]
    <P ID="edit"><FONT SIZE=-1>Edited by Grim-Reaper on 04/03/01 10:21 AM (server time).</FONT></P>
     
  8. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Just an extra set of eye's to help Steve monitor the board. If somebody were to get out of hand I can kill their post if needed and put an end to any problems.

    Diging it in the dirt with my K5's
    Grim-Reaper
    <A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
     
  9. dyeager535

    dyeager535 1 ton status Premium Member

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    Check out the Q-jet links on my page link below. There is a very good picture that I edited showing the difference between the two. FWIW, GM used the Q-jet on straight sixes as well....Most are mechanically limited well below what they can actually flow by the tang on the passenger side of the air valve shaft that prevents them from opening fully. Generally, the newer the carb, the more that tab restricts secondary opening. The tang can be easily removed.

    795/800 CFM (no one seems to agree what the number is really) carbs seem to be MUCH more rare on the pre '75 carbs. I have yet to see one as a matter of fact, and I have carbs from SD455's from the early 70's in my "collection" Almost all Oldsmobile carbs from about 77-90 are 800 CFM units, but they do not have the same fuel inlet.

    At least on an '82 K5, 700, AC, 305, the stock carb was an 800CFM unit. Not sure about the 350's and 454's, but I would think GM would have used them across the board at that point, but who knows. Haven't got to look at any but the 305's so far.

    Bigger is not always better, as MPG will be affected negatively, and so can throttle response, by going with a larger CFM Q-jet.

    Dorian
    My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
     
  10. HarryH3

    HarryH3 1 ton status Author

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    I know a guy that has an old Jeep that he put a 4.3L V6 in. The engine came from a Chevy truck and has the factory installed Q-Jet sitting on top of it! Blew my mind the first time I saw it! It looks huge sitting on top of that little engine. Apparently that was the stock engine in 3/4-ton trucks for a while. [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
    It's a great day to be alive...
     

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