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Edelbrock750 VS Q-Jet

Discussion in 'The Garage' started by CHEVY 4WD, Jan 24, 2004.

  1. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    well im running a pretty healthy SBC(350) and though that I might be able to pick up a little HP by switching to a edlbrock so I picked one up last night and put it on tonight after work... I dident really get to play with it much but my truck seems WAY slower then it was before /forums/images/graemlins/eek.gif it actually seems to rev up faster in neural but when I put it on the ground and try to kick the 4barrel open it dosent even really seem like it is, and my cable IS opening the carb all the way... like I said I dident get to play with it much but I just wanna make sure theres nothing yall can tell me that will make it a 5min job instead of a 5hour job... I do know on a quadra jet the secondarys are HUGE compared to the primarys... think that could have anything to do with it? /forums/images/graemlins/thinking.gif
     
  2. SUBFAN

    SUBFAN 1/2 ton status

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    Read that book that came with the box, all your answers are probably in there /forums/images/graemlins/thumb.gif

    Otherwise I can't be of any help. /forums/images/graemlins/confused.gif
     
  3. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    yea... Im reading it now but It dosent say if the q-jet should be faster then the edelbrock or not though /forums/images/graemlins/rotfl.gif
     
  4. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    Interesting...if I had to bet I would have said the edelbrock AFB style would be faster. Are the secondaries vaccuum or mechanical? Are they opening right? /forums/images/graemlins/thinking.gif
     
  5. Confedneck

    Confedneck 3/4 ton status

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    its a mechanical secondary carb.. im gonna go check out the problem in the AM and hopefully have it solved rather quickly.. /forums/images/graemlins/thumb.gif
     
  6. bablazer73

    bablazer73 1/2 ton status

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    750 cfm on a 350 is too much. You would have been better with the 625 edelbrock. I know the bore of the Q-jets look big, but it's the Venturi size that matters. The larger the carb, the higher the rpm range. BUT the larger carb will have less throttle responce. That is most likely the problem. What rpm does this engine run at? the 625 will carry most 350's to 5500.
     
  7. Confedneck

    Confedneck 3/4 ton status

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    625 edelbrock? hmm..
    he got the carb because he is getting a good deal on it,unless it doesnt work right, in which case the guy will take it back.. its a brand new reman unit..
    the motor is, lets say, very healthy.. healthy enough to make my 350 with holley intake, carb and dual exhaust feel like a 4 cyl toyota motor.. a 750 is actually probably about the right size for this motor.. ive talked to nick and we think the 4 barrel may not be opening correctly..
     
  8. bablazer73

    bablazer73 1/2 ton status

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    This may sound dumb, But think of the engine as a giant air pump. If it's 350 ci, it really doesn't matter how it's built. it's still 350 ci. The CFM(cubic feet per minute) is the valve going in. The engine can only take so much air in, so with a 750 cfm carb or a 625 cfm carb, at 6,000 rpm, it's going to use the same amount of air. By all flow charts, the normal 350, running at 100% volumetric effiency, will only need 600 cfm. 750 cfm will carry it to almost 8,000 rpm. Most street engines will run at 80-85 % built engines, 90-95% dyno tuned?? When the CFM is larger than needed, The airflow through the carb actually slows down and fuel atomization Is not as efficiant as it would be if a smaller carb was used. But at a higher RPM, as a larger would be needed, that wouldn't be a problem because air speed would already be there. did I make that so you can undertsand?? or did I confuse you more!!
     
  9. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    yea.. that make SINCE.... BUT then why did GM have 750 Q-Jets and the carb seems to make the power down low About the same but its up high in the RPM where it tends to feel like a dog /forums/images/graemlins/confused.gif
     
  10. bablazer73

    bablazer73 1/2 ton status

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    Thats always been one of the glories of the a-jet. If you play with the way the secondary air valve(flapper on top) opens,you can change the way the carb works. It resricts air flow and doesn't allow the carb to flow its full amount until higher RPM's are reached. This is also how the Q-jet also got the name "quadrabog". If the valve opens to quick or to slow, well we bog! most people don't know that the tension on that can be adjusted. There is a small set screw, that is only visable if the carb is turned over.
     
  11. dyeager535

    dyeager535 1 ton status Premium Member

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    Primary venturis on the 750 Q-jet are approx. 250CFM.

    Mechanical secondary carb opens the secondaries based on throttle position, nothing else. If the engine can't ingest that much air (750CFM at WOT for instance) then it bogs, because it has a huge vacuum leak.

    Now, talking about a properly tuned Q-jet, (which has vacuum/spring controlled secondaries) you can't have high vacuum and WOT, so if the engine isn't demanding that much air (flow through the secondaries) but the throttle is wide open, the air valve spring keeps them shut, even though the linkage at the base on the secondaries opens.

    The upper and lower secondary "controls" are independent, but are made to work together. A mechanical secondary carb eliminates the "independent" (the vacuum/spring controlled air valve up top) part of a vacuum secondary carb.
     
  12. R72K5

    R72K5 Banned

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    i wouldnt evne mess with putting a edelbrock on the engine, id leave the Q jet on there, its not worth the trouble of getting rodded and jetted and all tuned for the specific engine its on, Q jet are very easily performance buildable, the mods are easy to do, rods and jets swap as easy as they do with edelbrock and alot of the parts are same anyways
    nothing wrong with Q jet, they only get a bad name by some ppl who know nothing about carbs and just happen to have a Q jet thats old and tired and needs cleaned out and instead they just call it junk and gho but a junk holley or edelbrock but those are never tund set up for any specific engine/application, and you wont gain hardly any hp, if any at all, probably will lose some

    and yeah an engine can only take advantage of so much CFM, putting a carb with more CFM rating than the engine can ever suck in is pointless.


    stock Q jet were 750 and 800

    there is an easy way to tell the diff between them,. the 800 have smnaller annular ring bulge inside the primary bores than the 750 do and are found of course on larger CId engines and mainly the older years ones. i have a few around here, 455vi and 425ci and such ones.

    750 are simply with more restricted primary bores


    good luck
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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    Just as a point of reference, early 80's truck 305's and all Olds 307's (found to this point) are 800CFM versions. Go figure.
     
  14. Confedneck

    Confedneck 3/4 ton status

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    i went over to nick's house (chevy4wd) this morning.. and had a look at it.. it has mechanical secondaries with what seems like vacuum operated butterflies inside the bores? not sure but they werent operating properly.. you could put your foot to the floor, the 4barrel would open and it would pop sputter and bog.. we swapped the qjet back on and voila.. runs like a champ.. think hes going to get an edelbrock reman qjet..
     
  15. bryguy00b

    bryguy00b 3/4 ton status

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    hmmm..yea just seems like from what you describe that its just to much carb for it..
     
  16. Confedneck

    Confedneck 3/4 ton status

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    how about it has a 750 qjet on it.. but i figure the qjet is jetted lower..
     
  17. bryguy00b

    bryguy00b 3/4 ton status

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    huh /forums/images/graemlins/blush.gif

    guess so..i dunno im no carb guru, just rebiulding my first right now actually..
     
  18. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    I think its just the fact that I like the spread bowl pattern ALOT better with the HUGE secondary and it seems to make the power Faster... Does anyone know how to easily identify the CFM of a Q-Jet? /forums/images/graemlins/thinking.gif
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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  20. CHEVY 4WD

    CHEVY 4WD 1/2 ton status

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    isent there a smaller qjet like a 600 or 650?
     

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