cfm carb for Buick 350...places sell 600 and 650...is that enough?

Discussion in 'Small Block Tech' started by buicks, Aug 22, 2013.

  1. buicks

    buicks Well-Known Member

    My 72 350 is stock except for headers and a TA 4bbl intake, I have 2.5 inch exhaust tubes with X pipe but that is all. How many cfm do I need? I hear people say 750 but many say less, and most places sell less.
    I am looking for bolt on easy to adjust.

    http://www.autopartswarehouse.com/shop_parts/carburetor/buick/gs_350.html

    http://www.jegs.com/webapp/wcs/stor...kMAKE=1530190&mkMODEL=1530215&mmySubSearch=on

    ---------- Post added at 10:25 AM ---------- Previous post was at 10:20 AM ----------

    Speaking of TA performance, their carbs:

    http://taperformance.com/products.asp?cat=51
     
  2. David G

    David G de-modded....

    Factory used a 750 cfm Qjet, they probably knew what worked best. I would not go smaller.
     
  3. gsgnnut

    gsgnnut Well-Known Member

    Enough is a relative term. given the factory used a 750cfm on 4bbl 350s of the era i would go with something close to that. Try looking at Edlebrock Thunder series, I use an 850 thunder series on my 455 but I believe they also offer a smaller cfm model around 700-750cfms. The Edlebrock is hands down the best street carb out there if you dont want to screw around with 40 year old tempermental q jet. Mind you, Im not a racer and I am aware of the advantages of the q jet, but I wont use another q jet on a street car. I like something that works without having to mess with it. i got close to 10 years use on my edlebrock, it still operates perfectly and I havent had to even open it yet. :3gears:
     
  4. carmantx

    carmantx Never Surrender

    David is right, and this gets discussed a bunch. The carb choice thing gets confusing when you are trying to determine your cfm needs. Trust only the people on this site, or other Buick specific locations, mostly because the Buick people on this site have tried and tested what works for a Buick 350, which is different than any other GM 350 engine. Many of the big box retailers have a belief that what works for 1 gm 350 works for all.

    I like the quadrajet, because, when built correctly, it provides the fuel/air your engine needs all the way through your driving needs. It only pulls the air fuel you need. Better street drivability and economy, and the performance side. While 600 or 650 cfm would have a level of performance for you, the quadrajet lets your engine use up to the 750 it needs when you want it.

    So, yea, I am a little partial to Quadrajets, I build them, and have had them on all my street Buicks. 350 included.
     
  5. techg8

    techg8 The BS GS

    600 and 650 is enough for a Chevy 350.

    the right carb for your buick 350 is a 750 cfm quadrajet.
     
  6. buicks

    buicks Well-Known Member

    So is it possible the Qjet is not measured at the same point or the same way?
    For example, Is a 650 Edelbrock equivilent to a 750 Qjet?
    Kind of like when two engine are dynoed and put in a book ...and one is measured at 4rpm and the other at 3k?
     
  7. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    the primary side is probably pretty close so a 600-650 edelbrock may work ok idle/cruise ( the edelbrock 750's are gonna have a larger primary - less velocity - and may hurt idle/tip in/cruise characteristics ) - but - like they said the q-jet 750 is a dif design and has alot of adjustment so that when you make changes to your combo you can fine tune the carb better .
     
  8. buicks

    buicks Well-Known Member

    You can, if you know jets have testing equipment and such right? If you take the carb off and take it apart to change rods/jets.. etc?
    Are you saying the 650 woould be fine at idle and "tip in" but not enough fuel for wide open throttle?
     
  9. David G

    David G de-modded....

    When I bought my 68 back in 2001, it had a newer Edelbrock 650 carb on it. It ran fine just driving around town, very good driveability in general. But at WOT, it really felt like it was lacking. After a couple years I swapped on a 750 qjet from a 455. The difference was very noticeable, off idle and at WOT. It responded quicker off idle, and felt much stronger when I floored it. Seat of the pants info here... not track #'s.
     
  10. DEADMANSCURVE

    DEADMANSCURVE my first word : truck

    i was just saying that because the edelbrock 600-650's have a similar primary size compared to a 750 q-jet the low speed charcteristics may be ok - until you get to say 3000-3500 rpm then you may run out of steam . plus you loose the adjustability of the Q-J . the edelbrocks are a nice simple carb to rebuild and adjust because there are only a few steps to take and switch some rods and jets to get the mix correct but -------------- going to a 750 edelbrock with larger primary's to match up with the Q-J 750 you may loose the low speed effectivness .
    edelbrock builds the performer carbs so that they bolt on to a SBC and are kinda ready-to-go as is . thats where the money is .
     
    Last edited: Aug 23, 2013
  11. Gary Farmer

    Gary Farmer "The Paradigm Shifter"

    Edelbrock's square bore carb (performer) has richer jets for primary than secondary (by default). This tells me the carb is designed to be basically a big 2 barrel. It'll have great off-idle and low-mid range, with smoother transition to full throttle, but don't expect it to act like a Quadrajet.

    If memory serves, the primary CFM on a 750 Qjet is 178 CFM, and 228 CFM for the 800 (which is the only real difference between the two). This is smaller than even the primaries on the 500 CFM square bores (250 CFM), which leaves a LOT of CFM on the table for the secondaries, which adjust to the engine's CFM demand.

    This will generally give the Qjet better fuel economy and plenty of performance potential for a wide range of CFM demands.

    The Buick 350 uses the 750 CFM core Quadrajet, but does not actually use the full 750 CFM potential. Generally, 600-650 CFM is adequate for a stock engine, even the Buick 350. Same principle for the 800 Qjet. It will only deliver as much CFM as the engine needs, so the secondaries on an 800 would open up even less than the 750, depending on combination.

    I know I'm gonna get flamed for this, but the math doesn't lie. Buick engineered the 750 Quadrajet used on the 350's to only open up so far from the factory, to 70* on the upper secondaries, vs the 80* on big blocks. These can be modified to permit a full 90* opening.

    Nice thing about the Qjet though, is if you beef your engine up a bit with a larger cam and some head porting, the Qjet responds in kind by opening up further for more CFM when it needs to.
     
  12. buicks

    buicks Well-Known Member

    With 2.56 a rearend and a 350TH auto I wonder if I ever see 3500 rpm. No tach to know :D

    I can do 750 cfm, whatever it needs... but the Qjets intricate features that are great if you know how to set them, are killing me. I may want an Edelbraock 750. I cant accelerate hard without "bog" and have been adjusting the spring and (Cliffs Qjets) adjustable vacuum set screw all over the place to no avail. The carb is to spec, Cliff rebuilt it. It also wont fire right up after it sits 2 days, I gotta pump the heck out of it.
     
  13. Gary Farmer

    Gary Farmer "The Paradigm Shifter"

    You can get a rebuilt 750 CFM Qjet for about $250--$300 if they supply the core, and it will be adjusted to your combination.

    A brand new 600 CFM Edelbrock Performer will run you about $335ish, plus the cost of an adapter plate for a stock Qjet intake manifold.

    Take note: the adapter plate primary holes narrow down to the diameter of the Qjet primary holes on the intake, restricting CFM on the primary side. This may require some adjustment on the jets to lean it out a bit, though the .85" spacer thickness might offset this. The secondary holes widen up from the carb to the holes on the intake, creating more plenum draw on the secondaries, so you might have to put larger jets on the secondaries.

    This might not be such a bad thing, permitting the Edelbrock to reverse its settings to have the secondaries richer than the primaries, though it would take some tuning to optimise it.

    Both would serve you well for a stock engine, but if you ever wanted to upgrade later on to more power, the Qjet will be the better choice.

    Both will get similar gas mileage, properly tuned, though the Qjet will probably get a couple more MPG.


    Edit: ok I need to pay more attention to the original post. You have headers and a TA intake.

    Sadly, the intake is hurting your low end for a stock engine. If you have room, get yourself a 1" 4 hole spacer and put under your Qjet.

    And the X pipe is hurting low end too...

    Those exhaust setups are designed for mid-top end scavenging. H pipe is better suited for mild-moderate.

    But anyway, try the 1" 4 hole spacer if you can, and make sure there is no divider gap on it. You want the divider sealed off for your setup.

    Not to be a naysayer or bearer of bad news, but your car would make more power with a stock intake and exhaust manifolds with 2 1/4" duals with H pipe.

    Don't take off what you have, just add more to it (it's what the parts vendors want! :p ) TA 212 cam, 2500 stall, and 3.23-2.42 gears would wake it right on up. OR Crower level 2 or 3 cam, keep stock stall and gears.



    Edit 2: Wait til later on when Sean gets on, get advice from him too. He's a good small block info source.
     
  14. Gary Farmer

    Gary Farmer "The Paradigm Shifter"

    For your setup, not changing anything else, here is what I recommend:


    Get that adapter plate and a 600 CFM Edelbrock. Then get this: ham-3219_xl.jpg

    Make sure the bottom divider seals well to the intake.

    It will have much better off-idle response and power, and as it revs, the spacer design will help the 600 CFM draw better and have a much better fuel charge distribution than no spacer at all.

    You shouldn't have any hood clearance issues. I've researched this myself some time back, and the TA intake with a 1" spacer clears Skylark hoods no problem. 2" spacers are iffy.
     
  15. alec296

    alec296 i need another buick

    I doubt the x pipe is hurting you but I would. believe you will see better economy and driveability with a properly tune qjet
     
  16. gstewart

    gstewart Well-Known Member

    I believe that the 350v8 4bbl, at least in 1970 came with a 750 qj
     
  17. alec296

    alec296 i need another buick

    The big thing about the carb is that a Buick qjet will be metered for your engine not a universal carb. and I believe 77 the Buick qjet is 800 cfm
     
  18. sean Buick 76

    sean Buick 76 Buick Nut

    This thread is absolutely disgusting.

    Use a properly rebuilt Q jet and move on with life....

    The reason that a Buick 350 needs a larger carb than a chevy 350 is the lack of intake and head flow. About 850 CFM is required to feed a stock 350 Buick at 6000 RPM.
     
  19. Gary Farmer

    Gary Farmer "The Paradigm Shifter"



    A stock 350 needs an 850 douple pumper and pulls to 6,000 RPM.


    Interesting...
     
  20. sean Buick 76

    sean Buick 76 Buick Nut

    I am serious and I am about fed up with explaining why.
     

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