Why can a marine carburetor not be used on a street application?

Discussion in 'Small Block Tech' started by Curmudgeon, Jun 16, 2013.


    WV-MADMAN Well-Known Member

    The trick to picking the ''right'' parts is being honest with yourself about what you need in order to get what you want.

    My point about my '69 dually wasn't that the Holley was better than the Q-jet.(even if I feel it is)

    The point was that if I asked ''what size carb do I need for a 427 BBC?'', no one would have suggested a 600...

    Even though that was the right answer... 600cfm is all 427ci requires at 4500rpm, which is the redline on my engine.

    If I asked ''what will get the best MPG in a vehicle with the same weight and gears, a 307/Q-jet or a 427/Holley?''

    How many would have said ''pretty much the same''?

    The 307 spent all its time almost floored and the MPG even dropped well into single digits under load.

    The 427 cruises around barely off idle and dosnt really care if its under load or not...

    I hauled 4-tons of gravel over 30 miles up&down the WV mountains two weeks ago and still got 11mpg.

    The best carb is the right carb for the job at hand.

    Now you need to figure out what that is:laugh:

    Good luck!

    ---------- Post added at 11:49 PM ---------- Previous post was at 11:15 PM ----------

    Oh, I almost forgot...


    Now don't let it go to your head.

    We here know that deep down youre just as grease monkey like the rest of us:bla:
  2. Gary Farmer

    Gary Farmer "The Paradigm Shifter"

    Cubes vs CFM, that 600 would be ideal for low RPM and good velocity. Running a crap 307 wide open with a Q-jet I can't see how it would do anything but suck gas lmao.

    Kinda like comparing apples to oranges, but the point is port velocity, which the 600 Holley would have with big cubes. It's pretty much the same principle with the primaries on a Q-jet.
  3. Fox's Den

    Fox's Den 27 years of racing the same 355 Buick motor

    Run the 750 Holley DP with the 4 corner idle system and be done with it. :TU:
  4. sailbrd

    sailbrd Well-Known Member

    X2 but get the annular booster version
  5. sean Buick 76

    sean Buick 76 Buick Nut

    Your story about the heavy truck makes is very true!

    I had a 4x4 92 Dodge Dakota with a Magnum MPFI 318, and a 5 speed that got 17 MPG, but before that I had a 4x4 92 Dak 3.9, 5 speed, and it was 15 MPG.

    I tried to buy an "economic" truck but the underpowered 3.9 and light duty dak was not saving me any money.

    Pick of the overloaded Dak....


    Then I sold the little daks and got a 91 4x4 GMC 2500 with 350 7004R and it got 17 MPG even with 200,000 miles on it, and now 19 MPG with a rebuilt 350 HP crate engine and fresh trans. At least this truck can haul a trailer, a 75 regal, and a few engines in the back without squatting plus it pulls way safer!

    Heck my 81 Silverado gets 15 MPG so might as well drive a cool Chevy instead of a "economy" truck with no balls LOL!:TU::TU:




    WV-MADMAN Well-Known Member

    That's a sharp K-20 you have there Sean:TU:

    And I hear you on the 3.9 Dakota:rolleyes:

    I had a '94 2wd 3.9 auto and I was only getting 14-15mpg:spank:

    I got rid of that thing and got an '87 Chevy 4x4!

    I figured if Im going to have a gas-hog truck, it better be a bad a$$
  7. exfarmer

    exfarmer Well-Known Member

    I may be out to lunch here, and if I am please explain why, but this is how I see it. All carbs, no matter what the design, only flow as much air as the engine needs even at WOT up to the max they can flow. The can't flow more than an engine can suck in. eg. If a NA 350ci engine is turning 4000 rpm then it will be pulling in approx 365 cfm (assumed approx 90% VE) as long as the carb is large enough. If you punch the 850-DP it may have the CAPABILITY to flow 850 cfm right away but it can't FEED 850 cfm because the engine will not be pulling that much in. I realize that this is somewhat simplistic, but isn't the basic theory right?
    If a Q-jet can better handle the varying cfm requirements of an engine wouldn't that make it a preferred choice for the street? I would agree with Madman that a smaller Holley would be necessary on the street for driveability.
    This is a great thread. Have learned a lot about carb selection

    WV-MADMAN Well-Known Member

    Youre right and I should have been clearer:Dou:

    But the concept isn't easy to get across some times.

    An 850-dp cant force ''feed'' the engine more than it can pull in...

    What I meant was that when you open a dp-Holley, it opens up to its full CFM rating instantly.

    You cant technically have too much carb, but you can give yourself unnecessary tuning issues by running more carb than you need.

    And to be clear, I don't hate Q-jets.

    I just don't think they are worth the hassle for what you get.

    When Q-jet cores were dirt cheap and everywhere that was a different story.

    The last Q-jet I built for myself ended up costing me almost $500 by the time I bought 6 ''good'' cores at $50-75 each that turned out to be junk:rant:

    I was running an old Holley 600-vs that I built out of spare parts and used gaskets to get me by until I got the Q-jet fixed.

    The ''correct'' Q-jet was better than that crappy old Holley...

    But not by NEARLY enough to justify the added aggravation.
  9. Gary Farmer

    Gary Farmer "The Paradigm Shifter"

    Q-jet's secondary 'butterfly' top flaps control a pair of needle valves that are tapered and lift proportionately via a small cam 'lobe' as the butterfly flaps open. This lets the fuel/air mixture blend proportionate to the needs of the engine, depending on the size of the valves and gas needed per CFM. Control over how fast or slow this transitory position occurs is regulated in part by a tension spring.
    Other carb designs (VS designs) control a lower pair of flaps held closed by mechanical means at partial throttle, which is then released on full throttle, and adjusted by a spring for tension vs CFM need, and the venturi's vacuum draws out the fuel needed through open jets that mixes with the air as the secondaries open.
    The Q-jet also locks the (upper) secondaries closed until the lower mechanical flaps are moved open, which are held closed by a rod on the top attached to the primary choke flap when cold, and releases when the engine warms up. This forbids the secondaries from being used when the engine is still cold.
    Theses are just the basics.

    I think the Buick's tall runner ports are there for a reason, and aren't considered a design flaw. They permit the intake side of the engine to retain good runner velocity while allowing the use of larger carbs. This works hand in hand with larger carbs when the plenum is being emptied out faster than a smaller carb can keep up with.
    Unlike other designs, where the volume is high and a smaller carb is actually beneficial because it has to get its optimum fuel/air charge from somewhere.
    Each design is dependent on combination for optimum efficiency.

    That's my take on it anyway.
  10. exfarmer

    exfarmer Well-Known Member

    Excellent explanations Gary & Madman. I've got 3 Q-jets hopefully one is rebuildable. It's almost like the debate over which truck is best Chevy, Ford or Dodge ( obviously Chevy:))

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