73-87 Chevy _ GMC Trucks > Engine/Drivetrain
A note about carburetor flow capacities
rich weyand:
There were two body styles of Qjet, one nominally 750 and the other a little over 800. But there were a ton of tuning parameters, air bleeds, and other details that differed depending on the engine size it was supplied on. It was basically sized internally. That is, they were tuned to an optimum performance at a lower cfm than the carb body could handle. That's how they handled that issue. Run the Qjet from a 350 on a 454 and it will not perform as well above 500 cfm as one that was supplied with the 454.
But if you are buying an over-the-counter carb, including a Qjet, that is oversized for your engine, you will get less optimal performance than you will with the proper sized carb. So if you are going to buy a replacement Qjet, you should get it from someone who knows how to set it up for the engine you are putting it on, not off the shelf or from the junkyard from a different size engine.
http://www.chevelles.com/forums/showthread.php?t=297789
bd:
The secondary air valve in a Quadrajet allows it to function like a variable venturi carburetor. It adapts the airflow and fuel delivery characteristics of the carburetor to the air load demand of the engine differently than conventional designs. Effectively, the greater the demand the larger the carburetor to the limit of 800 CFM. The load controlled air valve was the game changer for the QJ, making it very adaptable and broadly applicable to the entire GM line.
BBM3:
--- Quote from: bd on July 22, 2015, 03:42:30 PM ---The secondary air valve in a Quadrajet allows it to function like a variable venturi carburetor. It adapts the airflow and fuel delivery characteristics of the carburetor to the air load demand of the engine differently than conventional designs. Effectively, the greater the demand the larger the carburetor to the limit of 800 CFM. The load controlled air valve was the game changer for the QJ, making it very adaptable and broadly applicable to the entire GM line.
--- End quote ---
Thanks for your excellent description.
I wish I understood that years ago.
rich weyand:
--- Quote from: bd on July 22, 2015, 03:42:30 PM ---The secondary air valve in a Quadrajet allows it to function like a variable venturi carburetor. It adapts the airflow and fuel delivery characteristics of the carburetor to the air load demand of the engine differently than conventional designs. Effectively, the greater the demand the larger the carburetor to the limit of 800 CFM. The load controlled air valve was the game changer for the QJ, making it very adaptable and broadly applicable to the entire GM line.
--- End quote ---
I've never been as impressed by that. The Edelbrock, for example, has an air door on the secondaries that functions the same way.
What I think made the Qjet so adaptable was all the pressed-in air bleeds and such that were tuned to the individual application.
ehjorten:
And that a QJ has no gaskets below the fuel level = no leaks!
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version