holley vs. edelbrock

[Steve_Chin]

Quadrajets came *only* in 750, 795, and 800 CFM configurations from GM. The carburetor that F*rd used on the 428CJ was a 795 CFM variant. GM used mainly 750 CFM units from '67-'74, leaving the 795 and 800 CFM varieties for specific large displacement, high-performance setups (such as the Buick Stage 1 455, Pontiac 455-HO and SD-455). From '75 onward, it was dependent on the division as to which variant was more common, with Olds and Pontiac using mainly 800 CFM versions and the other divisions using 750 CFM versions for the most part (except for certain large cube or hi-po applications).

The 850 CFM unit is a new casting that Weber USA is making and selling through Edelbrock.

 

[azheep]

Is there a way to tell what cfms your carbs? can you do it by the carb #. I am thinking about the holley truck avenger off road carb. does anyone have any opinions on this carb?

 

[Steve_Chin]

The easiest way to tell them apart is to look down the primary bores:

750 CFM carburetors have two booster rings and pretty much perfectly round bores.

Some 795/800 carburetors have only one booster ring (basically the same as a 750 with the outer booster ring milled off - this is how Rochester did it for most of the early Quadrajets and how a lot of racers modified 750s to flow better).

Most 795/800 CFM carburetors have two booster rings and a larger main venturi diameter. The larger main venturi are identified by a couple of bumps in the ID of the bore near the outboard side of each bore. A lot of the later versins of this type don't really flow to 800 CFM because the secondary air flap does not open all the way (which is curable by modifying the linkage and link arm). These can be modified to flow in excess of 900 CFM.

 

[Roostr84]

Steve didn't the throttle stops on the carb afect the available cfm as well. I belive that on my Monte SS the carb was a 750 cfm however the stops only allowed it to flow around 565 or so. Many guys ground them down when swapping from the 305 to a larger engine.

 

[Steve_Chin]

GM put throttle stops on some engine combinations for various reasons, usually to limit horsepower (in '67-'69 they put throttle stops on the Firebird 400 engines so that they wouldn't be faster than the GTOs with the same engines). Yes, they limit CFM, but they're usually a part that's bolted or glued onto the carburetor.