Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by hailey, Jan 29, 2003.
how big (cfm) is the stock q-jet on a 72?
I believe stock was a 650
I've got my org. sitting in a box at home...650cfm
I've never heard of a 650 cfm Q-jet before. Only 750 and 795 cfm.
If you are going to replace it with something other than a Q-jet go with a 650.
750 CFM would be correct for any 4MV Quadrajet other than one that was intended for an ultra-high performance application on a big engine like the Pontiac 455-HO, SD455, Buick Stage 1, Olds W-30, Cobra-Jet 429, etc. 795/800 CFM versions of the 4MV can be found, but they are exceedingly rare. They can be identified by either the lack of an outer booster ring or larger primary venturi (identified by bumps along the outboard wall of the primary bores).
It is much more common to find 800 CFM units in the M4M series of Quadrajet. They were extremely common in Pontiacs of the '75-'80 era, and I've found a few on Chevy trucks with big blocks in them. I haven't found any on small-blocks in the yards, but I'm pretty certain that the L-82 Camaros and 'Vettes had 800 CFM Quadrajets on them. These later 800 CFM castings can also be identified by a small bump in the primary bore toward the outboard side of the bore.
i think my dad said his 72 jimmy came stock with a 2 barrel carb.he replaced it with a holly economiser which was 400 cfm.i dont think this is the answer you were looking for though.
Two barrel carburetors are rated at a higher vacuum drop than four barrels, which is why you can find a 600 CFM two barrel that doesn't look like it has nearly the cross-sectional area of a 600 CFM 4 barrel (it doesn't because it is smaller in reality).
so if the 2 barrel has a faster vaccuum it has a bigger venturi or what?could you get a better low end torque from a 2 barrel with 600 cfm than a 4 barrel in stock form?or would the added fuel from haveing the big secondaries be easier to overcome the flooding that can accur with too much WOT and not enuf engine speed?i never owned a 2 barrel anything but my dad asaid his ran great in the snow.if it got stuck he would just put it in 4 low in first gear and let the tires spin while he dug out the front of the tires and the truck would just chug along and pull itself out.
No such thing as "faster vacuum." Two barrels are rated for flow at a higher vacuum drop than 4-barrels, which makes them look bigger on paper unless you realize that the CFM ratings are different due to that. Part of the reason why is that back when they were beginning to rate carburetors they didn't have good (i.e. consistent) pumps large enough to deal with the 4-barrel carburetors that were coming out.
Whether a 2-barrel carburetor would give better low end torque than a 4-barrel is entirely dependent on the design of each carburetor, the design of the manifolding, and how well each carburetor is tuned to the engine in question. Given that the manifolds are more or less identical and the carburetors are tuned properly for the engine combination, I don't see where a 2-barrel would be any better than a Quadrajet, and I'd expect it to be worse, given the Quadrajet's very small primary venturi. If you are comparing to a square bore, that would be entirely dependent on the venturi cross-sectional area in that carburetor.
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