Just for grins
G.M.C. used, for the most part, Zenith carburetors; and Zenith was a company that actually posted CFM ratings.
So just what size carburetors did actual GM engineers think was
necessary for various 1952 G.M.C. 6 cylinder engines?
Since these are all 1 barrel carbs, the standard rating scale at 3 inches mercury (Hg) was used.
228 CID - 188 CFM
248 CID - 210 CFM
270 CID - 214 CFM
302 CID - 224 CFM
But wait, as the obnoxious TV commercials burn into our brains; we are now speaking of a 4-barrel conversion, and by convention, 4-barrels are rated at 1 1/2 inches of mercury (Hg).
So doing a conversion by dividing the 1 barrel rating by the square root of 2 (1.414) we obtain:
228 CID 188 -> 133 CFM
248 CID 210 -> 148 CFM
270 CID 214 -> 151 CFM
302 CID 224 -> 158 CFM
More difficult to deal with Rochester, as venturi sizes are not specified, but the largest Rochester monojet was rated at 250, converting, this would be 177 CFM on the 4-barrel scale. I do not know if this large monojet was used on the normal 250 CID Chevrolet engine. I would guess this venturi was used on the governed truck engines.
So a 390 CFM Holley is more than double the very largest Monojet, and 2 1/2 times the size of the carburetor GM engineers thought was adequate for a 270 CID. And the 600, to paraphrase my college calculus instructor from so many years ago "the proof is intuitively obvious, and the solution is left to the reader".
Jon.
