[carbking]

There is a chart of Rochester flow rates in Doug Roe's book 'Rochester Carburetors'. As Doug at one time was with Rochester engineering, you can pretty well take these numbers as gospel. Noting that the 1966 carbs are S.A.E. size 3 (1.5 inch nominal - this is an 'artifical' size, and does not appear anywhere on the carb, the actual throttle bores are 1.6875) WITH internal venturi size (this is important, as there are several) of 1.3125, the chart states these carburetors will flow INDIVIDUALLY 423 CFM at a pressure drop of 3.0 in mercury. 4 barrels are rated at 1.5 in mercury. To convert the two-barrel rating to a four barrel rating, divide the two-barrel rating by the square root of 2 (1.414). Doing this conversion means each of the three individually would flow 299 CFM on the four-barrel scale. CFM of multiple carburetors is NOT additive. Several have correctly mentioned that the intake manifold is the limiting factor. I think Pontiac Dude mentioned a figure of 800 CFM for the set. I read somewhere a figure of 780, but can't remember where I believe 800 is close enough to 780 to use as a round figure. While 3 deuces are 'neat on the street', for serious racing remember Pontiac tried them on the 1960 SD 389 (manifold 535552 cast iron), and again in 1961 on the SD 389 (aluminum manifold 541510), but the 421 SD's used either single four barrels, 2x4 barrels, or for NASCAR the trap door '3 barrel'. You can bet they had a good reason. As far as finding casting numbers (the raised numbers on the carburetor castings) in some listing of Rochester carburetors, you won't. Casting numbers are the number for the casting blank before machining. These can be machined many ways, and used in many different carburetors. Once the tag has been removed on pre-1968 Rochester carburetors, identification, while not impossible, can be quite difficult. Two barrels produced in 1968 and later have the part number stamped horizontally on the passenger side of the bowl. Jon.