How many dual four-barrel Corvettes were sold circa 1956-1961? Exactly 21,000. We estimate the '56 and '57 passenger-car 2x4 265 and 283 to be 7,500. This induction was a very popular bolt-on from 1957 through the late 1960s. Many more manifolds and carbs were bought new over-the-counter at dealerships. Each Carter WCFB carb flowed about 380 cfm. A pair meant 760 cfm, plenty for most small-blocks.