The 1960s were a time when top-named muscle cars and automakers raced for horsepower supremacy. While drag racers often turned to large-displacement big-block engines for power, others preferred a little more finesse and looked for ways to increase the small-block V8’s power.
The small-block Chevy (SBC) V8, the engine that powers more cars than any other, was a leader in the small-block horsepower race. Chevrolet – and General Motorsm – produced several iterations of the SBC V8 with different displacements and successive generations. But it all started with the 265 cubic inch “Turbo Fire” V8 that powered the 1955 Corvette and Bel Air models.
If we consider the SBC as one of the founders of the small power movement, the double-hump cylinder heads were the statement of power. These heads (named for the shape of the raised casting mark that identifies them) were first seen on the 327 cubic inch small block 1962 Chevrolet Corvette. Hot rod magazine calls Chevy’s twin bump heads “the gold standard” and says they are “among the best stock heads ever allowed in racing.”
Chevy’s twin bump heads have sometimes been called “Fuelie” heads since their first appearance in the fuel-injected 327. With its 360 hp, that engine was a shot across the bow of the other American car manufacturers. Other small-block V8s from the same period, or released the following year, include Ford’s 271 hp HiPo K-Code 289 and Mopar’s 260 hp 318 Power Pack.
What’s special about Chevy’s double bump heads?
Over approximately 30 years of manufacturing double-hump cylinder heads, millions of units were made using a number of unique castings. The different castings are identified by the last three digits of their casting numbers, with examples including the 291, 461-X and 462. The best Chevy double-hump heads have large valves measuring 2.02 inches for the intake and 1.60 inches on the exhaust side, as opposed to the smaller 1.94- and 1.50-inch valves found on more basic double-humps.
Another advantage of Chevy’s twin-bump heads is their small combustion chamber, which often ranges from 62cc to 64cc. While other Chevy heads can be machined to achieve 62cc chambers, the twin bump heads feature a closed chamber design that provides more optimized fuel combustion.
While there are pros and cons to porting your cylinder heads, twin bump heads rarely need more than a touch-up. In a story about rebuilding a 1962 Corvette shared by MotorTrendMaster engine builder Joe Sherman thought the ports on the stock 461-X dual bump heads were impressive. The 461-X has more intake flow potential than stock 461 dual humps due to the “higher floors and roofs” present in the intake ports. During flow testing, Sherman found that the flow volume of the 461-X head was about as good as that of a late model Vortec head.
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