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Chevrolet Big-Block engine


The Chevrolet "Big Block" is a term for a series of large displacement V8 engines that were developed in the United States during the 1950s to the early 1970s. As American automobiles grew in size and weight following the Second World War, the engines powering them had to keep pace. Chevrolet had introduced its popular small block V8 in 1955, but needed something larger to power its medium duty trucks and the heavier cars that were on the drawing board.

The first version of the "Big Block" V8 Chevrolet engine, known as the W-series, was introduced in 1958. Chevrolet designed this engine for use in passenger cars and light trucks. This engine had an overhead valve design with offset valves and uniquely scalloped rocker covers, giving it a distinctive appearance. The W-series was produced from 1958 to 1965, and had three displacement options:

The W-series engine was made of cast iron. The engine block had 4.84-inch (123 mm) bore centers, two-bolt main bearing caps, a "side oiling" lubrication system (the main oil gallery located low on the driver's side of the crankcase), with full-flow oil filter, and interchangeable cylinder heads. Heads used on the high performance 409 and 427 engines had larger ports and valves than those used on the 348 and the base 409 passenger car and truck engines, but externally were identical to the standard units. One minor difference between the 348 and 409/427 was the location of the engine oil dipstick: it was on the driver's side on the former and the passenger's side on the latter. No satisfactory explanation was ever offered for why this change was made. However, it did provide a fairly reliable way to differentiate between the smaller and larger versions of the engine.

As with the 265- and 283-cubic-inch "Small Block" engines, the W-series valve gear consisted of tubular steel push rods operating stud-mounted, stamped-steel rocker arms. The push rods also acted as conduits for oil flow to the valve gear. Due to the relatively low mass of the valve train, mechanical lifter versions of the W-series engine were capable of operating at speeds well beyond 6000 (RPM).

Unlike many of its contemporaries, the combustion chamber of the W-series engine was in the upper part of the cylinder, not the head, the head having only tiny recesses for the valves. This arrangement was achieved by combining a cylinder head deck that was not perpendicular to the bore with a crowned piston, which was a novel concept in American production engines of the day. As the piston approached top dead center, the angle of the crown combined with that of the head deck to form a wedge-shaped combustion chamber with a pronounced quench area. The spark plugs were inserted vertically into the quench area, which helped to produce a rapidly moving flame front for more complete combustion.


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