What is a Big Block Chevy Engine?
"Big block" is the term used to describe the large displacement V8 engines that were developed in the USA during the 1950's and 1960's. As American automobiles grew in size and weight following the Second World War the engines powering them had to keep pace. Chevrolet had introduced their popular small block V8 in 1955 but needed something larger to power their medium duty trucks and the heavier cars that were on the drawing board. The decision was made by Chevrolet to develop an all-new design for large-displacement use. This engine family had two generations, the "W" series, and the Mark IV series., being this design, the one used on First Generation Chevy Camaros.
Development of the second generation big-block started with the so-called Mystery Motor used in Chevrolet's 1963 Daytona 500 record-setting stock cars. This "secret" engine was a substantially modified form of the "W" engine, and was subsequently released for production use in mid-1965 as the Mark IV, referred to in sales literature as the "Turbo-Jet V8."
Where the Mark IV differed from the "W" engine was in the placement of the valves and the shape of the combustion chambers. Gone was the chamber-in-block design of the "W" (which caused the power curve to drastically sag above 6500 RPM), and in its place was a more conventional wedge chamber in the cylinder head, which was now attached to a conventional 90 degree deck. The valves continued to use the displaced arrangement of the "W" engine, but were also inclined so that they would open away from the combustion chamber and cylinder walls, a design feature made possible by Chevrolet's stud mounted rocker arms. This alteration in valve placement resulted in a significant improvement in high RPM volumetric efficiency and resulted in a substantial increase in power output at racing speeds. Owing to the appearance of the compound angularity of the valves, the automotive press dubbed the engine the "porcupine" design.
As part of the head redesign, the spark plugs were relocated so that they entered the combustion chamber at an angle relative the cylinder centerline, rather than the straight in relationship of the "W" engine. This too helped high RPM performance. Due to the new spark plug angle, the clearance provided by the distinctive scalloped valve covers of the "W" model was no longer needed, and wide, rectangular covers were used.
In all forms (except the ZL-1 Can-Am model) the "rat motor," as it was later nicknamed (the small-block engine being a "mouse motor"), was slightly heavier than the "W" model, with a dry weight of about 685 pounds (310.7 kg). Aside from the new cylinder head design and the reversion to a conventional 90 degree cylinder head deck angle, the Mark IV shared many dimensional and mechanical design similarities with the "W" engine. The cylinder block, although more s ubstantial in all respects, used the same cylinder bore centers and main bearing dimensions as the older engine (in fact, the shorter stroke 348 and 409 crankshafts could be installed without modification). Like its predecessor, the Mark IV used crowned pistons, which were castings for conventional models and impact extruded (forged), solid skirt types in high performance applications.
Also retained from the "W" design were the race-proven Moraine M400 aluminum bearings first used in the 409, as well as the highly efficient "side oiling" lubrication system, which assured maximum oil flow to the main and connecting rod bearings at all times. These features, along with the robust crankcase design, sturdy forged steel crankshaft and massive four bolt main bearing caps used in the high performance versions, resulted in what many have considered to be the most rugged and reliable large displacement automotive V8 engine design of all time.
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