The 350 cubic inch engine, specifically the Chevrolet 350, is definitively classified as a small block engine. This classification is based on its physical architecture, not its displacement (the volume of all its cylinders). Confusion arises because the 350 cubic inch displacement is substantial, leading many to assume it belongs to the larger engine family. This article clarifies the technical criteria that separate small block and big block engines.
The Defining Characteristics of Engine Blocks
The distinction between a small block and a big block engine lies in the fundamental dimensions of the engine casting, not the cubic inches it displaces. The most significant technical criterion is the bore spacing, the fixed distance between the centerlines of adjacent cylinder bores. This measurement dictates the maximum cylinder bore size and the engine’s ultimate displacement potential.
Engine manufacturers, primarily Chevrolet, created these two distinct engine families for different purposes. The small block was designed as a compact, lightweight V8 for passenger cars, while the big block was intended for heavy-duty applications requiring maximum torque. The physical size of the block, including its deck height (the distance from the crankshaft centerline to the top of the block), is also a defining factor, reflecting the overall external dimensions.
The Chevrolet 350: A Small Block Engine
The Chevrolet 350 cubic inch engine is a member of the original Small Block Chevrolet (SBC) family, which began production in 1955. It is classified as a small block because it shares the foundational physical dimensions of that engine family. The defining characteristic of the SBC is its 4.4-inch bore spacing, a measurement consistent across all displacements in the Gen I family.
The 350 cubic inch variant, which uses a 4.00-inch bore and a 3.48-inch stroke, is one of the most widely produced engines in automotive history. Its widespread use and reputation for performance likely cause the misclassification, as it was powerful for its compact size. Every engine in the Gen I SBC lineage shares the same external block dimensions and the 4.4-inch bore spacing, classifying them all as small blocks.
Identifying True Big Block Engines
A big block engine is defined by its significantly larger physical architecture, which allows for greater displacement potential. Using the Chevrolet Big Block (BBC) as the primary example, the difference in bore spacing is immediately apparent. The BBC family, which includes displacements like the 396, 427, 454, and 502 cubic inches, utilizes a 4.84-inch bore spacing.
This larger spacing provides a substantial increase in the distance between cylinders, allowing for larger cylinder bores and longer strokes than the small block can physically accommodate. The larger bore spacing and taller deck height result in a physically heavier and larger engine overall. This design was necessary to achieve the high-torque output required for heavy vehicles and performance applications.