In essence, the difference between a big block engine and a small block engine is dependent on the engineers who designed them. Certainly, the big block engine is larger in size overall. It’s physically larger, it weighs more, and the valve bores are larger. Furthermore, the big block comes in both 2-bolt and 4-bolt mains. But don’t confuse “cubic inches” (as in displacement) with the physical size of the engine. The first version of the big block engine was introduced in 1958, the “U” Series, and was designed to power light-duty trucks and some of the newly designed passenger cars. The W-series, made of cast iron, was produced from 1958 to 1965, with three displacement options: 348 cubic inches (5.7 L); 409 cubic inches (6.702 L); and 427 cubic inches (6.9973 L).
Prior to that, the small block V-8 engine was standard in most Chevy cars, varying over the years in displacement. The Chevy 350 cubic inch (5.7 L) small-block eventually became the GM corporate standard until its manufacture was discontinued in 2003. Currently, the small block engine is built in Mexico as a crate engine for hot rod and restoration enthusiasts. The small block engine is commonly referred to as a “mouse motor.”