* Bore and Stroke: The 327 achieved its larger displacement through a larger bore (the diameter of the cylinder) compared to the 307. The stroke (the distance the piston travels) might also be slightly different, but the bore is the primary differentiator.
* Block Casting: While both are small-block Chevys, they have different block castings. This means the overall block design, including features like the water jacket design and the location of bolt holes, will differ. This often leads to different bolt patterns for accessories.
* Parts Interchangeability: While they share the small-block family architecture, many parts are *not* interchangeable. For example, pistons, connecting rods, and even some crankshaft components will be unique to each engine.
* Cylinder Head Design: While both used small-block heads, the specific design of the cylinder heads (valve arrangement, combustion chamber shape, etc.) varied over the years for both engines, leading to differences in performance characteristics.
* Year of Production: The 327 was produced earlier and for a longer period than the 307. The 307 was introduced later as a more fuel-efficient option, sacrificing some performance for economy.
In short, while both are members of the Chevrolet small-block family, they are distinct engines with different internal dimensions, castings, and thus, different performance characteristics. Just focusing on horsepower misses the broader differences in their design and suitability for various applications.
