Engine pistons are available with three basic designs, domed, flat-top and dished. Dished piston tops have the effect of lowering the compression ratios. The compression ratio for a Chevy 350 engine with dished pistons depends on several factors.
Compression ratios reflect the amount of full, unswept volume (piston at the bottom of the stroke) to the swept volume (compressed, piston at the top of the stroke). If a cylinder has a volume of 45 unswept cubic inches and a compressed volume of 5 cubic inches, it is said to have a compression ratio of 9-to-1.
The shape of the piston top is not the only factor that affects compression ratio. The shape and volume of the cylinder head's combustion chamber and valve faces, and the thickness of the head gasket also have a meaningful role.
Replacement dished pistons for the 350 engine are available in differing volumes, most commonly varying from 10 to more than 25 cc. A larger dish results in a lower compression ratio.
Using a compression ratio calculator with two dish volume inputs (20.3 versus 12.5 cc) and assuming stock 350 bore/stroke, a 64 cc combustion chamber and 0.041-inch thick head gasket, the compression ratios are 8.73-to-1 and 9.44-to-1, respectively. Comparatively, a true "flat-top" piston would have a compression ratio of nearly 11-to-1.
