The Edelbrock Flathead Cylinder Head is made of A356 aluminum T-6 tempered cylinder head. They are rugged and ribbed internally and externally for more strength. Flat cylinder heads have large water capacity and excellent breathing characteristics. The compression ratio is 8. 0: 1 on blocks with a 3/16 base relief (stock bore and stroke). The compression ratio increase is approximately 3/10 of a percent with each 1/8 stroke.