The combustion chamber shapes are different. The d16z6 has a hemispherical shape and the d16y8 combustion chamber is squared. The d16z6 combustion chamber holds more volume than the d16y8. Because of the d16y8 smaller combustion chamber, the head yields higher compression than the d16z6.
hey, just a note, d16z6 is not a hemispherical chamber but rather pent roof. the y8 is also but with different quench design