The Hemi engines drag racers know and love get their name from the hemispherical (half of a sphere) combustion chamber that positions their valves 180 degrees from one another. Those monstrous valves are held in place by titanium retainers and steel keepers.
From the NHRA Rulebook
For Nitro Funny Car engines:
Only one cylinder-head design is acceptable: Intake valve angle of 35 degrees, + or - 1 degree Intake valve size maximum: 2.470 inches Exhaust valve angle of 21 degrees, + or - 1 degree Combined intake and exhaust valve size maximum: 4.395 Inches