Hemi History X

The Hemi engine was designed by Chrysler in 1951 and launched in a new line of V8 engines that year. The name was evolved from the shape of the cylinder heads, which had a hemispherical design. There are two Hemi engine designs...  One where the combustion chamber is hemispherical and one where the top of the cylinder head has that shape.

The hemispherical design helps improve thermal and volumetric efficiency when fuel is ignited in the engine's combustion chamber, creating more power without requiring a higher octane fuel to create more compression.

As well, the design places valves across from each other instead of side-by-side, to help improve air intake and exhaust gas flow. Spark plugs also are placed differently, angled to provide maximum ignition combustion in the chambers. More

Is the question of who invented the hemispherical combustion chamber a rather silly way to waste time.
Well... Chrysler didn't, and never claimed otherwise. But Zora Arkus-Duntov didn't either, and I don't believe he ever claimed to. The advantage of the hemi layout, with valves angled away from each other, lies with increased valve size, and greatly enhanced porting...

In The Grand Prix Car Volume 1 by Lawrence Pomeroy, the first mention of angled valves and overhead camshafts is in his description of the 1912 3 liter Peugeot Grand Prix car, designed by a Frenchman named Henri. This was not a hemispherical chamber, it had four valves in what's known as a pent-roof chamber. So it's a level above the hemi. Hence, the inventor of the hemi, if he exists, came up with a way to reduce the potential of the overhead cam, multiple valve design. More/Source

Some Hemi's in cars between 1952 and 1959

1952	DeSoto FireDome 276 ci - 160 hp / 250 ft-lb
1953	DeSoto FireDome 276 ci - 160 hp 250 ft-lb
1953	Dodge Red Ram 241 ci - 140 hp 220 ft-lb
1954	DeSoto FireDome 276 ci - 160 hp 250 ft-lb
1954	Dodge Truck 270 ci
1954	Dodge Red Ram 241 ci - 140 hp 220 ft-lb
1954	Dodge Red Ram 241 ci - 150 hp 222 ft-lb
1955	DeSoto FireDome 291 ci - 185 hp 245 ft-lb
1955	DeSoto FireFlite 291 ci - 200 hp 274 ft-lb
1955	Dodge Red Ram 270 ci - 183 hp 245 ft-lb
1955	Dodge Red Ram 270 ci - 193 hp 245 ft-lb
1955	Dodge Truck 259 ci - 134 hp 243 ft-lb
1955	Dodge Truck 259 ci - 134 hp 243 ft-lb
1955	Dodge Truck 270 ci - 142 hp 256 ft-lb
1955	Dodge Truck 331 ci - 201 hp 311 ft-lb
1956	DeSoto FireDome 330 ci - 230 hp 305 ft-lb
1956	DeSoto FireFlite 330 ci - 255 hp 350 ft-lb
1956	DeSoto Adventurer 341 ci - 320 hp 356 ft-lb
1956	Dodge Red Ram 315 ci - 260 hp 330 ft-lb
1956	Dodge Red Ram 315 ci - 295 hp n/a ft-lb
1956	Dodge Truck 259 ci - 134 hp 243 ft-lb
1956	Dodge Truck 259 ci - 134 hp 243 ft-lb
1956	Dodge Truck 270 ci - 142 hp 256 ft-lb
1956	Dodge Truck 331 ci - 201 hp 311 ft-lb
1957	DeSoto FireDome 341 ci - 270 hp 350 ft- lb
1957	DeSoto FireFlite 341 ci - 295 hp 375 ft-lb
1957	DeSoto Adventurer  345 ci - 345 hp 355 ft-lb
1957	Dodge Red Ram 325 ci - 285 hp 345 ft-lb
1957	Dodge Red Ram 325 ci - 310 hp 350 ft-lb
1957	Dodge Truck 315 ci - 197 hp 284 ft-lb
1957	Dodge Truck 315 ci - 197 hp 284 ft-lb
1957	Dodge Truck 331 ci - 201 hp 311 ft-lb
1957	Dodge Truck 331 ci - 201 hp 311 ft-lb
1957	Dodge Truck 354 ci - 218 hp 319 ft-lb
1957	Dodge Truck 354 ci - 218 hp 319 ft-lb
1958	Dodge Truck 315 ci - 197 hp 284 ft-lb
1958	Dodge Truck 354 ci - 218 hp 319 ft-lb
1958	Dodge Truck 354 ci - 218 hp 319 ft-lb
1958	Dodge Truck 354 ci - 218 hp 319 ft-lb
1959	Dodge Truck 315 ci - 197 hp 284 ft-lb
1959	Dodge Truck 354 ci - 218 hp 319 ft-lb
1959	Dodge Truck 354 ci - 218 hp 319 ft-lb
Crate Mopar	Gen III Crate 392 HEMI 392 ci - 540 hp 490 ft-lb