RE: Hemi Engines?
The 426 WAS a true Hemi design. Hence the name. otherwise, since Chrysler was the first to use it in production vehicles, they would have named it something else. In fact, they almost didnt use the word Hemi-it wasnt "catchy enough" for them back then.
When Ford came out with the BOSS 429, it too used a hemisphericle design combustion chamber, but since MOPAR had the "dibs" on the word Hemi, Ford had to call their motor a "crescent" design.
There are many different parts of an engine's design that control the amount of power you can extract from each combustion stroke. For example:
You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
You want to waste as little of the engine's energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.
The last item in the list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved.
