A hemispherical head ("hemi-head") gives an efficient combustion chamber with minimal heat loss to the head, and allows for two large valves. ... Since the combustion chamber is virtually a hemisphere, a flat-topped piston yields a lower compression ratio unless a smaller chamber is utilized.

What is so special about a Hemi engine?

What is so special about a Hemi engine? The Hemi engine's unique design, which features less surface area, results in less heat loss. This means the engine's peak pressure can be higher, which can result in better efficiency. Additionally, larger valves mean it's easier for air to flow through a Hemi engine.

Why do Hemis have 2 spark plugs?

The extra set of spark plugs on the HEMI and on previous engines are designed to reduce emissions before a catalyst is needed. They add some horsepower, but not very much.”