They usually outflow other designs especially at high RPM or under boost. But the design is more efficient than othe designs in certain situations.
Listed from what makes a hemi unique:
Quote:
HEMI has a super-efficient combustion chamber, making it markedly more powerful than other engines. Unlike a flat head piston on the engine you might find in your lawnmower, HEMI pistons usually have a dome-shaped head with open, angled valves and twin spark plugs per cylinder.
This configuration allows for:
Extreme intake of air
Optimal mix of fuel and air
Higher compression
More power output
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FWIW, The Chinese clone honda small engine just came out with 2 hemi head designs. One on the 5 horse and one on the 13 horse engines. The 5 horse has a definite power advantage while the 13 horse is just about the same power as the previous wedge design. If all things aren't properly designed the advantage of a hemi is lost.