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To properly identify a combustion chamber, all its aspects including shape need to be considered. Hemispherical or Pentroof - The "Hemi"

A chamber of this design is considered to offer the least amount of compromise for the efficiency gained. Effectively, a hemispherical combustion chamber is one half of a sphere cast into the bottom of the cylinder head. The valves are placed at the outside of the bore area and at a specific angle from the crankshaft centerline. Optimizing this position allows for huge airflow gains, since it moves the valve away from the wall to keep it from creating turbulence. This creates a more efficient cross-flow movement of the charge during valve overlap and limits thermal transfer from the exhaust valve to the fresh charge. As mentioned previously, this design offers the best surface-to-volume ratio and also creates a very short direct exhaust port, essential in limiting heat rejection into the coolant. Hemispherical chambers generally have a central spark plug, which offers excellent octane tolerance. An additional benefit is the distance between the intake and exhaust valves, which further limits heat transfer.

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