In addition to the ability to use larger valves, the hemispherical combustion chamber design can yield more efficient combustion action. Chrysler started working with the hemi concept at least as far back as the fifties, with engines under 400 cubes. The 426 was the culmination of the approach and very few 'stock' U.S. motors could touch it for power. Just about anything Chrysler dropped a 426 hemi into made the top 10 fastest production quarter-mile lists in the sixties. A few other makers (including BMW) used hemi (or multiple hemispherical dome areas, 3 in BMW's case) within the combustion chambers of their head designs for this reason in addition to the valve advantages.

It's not all hype, although it's not the only way to skin that cat.

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