In the early 1960s Dodge and Plymouth NASCAR teams were regularly getting beat by the competition and to deal with that bad publicity (when Win on Sunday, Sell on Monday was a reality) there was an all-out effort by Chrysler engineers to find the required extra power.
How they did it was to reach back into their engineering from the 1950s and come up with a new cylinder head design for their existing wedge big block engine. Returning was hemispherical combustion chambers on new cylinder heads. The design featured large valves, free-flowing ports and centrally-located spark plugs for optimum spark.
Soon after its release the new 426 Hemi engine dominated the NASCAR super-speedway tracks, the drag racing scene and today this basic engine design is still incorporated into the billet aluminum nitromethane-fueled powerplants of today's Top Fuel and Funny Car projectiles.