Here is some information regarding the modern Hemi from allpar:
Tom Hoover one of the engineers responsible for the 426 Hemi, told Hot Rod that he had discussed the Elephant Engine’s design with new-Hemi engineers. Three of his major suggestions — raising the camshaft (to shorten the pushrods, reducing valve-train inertia and simplifying the rocker arms), using twin spark plugs, and adding squish area (to make light load/low speed efficiency better and reduce emissions) were immediately adopted.
The 2009 Hemi was extensively modified to increase both power and gas mileage. Changes included a higher compression ratio, better-flowing heads, intake, and exhaust, and an active intake manifold. This is a new technology that takes advantage of Chrysler advances in the 1960s; it switches from long runners to short runners by moving a flapper door, to switch from better torque at low engine speeds to higher horsepower as rpms rise.
The new variable valve timing system used hydraulic cam phasing, relatively simple and inexpensive. The hydraulic roller camshaft had oil passages in front to drive the cam phaser sprocket, and increase valve lift. A new piston/rod assembly had thinner walls with a stronger-alloy pin.
All the ports were redesigned, with a raised exhaust floor; the new intake valves were 2mm larger. Other upgrades were a stronger crankshaft, dual-mass crankshaft damper, floating pin pistons, new valve springs, and higher oil pump capacity. The results were better reliability and gas mileage, with more horsepower and torque at every point in the engine’s speed range.
Each cylinder has an ignition coil pack over one spark plug, and a regular plug wire connected to the other spark plug. Further, the coil pack also has a plug wire attached to it that extends to the opposite cylinder bank. Each cylinder shares a coil pack with another cylinder. Each of the two plugs on a given cylinder is fired by a separate coil. One plug has a coil directly attached, and the other is fired via an ignition wire connected to a coil located on another cylinder on the opposite bank. The benefits would be one-half the number of coils (8 vs. 16) compared to each plug having its own coil, and of course less weight.
The extra plug fires during the power stroke to more fully burn the hydrocarbons. ... the second ignition allows additional power in the down stroke while lowering the need for restrictive catalyst plates in the converter. ... [using] dual fired plugs on each cylinder allows the firing to take place closer to top dead center, and then again when the piston is on the back side of the power stroke. The extra set of spark plugs on the Hemi and on previous engines are designed to reduce NOx and ozone emissions before a catalyst is needed. They add some horsepower, but not much.