Each cylinder on a Hemi engine 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.
In the 1980s Japanese manufacturers reduced unburned hydrocarbons by placing spark plugs either in the exhaust pipe (which fired with every piston ignition) or in the exhaust manifold (which fired each time their corresponding cylinder fired). This idea was championed by Tom Hoover, the “father of the 426 Hemi. He told Hot Rod that he had discussed the Elephant Engine’s design with the new-Hemi engineers. Three of his major suggestions- using dual spark plugs, raising the camshaft (to shorten the pushrods, reducing valve-train inertia and simplifying the rocker arms), and adding squish area (for more light load/low speed efficiency and reduced emissions) were adopted.
Dual plugs on each cylinder, allowed 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. This (also reduces) NOx and ozone. Full combustion results in heat, water, and carbon dioxide. NOx emissions are only significant during incomplete or partial combustion, due to the lack of available oxygen, high temperatures, and various chemical reactions. That's why catalytic converters have been standard on cars for the past three decades. The extra set of spark plugs on the HEMI and on previous engines are designed to reduce emissions before a catalyst is needed. They add some horsepower, but not very much.
Here are the spark plugs used on the Challenger engines:
3.5L- Champion Copper CC3034. Change at 30,000 miles.
3.6L- Champion Copper CC9407 (CPN-9407). Change at 30,000 miles.
5.7L- Champion Copper CCH570 (SPRE14MCC4). Change at 30,000 miles.
6.1L- NKG Platinum 4998 (PLZTR5A-13). Change at 100,000 miles.
6.2L- Mopar SP196724AA. Change at 36,000 miles.
6.4L- Champion Platinum CCH3405 (CPN3405). Change at 100,000 miles.
Note: Spark plugs should be torqued to 13 lb/ft.