Given the huge expense Mopar has spent over the years in development and production, I have to say they had to have seen an overwhelming benefit of the dual plug design. Whether that benefit is in power or emissions, I have not determined, but whatever it was had to offset quite significant cost increases. A
s I am in production of vehicles, I know those engineers had to make quite a case to management/penny-pinchers to get those extra 8 spark plugs per car, the added raw materials into the head and a coil to handle both plugs as well...and they won. I'll carryover their lessons learned and just run the 16 plugs, if it's just for emissions...well that's fine too. The cost is insignificant.
A little column A a little column B.
The flame front can only travel so fast. Putting 2 plugs in the chamber the flame front doesn't have to travel so far (there is no "penalty" for the flame fronts crashing together) so you get a more complete burn which is good for both performance and emissions.
The center plug is one of the benefits of the original HEMI design. For instance, if you have a 4" bore the flame front only has to travel 2" in any direction, with a wedge head, with the plug to one side of the cylinder, the flame front may have to travel up to 3.5" in one direction.
As you can imagine, at higher RPMs the ability of the flame front to travel to the edges of the cylinder become more critical as there is less time to complete the burn.
I suspect that they couldn't fit the plug in the center of the chamber without sacrificing valve size, so the nested a plug between the valves on either side. It probably didn't add too much to the cost of the coil packs, and at the volumes they buy spark plugs that probably wasn't a big deal either.
