Looks like they taking a cue from Chrysler. On the early A exhaust high performance exhaust, I’ve read that the Commando/Charger 273 benefited more from a well designed large diameter single exhaust than a smaller diameter dual exhaust. From the crossover back it’s something like 2 1/4 or maybe it’s 2 1/2 ID on the headpipe, muffler, over-the-axle pipe and resonator.