The Merlin/Argonaut exhaust mod had a parallel in R-2600 installations. The TBM (nee TBF) had a collector ring, like many radials; but the same engine in the B-25 had short stacks. The difference in noise level is dramatic.
But postwar most B-25s had a half-collector ring installed, for a different reason: the short shacks on the upper cylinders were rain collectors, leading to exhaust valve rusting and failure. Engine covers were mandatory ground equipment.
The half-ring mod alleviated this problem, exhausting low on the outboard side. But the lower cylinders retained their short stacks, and so remained noisemakers.
So if you plan on a joy ride in a B-25, do yourself a BIG favor and stop off at the local apothecary for some ear plugs!
