The larger "exausts" close to the aircraft centre line are actually to help with engine bay cooling. The exhausts pipes are inside ejectors which use the exaust gases to cause a venturi effect pulling hot air from around the engines. The S76 with the Arriel uses the same principal.
The 105 tends to suffer very badly from reinjested exaust soot being sucked in by the generators, working on the engines can leave you looking like a coal miner.
We used to swop the left and right outboard exausts over which changed their angle away from the generator air inlet. Not sure if it did any good but it made us feel better!!!!