Complex sentence: The early models had two turbocharged Continental TSIO-520-K engines with 285hp each, but in 1976 the engines were upgraded to the more powerful Continental TSIO-520-NBs with 310hp.
Split into simpler sentences: The early models had two turbocharged Continental TSIO-520-K engines with 285hp each. In 1976 the engines were upgraded to the more powerful Continental TSIO-520-NBs with 310hp.