At the time, IMSA was struggling to to keep the American V8 alive in this time of rapid technological improvement, and so the rules were shaped around limiting engine sophistication. Therefore, many of their prototypes ran two-valve, single-turbo "stock block" motors.
Though it's thought that the twin-turbo configuration was banned from running in IMSA, that's not exactly true. Twin-turbo engines were not ruled out of IMSA GTP, however, like four-valve engines, they were considered "race engines," and the twin-turbocharged motors were penalized with a lower displacement limit.
To conform with IMSA regulations and remain competitive, the Type-935 2.65-litre twin-turbo flat-six was replaced by a single-turbo flat-six with a capacity that varied according to the year and rules (from 2.8 to 3.2-liters). This car features an air-cooled turbocharged 3.2-liter flat-six making a conservative 590 horsepower—though it was able to produce up to 700-750 horsepower in its heyday.