The Corvette C6.Rs are powered by 5.5-liter, naturally aspirated GM small-block V-8 engines. They’re based on the 7.0-liter LS7 powerplant and use production aluminum blocks along with heavy-duty cylinder heads. The engines feature a smaller cylinder-bore diameter (4.090-inch) and a shorter stroke (3.185 inch) than the production LS7 to reduce displacement to meet GT-class rules. The series also “equalizes” the performance of engines of various types and sizes by requiring intake-air restrictors. Thus the Corvettes’ engines breathe through a pair of 28.8mm restrictors that are located in front of the sealed airbox, which limit engine output to 485 horsepower at 5,800 rpm.