From the GM website:
Z06 Exhaust System
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The three-inch-diameter exhaust system features a bimodal shutoff, which reduces noise output at idle and during moderate driving, while providing reduced back pressure at speed for race car-like performance. A vacuum-operated valve varies the exhaust note and volume according to engine load. At wide-open throttle, the sound emanating from the four trumpet-shaped exhaust outlets at the rear of the car is nothing short of sensational.
Hmmm....wheels turning.