Introduced on the C6 Corvette, the Dual Mode Performance Exhaust - option code NPP on the ECU touchscreen - consists of a vacuum-activated butterfly valve in the exhaust tube. When engaged, the flaps open at 3,500 RPM, changing the exhaust noise and making it louder. GM first introduced the NPP dual mode Corvette exhaust on the following C6 Corvette models: the 2013 Z06, the 2009-2013 ZR1 and the 2010-2013 Grand Sport.