Jeff Luke, Executive Chief Engineer for the Chevy Silverado talked to www.drivingthenation.com about passive flapper valves. Chevy uses Active Fuel Management to shut off cylinders in their V-6 and V-8 engines. To reduce the noise that would occur during that transition they included a passive flapper valve that is used for exhaust tuning. When the engine is making the transition there is no indication that the engine is going in V-4 or V-6 from a V-8 mode.
The only way one knows the valves are shutting off is through an indicator light on the screen. Luke takes us through some of the hidden upgrades for the 2013 Chevy Silverado including hill ascent/descent control and hydraulic body mounts.