Ferrari has employed this for many years too, not only for noise control but to vary backpressure and pressure waves for scavenging via servomotors.
And yes, the guy with the trick AMX has this too, although he uses them as bypass valves to run the exhaust through smaller pipes while cruising to keep the noise levels down.
I believe "Power Effects" was the vendor making variable mufflers for the C4 crowd.
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1969 Chevelle
Old setup: Procharged/intercooled/EFI 353 SBC, TKO, ATS/SPC/Global West suspension, C6 brakes & hydroboost.
In progress: LS2, 3.0 Whipple, T56 Magnum, torque arm & watts link, Wilwood Aero6/4 brakes, Mk60 ABS, Vaporworx, floater 9" rear, etc.
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