HappyAndy wrote: A long time ago there was a thread here about DIY exhaust cutouts. Someone linked to a thread from another forum in which someone made vacuum operated cutouts in a dual chambered muffler. They repurposed vacuum solenoids from an old style air cleaner to operate them. They had a switch inside the car to deactivate the system. When the system was active the engine vacuum kept the solenoid closed. With mild to hard acceleration the vacuum dropped off and the solenoid opened up, passing the exhaust through the second, basically glass pack chamber for glorious noises. The same setup could be used for a straight bypass too, I guess. Sorry, I don't remember who who posted the link, it what forum it cam from. It was probably 6 or 7 years ago.
Funny, you've described the Hush-O-Matic exhaust from Flyin' Miata for the ND 
Vacuum operated cutouts are used in some OE applications. Rock solid reliability. The nice thing about having them hooked to engine vacuum is that the transfer from muffled to unmuffled is a nice smooth analog, not a step change.
