In the mid-1970's smog-equipped engines went to vacuum operated heat riser valves that got the vacuum motor triggered by a thermal sensor in the thermostat housing. When the thermostat opened, usually at 195 degrees, the heat riser would snap fully open and allow the exhaust gas to go directly down the pipe instead of being routed under the intake manifold.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!