i think you're thinking of the hot air intake. it works off a heatshield around the exhaust manifold, and scavenges excess heat.

the heat riser on a v8 blocks half the engine's exhaust with a butterfly at the end of one exhaust manifold, and redirects it thru the intake manifold crossover, to come out the other exhaust manifold.

they can either be built into the manifold, near the pipe flange, or are a separate piece between manifold and pipe. usually are bimetal spring operated, but sometimes vacuum too.