I have a 289 on my 66 and was looking to buy an air cleaner assembly and saw that there was a heat riser tube or heat shroud for sale and was wondering what that part was used for. I know the car came out of the factory with it but all the mustangs I've seen don't have one. Just curious.
There was a valve in the aircleaner inlet tube that was vacuum operated and the heat riser tube attached to that and ran down to the passenger side exhaust manifold to draw warm air off the manifold for easier cold weather warmups. When the engine reached a certain temp, the vacuum valve was closed by a vacuum switch on the intake mainfold (coolant)runner or one on the T-Stat housing.
D. Hearne explained the basic process well (as he usually does), but (if I can recall correctly) I think the 66 will not have vacuum lines running to the inlet tube like the later models did. The air cleaner inlet tube valve was opened with heat from the exhaust manifolds through the heat riser only. The only line coming from the exhaust manifolds was plumbed into the choke on the carb.