According to
Corolland's website, a "cheap exhaust trick" is to wrap the exhaust manifold with exhaust wrap. The gases maintain higher temperatures, which translates into lower density, greater volume and thus higher velocities. They claim, with out backup, that this increases fuel economy.
My initial reaction is mixed. Yes, the density will be lower and so forth. But the dynamic viscosity of a gas increases with temperature. Anybody have any experience here?