I found some information about Smokey Yunick's "Hot Vapor Cycle Engine" here.
Adiabatic Engine - Hot Vapor Cycle Engine - Henry "Smokey" Yunick
He was able to get a big increase in efficiency. His engine used a low pressure turbocharger and very hot intake air. He had a secret method of avoiding knocking. It may have been water injection.
I think I have an article about this in a Popular Science magazine.
I found another forum discussing this topic here.
Engine & fuel engineering - Smokey's Adiabatic Engine