I've been reading about this seemingly 'new' idea about insulating your exhaust headers and pipes using a fiberglass-based product called 'Thermo Tec' in order to boost horsepower by 5-7%. You can purchase 50 feet of this stuff on Ebay for less than $40, and it's supposed to withstand temperatures up to 2000 deg F. The physics behind the horsepower gains are as follows:

"Wrapping headers maintains hotter exhaust gases that exit the system faster through decreased density. Increased exhaust scavenging is produced, along with lower intake temperatures."

Assuming the horsepower bit is true, I wonder what kind of mpg gains are possible. Has anyone tried this?

Sorry, I forgot to post the link to the Thermo-tec website. Here's the "how-to" of exhaust wrapping technology:

http://www.thermotec.com/technology/how/how.html