Originally Posted by
Vadim Yup, air that cools down becomes heavier and doesn't flow out as easily thus results in back pressure.Nope. Common misconception. Air cooling down and slowing down resulting in back pressure is not the issue. The faster the air moves past an adjacent merging pipe the more it will scavenge from that pipe (local pressure in adjacent pipe will be lower). After all merges are made, exhaust temps mean nothing for flow. You could put refrigerator coils around your exhaust and it wouldn't make a difference.
Keeping the exhaust in the header (specifically) hot is for increased scavenging effects. Also lower under hood temps as mentioned earlier. It could also
help or hurt catalytic converter and O2 sensor operation depending on placement in exhaust. After the collectors are merged, temps mean nothing.
On topic: header wrap promotes rust because it holds water and other crap next to the (bare?) metal allowing rust to form easier. It can also rub through paint and coatings to help rust. I painted my downpipe and wrapped it if you're curious. I'm too cheap for ceramic coatings or stainless steel/titanium/inconel. =/