Originally Posted by
BenFenner 
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. =/
Well I did wrap mine primaries, I didn't bother with secondaries because Im cheap and it didn't seem useful.
I got my information from CarBibles.com. Check the bottom of
this page for more information.
This little section almost needs to be considered along with turbos, above, because the two typically go hand-in-hand. When gas gets hot, it gets less dense. Less dense means less resistance to flow. It figures, then, that people who are looking at every tiny minutae of performance would want to wrap their exhaust headers. Why? Well exhaust gas exits the combustion chamber extremely hot (duh!) but it cools rapidly as it travels through the exhaust system. In doing so, two things happen. First, the gas becomes more dense and begins to resist flow, and second, as it does this, it disperses heat into the metal exhaust pipes, which in turn radiate the heat into the engine bay, raising the under-hood temperatures. The problem with the gas cooling down is obvious - it begins to slow down and provide resistance in the exhaust system. The problem with the under-hood temperatures going up is that it makes it more difficult for the engine to get a good, cold charge of air. (Colder air is more dense, which means better, more powerful combustion.) This is why you sometimes see vented hoods on cars; they're designed to let the hot air out and keep the under-hood temperatures down. So wrapping the exhaust headers with exhaust wrap helps because it basically insulates the metal exhaust pipes. This means they retain the heat better which in turn means the exhaust gas remains less dense and keeps up it's high flow rate. For turbos, this is a good thing because it means the exhaust reaching the turbo is travelling faster, which means the turbo spins faster, which means more air forced into the engine. Everything is connected, you see? So the ideal system would be a turbo, with wrapped exhaust headers, a vented hood, a cold-air unduction and an intercooler. That combination, whilst expensive, will give the coldest (and thus densest) fuel-air charge into the engine, whilst insulating the exhaust and ventilating the engine bay at the same time.
It's worth pointing out that not all exhaust wraps are made equal. If the wrap insulates too well, then the exhaust pipes get too hot and that can cause all it's own problems from engine bay fires to structural failure of the exhaust or turbo.
Point to note: It's only a rumour that exhaust wrap absorbs water and can encourage your mild steel headers to crumble away prematurely. If anyone tells you this, they're fearmongering.
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