Originally Posted by Byprodriver

Less heat disapation from the exhaust keeps the engine cooler & the exhaust gases hotter for faster flow.

Originally Posted by alpine003

Generally anything close to the exhaust, you'd want to retain heat for better exhaust scavenging characteristics and velocity. Hence the reason why a lot of hardcore racer guys heat wrap their headers. On front engine car, the mufflers don't make a big difference because they are located further downstream where the heat has cooled off but on rear engined Porsches where the muffler is very close to the exhaust manifold, there might be some difference there.