Mufflers for 2005 workhorse with GMC 8.1

rgatijnet1 wrote:
Yes, mufflers are usually designed to fit the space available and provide the correct amount of back pressure. Too little back pressure can eventually burn the exhaust valves and too much can cause the engine to lose power, etc.

This is another old wives tale. Your valves work better with no back pressure. The better exhaust systems/headers actually create lower pressure at the valve that promotes better exhaust scavenging.
Bill