[seanp2k2]

This is not strictly true. Some engines work better with back-pressure, and removing or altering the exhaust without re-tuning usually loses power: http://www.formula1-dictionary.net/exhaust_road_perf.html

[teh_klev]

I can attest to that, my old Saab performed terribly (performance and fuel economy) during the time I was running around with a pierced and corroded exhaust mid-section flexi-joint.