Mufflers are designed to reduce sound, but they do it at the cost of horsepower and fuel efficiency. Removing the muffler can only stand to increase engine performance.
Most mufflers use fiberglass or steel wool packing material to absorb sound energy. Exhaust flows through a perforated tube and exhaust noise exits through the holes in the tube to be absorbed by the packing material. Some performance "chambered" mufflers use a series of metal baffles and chambers that cause sound waves to bounce back on themselves and cancel out.
Anything but a completely straight section of large diameter tubing will act as a restriction in your exhaust system. This includes bends in the tubing, catalytic converters and mufflers.
Exhaust restrictions increase pressure in the system, which keeps used gases trapped inside of the engine's combustion chambers. These gases take up space that should be occupied by fresh air and fuel. The end result is a loss of engine performance and fuel economy. Removing the muffler or replacing it with a higher-flowing unit will reduce system pressure and increase performance.
