Modifying a stock exhaust is a popular option when you want your car to emit a deeper, richer, more throaty exhaust note. This can be done through a number of methods that will preserve the stock look of the exhaust. You can modify the muffler itself, with varying levels of external and internal modification, modify the exhaust tubing leading to the muffler or modify the engine itself.
Drill a hole through the internal baffles inside the muffler so there is a clear path from the front to the end of the muffler. A muffler normally "muffles" the engine note by forcing the sound waves to pass around a set of baffles. By making a hole in the baffles, the sound waves can now propagate through the baffles and lose less kinetic energy inside the muffler. This can be done with a rotary hammer (high powered drill) and a metal drill (do not use a wood drill as it will unduly dull the tip) while the muffler is still attached to the car. Be careful to keep the line straight, as the more off centre the new holes are, the less effective the modification will be.
Perforate the external casing of the muffler. Drilling multiple holes on the outside of the muffler produces many more paths for sound waves to take, leaving that much less baffling between the sound and the outside. This modification requires hydraulic lifts to lift the car and a rotary hammer to perforate the muffler casing. Holes should be made in a regular pattern, with a thumb's space between each hole to preserve structural integrity.
Bypass the muffler entirely by installing a bypass vent. This will entail installing a Y-branch in the exhaust tubing leading from the engine to the muffler. One branch will go to the muffler while the other will be open ended. This will allow some of the exhaust, and sound, to exit the car without going through the muffler. This is undetectable on the outside, but the exhaust noise will obviously come from a modified exhaust system. The car will need to be lifted on hydraulic lifts, and a Sawzall should be used to take out tubing. The Y-branch has to be attached with welding, as the under body is too exposed to use O-ring clamps.
Reprogram (commonly referred to as flashing) the Engine Control Unit (ECU) of the car, the electronic device that controls how the engine runs. Determine the make, model and power train of your car. Then buy an ECU flasher, which is a cell-phone-sized module that reprograms the ECU, for your car. Many will plug into the diagnostic port and do a one-time installation of the upgraded software. This software will then tell your car's engine to run better, giving it greater horsepower and torque in addition to making the exhaust sound deeper due to richer fuel-air mixtures. No tools are required for this modification.