It depends on what you are starting with and what your motor setup is. For most typical street setups where you are running a modified engine made to be run on the street, your best performance option is usually going to be a 2 into 1 exhaust system. These systems help balance out the pressure on V-Twin applications producing the right amount of back pressure your engine needs to perform. If you are running a heavily modified engine made for racing, then a straight pipe exhaust system may be a good option. Typically, a straight pipe exhaust system will add horsepower over your average stock exhaust system.