the appearance of high performance—from a stock motor vehicle through the addition, alteration or outright replacement of parts. Although this largely involves modifying the engine and management systems of the vehicle to increase power output, additional changes are often required to allow the vehicle to handle such power, including stiffened suspension, widened tires, better brakes, and improved steering and transmission modifications (such as the installation of a short shifter). Although largely insignificant in terms of appearance, certain modifications such as low-profile tires, altered suspension, and the addition of spoilers can change the overall appearance of the car, as well as adding downforce to increase traction. Audio A stock audio system is one specified by the manufacturer when the vehicle was built in the factory. A custom audio installation can involve anything from the upgrade of the radio to a full customization based around specific audio equipment. Events are held where entrants compete for the loudest, highest-quality audio reception or most innovative sound systems. Some common modifications include higher quality speakers and subwoofers, amplifiers, and better wiring. Interior Race cars competing in various classes must adhere to a strict set of regulations. As in some well-known racing events, like NASCAR and NHRA, sanctioned events often require a minimum vehicle weight. In such cases, the interior is stripped, and the required weight is achieved by adding ballast, allowing precise control over weight distribution. Along with weight requirements, safety requirements are present. Requirements differ for different classes. Roll cages, fire extinguishers, reinforced bucket seats, seat harnesses, and the like are some of the required safety modifications. Roll cages may be difficult to install when the stock interior is present. Some tuners will have "gutted" interiors, or omit features that many ordinary drivers would find desirable or necessary, such as audio systems, air conditioning and soundproofing, in order to reduce vehicle weight. Engine tuning Engine tuning is the process of modifying the operating characteristics of an engine. In a typical engine set-up, there are various mechanical and electronic elements such as the intake manifold, spark plugs, and mass air flow sensor. Modern engines employ the use of an engine control unit to provide the best balance between performance and emissions. Via the OBD communications protocol, electronically controlled aspects of the engine can be modified in a process known as mapping. Mapping can either be performed by changing the software within the ECU (chip tuning