Cars have always been subject to after-market modification. The golden age of car tuning was the decades between World War II and the beginning of air pollution restrictions. Both moderate and radical modification have been commemorated in the popular songs Hot Rod Race and Hot Rod Lincoln. The names of Abarth and Cooper appear on models styled after the cars they modified. Cosworth went, with support from Ford, from modifying English Ford Flathead engines for Lotus Sevens to dominating Formula One.
In the 1970s and 80s, many Japanese performance cars were never exported outside the Japanese domestic market. In the late 1980s and early 1990s, grey imports of Japanese performance cars, such as the Nissan Skyline, began to be privately imported into Western Europe and North America. In the United States, this was in direct contrast to the domestic car production around the same time, where there was a very small performance aftermarket for domestic compact and economy cars; the focus was instead on sporty cars such as the Ford Mustang and Chevrolet Corvette, or on classic muscle cars.
Because of their light weight and the increasing availability of low-cost tuning equipment, economy and compact cars exhibit high performance at a low cost in comparison to dedicated sports cars. As professional sporting and racing with such vehicles increased, so did recreational use of these vehicles. Drivers with little or no automotive, mechanical, or racing experience would modify their vehicles to emulate the more impressive versions of racing vehicles, with mixed results.
The essence of modification of a tuner car is an attempt to extract the greatest possible performance—or the appearance of high performance—from the base 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 the power output, additional changes are often required to allow the vehicle to handle this power, including stiffened suspension, widened tires, better brakes, improved steering and transmission modifications such as the installation of a short shifter. Although largely invisible from outside the vehicle, 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.
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-blown customization based around the audio equipment. Events are held where entrants compete for the loudest, highest quality reception or most innovative sound systems. Some common modifications include higher quality speakers and sub woofers, amps, a better wiring system, etc.
All cars competing in each class 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 that allows 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 original equipment 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 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, Mass air flow/ Volume air flow, etc. Modern engines employ the use of an ECM to provide the best balance between performance and emissions. Via the OBD communications protocol, the electronically controlled aspects of the engine can be modified in the process known as 'mapping'. Mapping can either be performed by changing the software within the ECU (chip tuning via firmware modification), or by providing false data via plug-in hardware. Mechanical components can also be replaced, such as turbochargers or superchargers.
Other standalone engine management systems are available. These systems replace the factory computer with one that is user programmable.
Improper, incorrect and poorly executed engine modifications can have a detrimental effect on performance. Mechanical and electrical components will suffer or simply fail as a result. An example would be the use of an air compressor such as a turbocharger to increase the volume of air used in power stroke of the otto cycle. In a typical chemical reaction, the air-fuel ratio must be a minimum of 14:1 (see Stoichiometry). If higher ratios are used, higher pressures and temperatures are observed in the cylinders, which can quickly push an engine beyond its intended design limits. Neglecting such operating parameters can lead to premature failure, such as warped cylinder heads and walls (temperature related), disintegrated piston rings, cracked or bent connecting rods and crankshafts (excessive amount of torque applied), total cooling system failure, engine fire, engine detonation, engine seizing, and even blowouts. This can all lead to very expensive repairs, as well as being very dangerous.
Suspension tuning involves modifying the springs, shock absorbers, swaybars, and other related components of a vehicle. Shorter springs offer greater stiffness and a lower center of gravity at the possible cost of unwanted changes of suspension geometry. Stiffer shock absorbers improve the dynamic weight shifting during cornering and normally have shorter internals to stop them from bottoming out when shorter springs are used. Stiffer sway bars reduce body roll during cornering, thus improving the grip that the tires have on the surface by reducing suspension geometry changes caused by roll; this also improves handling response due to faster weight shifting (similar to stiffer springs.) The danger with overly stiff swaybars is the lifing of the inner wheel, which changes traction of that end at a discontinuous rate. By increasing the roll resistance of on end, weight transfer is concentrated at that end, causing it to slip more than the other. This effect is used to control the over/under steer characteristic as well as to reduce roll. Other components that are sometimes added are strut bars, which improve the body stiffness and help better maintain the proper suspension geometry during cornering. On some cars certain braces, anti-roll bars, etc., can be retrofitted to base model cars from sports models.
For offroad vehicles, the emphasis is on lengthening the suspension travel and installing larger tires. Larger tires, with or without larger wheels, increase ground clearance, ride over short obstacles and holes more smoothly, provide more cushioning and decrease ground loading which is important on soft surfaces.
These suspension modifications are in contrast to Lowriders with hydraulic or pneumatic suspensions. Lowriders use another type of suspension tuning in which the height of each individual wheel can be rapidly adjusted by a system of rams which, in some cases, makes it possible to "bounce" the wheels completely clear of the ground.
Body tuning involves adding or modifying spoilers and a body kit in order to improve not just the physical looks of the car, but most importantly, the aerodynamic performance of a vehicle. Through the generation of downforce, cornering speeds and tire adhesion can be improved, often at the expense of increased drag. To lighten the vehicle, bodywork components such as hoods and rear view mirrors may be replaced with lighter weight components.
Often, body modifications are done mainly to improve a vehicle's appearance, as in the case of non-functioning scoops, wide arches or other aesthetic modification. Aftermarket spoilers or body kits rarely improve a car's performance. The majority, in fact, add weight and increase the drag coefficient of the vehicle, thus reducing its overall performance.
Increasing the wheel track width through spacers and wide body kits enhance the cars cornering ability. Lowering the center of gravity via suspension modifications is another aim of body tuning. Often, suspension tuners unfamiliar with spring dynamics will cut stock springs, producing a harder, bouncy ride. It is also common to lower the car too far, beyond the optimal height for performance, purely for appearance.
Competition cars may have light weight windows, or the windows may be completely removed, as auto glass adds significant weight high up. Plastic windows are much more vulnerable to scratches which reduces service life.
Tires have large effects on a car's behavior and are replaced periodically, therefore tire selection is a very cost-effective way to personalize an automobile. Choices include tires for various weather and road conditions, different sizes and various compromises between cost, grip, service life, rolling resistance, handling and ride comfort.
Detuning is returning a modified car to its original factory status or reducing its performance in a particular area of tuning. For example, a car may be "detuned" to allow increased traction where the track grip is not sufficient to handle the increased power of the tuned engine.
Modified cars can be significantly different from their stock counterparts. A common factor among owners/modifiers is to emulate the visual and/or performance characteristics of established styles and design principles. Sometimes these similarities are unintentional. Some of the many different styles and visual influences to car modification are:Rat rod /Rat style: Style of hot rod and custom cars, imitating the "unfinished" appearance of some hot rods in the 40s, 50s and 60s. Rat style also defines a car that is kept on the road no matter what,and at a low cost
Euro style: One off paint/small wheels and lowered or stance'd with shaved features to define car body lines.
Lowrider: Hydraulic setups, flashy paint, custom interior, bling wheels. Others may look like straight restorations, aside from a low stance.
Import scene /JDM: Japanese Scene that uses Japanese vehicles, aftermarket parts and Race details.
DUB/DONK: Extremely large designer wheels, loud speaker set-ups and abnormally high ride height.
SLAB: Originated in the Houston, TX area since the mid-1980s - usually a full-size luxury car (primarily a Cadillac, Buick, Oldsmobile, or Lincoln inclusive with several lowered-tier brands e.g. Chevrolet, Ford, Chrysler; in recent years some malaise era GM front wheel drive luxury sedans and imports e.g. Mercedes-Benz, Jaguar, Lexus are included), custom wire wheels (primarily the discontinued Cragar Star Wire originally optioned for the 1983 and 1984 Cadillac Eldorado now manufactured by Texan Wire Wheels) fitted with Vogue Tyres (known as 83s, 84s, Swangaz, Elbows), loud speaker set-ups (same as the DUB/DONK), neon signage inside the trunk panel, hydraulic-actuated trunk panels (known as a pop trunk), flashy paint (several coats of clearcoat on a paint job known as candy paint (named after the Jolly Rancher or Now & Later candy products), vertical stainless steel trim on the trunk panel (known as belt buckles) aftermarket grille (similar to a Rolls-Royce or Bentley as homage to the 1970s-era pimpmobiles), and the use of a Cadillac front end sheetmetal conversion (from the use of 1990-92 Brougham sheetmetal or if a large truck or SUV, the front sheetmetal from an Escalade). Vehicle interior usually clad in beige or tan (known as peanut butter interior), and a Grant GT steering wheel clad in woodgrain (known as a Wood Grain Wheel). Usually connotated with Houston Hip Hop music.
Bōsōzoku/zokusha: This Japanese motorbike style features additional fairings and exaggerated exhausts.
VIP style: Japanese style that evolved from Bōsōzoku that emulates the Extreme style with a more modern vibe, e.g. ridiculously low ride heights, camber, body kits, lights.
Stance (vehicle): – This style is mostly associated with lowered sport cars, sedans, hatchbacks, vans and other body styles of passenger cars. Such cars are lowered with a help of sport springs, coilovers or air suspension components. Custom wheels with low profile tires (sometimes stretched) play a big role in this style and often feature aggressive sizes, offsets and camber.
Cal Look: Classic beach look (bright colours and polished) and subtle features added to give a Cali sense.
Military/service style: Cars designed to look like certain service vehicles (e.g. Military, Navy, Police).
Hot rod style largely consists of period specific vehicles, components and finishes to reproduce characteristics of early Drag-rods from the 1930s and 1940s.
Custom cars : Cars built for the showroom floor can be any manner of car
Sleeper (car): This is where a car owner will put every effort into performance and try to keep the car looking stock, usually to avoid raising suspicion.
Rallying: Cars built to be driven on dirt.
Drifting (motorsport): Cars engineered to drift (skid in controlled manner).
Drag racing: Cars engineered for straight line speed.
South London Look: Subtly modified 50's-70's British Fords that are lowered, pastel paint, 13 inch Lotus Cortina steels or RS, Minilite, Revolution mag wheels. Often running a tuned Ford Kent, Pinto engine.
German Look: A VW Type 1, Type 3 or Karmann Ghia lowered and fitted with late model Porsche mag wheels and touring car influenced styling. Heavily modified suspension and drivetrain with emphasis on handling and cornering.
Many countries or municipalities have legal requirements which govern vehicle modifications. For example, all vehicles in Victoria, Australia, must conform to construction standards to ensure vehicle safety. There are also restrictions for P Plate drivers which can prevent young drivers from driving modified vehicles.
Many developed countries have "smog rules", which generally forbid any modifications to engines or related components, unless the modifications themselves are type smog certified, like production car models. Such modifications, even if they do not actually result in increased emissions, prevent legal use on public roads.
Many organizations involved in competitive motorsports establish safety guidelines that far exceed legal requirements placed on street legal vehicles. The NHRA, IHRA and SOLO programs all require that vehicles pass inspection to ensure that all regulations are being complied with.