A person that overcomes the factory limitations of their vehcile to gain more power, lower times at races, and/or better handling.
When emmissions standards were introduced, many tuners gave up, but others learned about
HC, CO,
NOx,
CO2 and were able to improve those numbers.
When electronic controls were introduced, many other tuners gave up. but others learned the way they worked, and were able to make performance gains in their cars.
What stands in the way of tuners now, is quite frequently, illogical laws that ban tuning outright, despite the fact that a well tuned engine can have lower HC, CO, NOx, CO2 and fuel consumption than one in factory specification. The same goes for
swaps, even though the emmissions of a
S15 SR20DET are less than a 1989 KA24E, many areas have laws that declare the Silvia Conversion illegal anyway. If you think that
Japanese cars are the only ones affected, think again. Many Chevy guys have fitted
LS1/
LS6 and
LT1/
LT4 engines to older cars, bringing lower emmissions and fuel consumption in the process. European imports are also affected, as a popular swap is to take a 1.8 turbocharged engine from a VW
MKIV chassis, and fit it to a
MKII Golf or
GTI. Still, many areas have laws written by people who don't know any automotive physics.
Example:
Any car with altered factory parts, fabricated parts, or specialty aftermarket parts is considered a
tuner car.
Any driver who builds or installs those parts is a tuner. Tuners also adjust things like the mixture screws on a
carb, or the programming of
EFI.