Originally Posted by shadoquad

A tune is actually a great mod. Your vehicle's engine is controlled by a computer, the ECU. A "tune" gives the ECU a newer image which opens the engine up for more power and quicker response. A stock ECU tune is geared for some performance, but also for engine efficiency and reliability. An aftermarket tune can improve performance.

Usually, a reputable garage performs dyno tests to calculate hp gains vs. stock.

They used to refer to this as "chipping" a car as well, because unless I'm mistaken, one used to literally replace a chip in the vehicle. Now it's all software through the OBD-II, if I'm correct.