pros
more power
more torque
improved mpg
smoother rev band
cons
void warranty
insurance increase
decrease engine longevity (debatable, but some tuners will say decrease lifetime of 3-5%...but you can't measure that really in real life, and even so 3-5% is pretty negligable)
decrease transmission longevity (especially if with a big power gain)
decreased mpg, if using the extra power a lot
Under cons, I'd also add the increasing risk of overfuelling and unburnt petrol going through the system, and increased smokey / sooty emissions. If you look at the inside of the backbox of a aggressively mapped N/A car, it will be much sootier than usual.
Under cons, I'd also add the increasing risk of overfuelling and unburnt petrol going through the system, and increased smokey / sooty emissions. If you look at the inside of the backbox of a aggressively mapped N/A car, it will be much sootier than usual.