A modern engine is controlled as much by software as by metal. The engine control unit, or ECU, decides how much fuel to inject, when to ignite it, how much boost a turbo should make, and how the car responds when the accelerator is pressed. Engine remapping changes that software so the engine works closer to its potential. Done well, a remap can make a car feel stronger, smoother, and more flexible. It can also help fuel economy, especially on turbocharged diesel and petrol