A retuned engine is an internal combustion engine that has been modified to improve its performance, fuel efficiency or reduce emissions. This can involve adjusting various parameters such as ignition timing, compression ratio and fuel delivery system among others. The process of retuning involves extensive testing and fine-tuning of the engine's components in order to achieve optimal results. Retuned engines are commonly used in automotive industry where manufacturers strive to improve the performance of their vehicles while reducing environmental impact.