Engine swap is a customization option you’ll gain access to as you advance REP levels. This swaps out the entire engine of the car for a better one, allowing you to tune any car to a much higher level than normal, making it so you can basically have most cars viable at higher levels with a swapped engine. To perform an engine swap go into the Performance menu for a car, then hit Triangle (PS4) / Y (Xbox One) to open the Swap Engine menu. Here you can choose between available engine models for your current car. Engine swaps aren’t cheap, so they aren’t something you typically want to do right away. They also come with pre-installed parts, but you can swap your parts from your current engine to the new one if you have better ones, you just have to swap them off the old engine first to put them into your inventory.