swap engine

It’s so you can get more power out of a car. For example, the BMW e30 m3 we get at the start it pretty slow even when fully upgraded but, if you throw a V8 in it than you get way more power. Each engine also changes the characteristics of a car. 

People do this in real life -  BMW E46 with LS V8 engines or a 1968 mustang with a 4.3 litre Ferrari V8. 

I think this addition to the game is great because why shouldn’t we be able to put a Ferrari engine in an e92 m3? I think it adds another element of customisation because you can literally have your car the way you want it. You want to leave it with a stock engine? You can do that. You want to throw a bmw i8 engine in there? You can do that. 

The game never forces to do an engine swap.