In real life, there are many different kinds of engines. They all sound different, depending on their construction, operation, and even how the intake and exhaust are routed. While there are general "base" noises each kind of engine makes, the V8 engine is the most unique in sound. Because of the lore of V8 engines, or maybe because they are a little behind the times, The Coconut Effect comes into play, and oftentimes film makers overdub the sound of a mundane car so it sounds like it has a V8. This, somehow, is more realistic for viewers who know nothing about cars.