I'll give it a 3 because it is a very good attempt and it does sound decent... if it was a 6 cylinder engine that is. Save for maybe the idle (I really like the idle) that's why 3 stars got a great V8 sounding idle(might also work as an old 20's and older v/inline 4-6 idle as well). Past idle though if you made a 6 cylinder for the car and used the sound. I'd give at least 4 stars.
Still I like the mod I like at least the effort to actually put some new sounds for default engines without replacing the engines them selves, but instead making clones that sound different.(That's reason I'm not rating below a 3 on this).
I'd say put some work into the engine sound above idle. A good example of a V8 engines going through the RPM range is in a game is Driver. Yeah an old game but the engines sound great(IMO).
Anyway keep working on this mod maybe use the way it sounds(past idle) as of this review for some non 8 cylinder engines.