You did nothing wrong. The sounds folder will only allow for a single engine noise across all mods, thus why everything sounds like a 4-cyl engine. If you have backed up the original sounds, I would recommend making a new folder for the 4-cyl sounds named "Sound - 4-Cyl" (or something similar) while retaining the original Sound folder as well. That way when you want to use a different soundpack, you can rename the folders (ie, the original sounds folder "Sound - Original" while the 4-cyl folder is then renamed "Sound") every time you want to switch to a different soundpack. It might be mildly inconvenient at first, but it will save so much time and is much more convenient than having to delete and then reinstall the 4-cyl sounds.