You just hit the basic limitations of BSP brushes.
BSP brushes are not meant to model game objects with. They are supposed to be used as blocking elements to roughly set up your level in terms of scale.
While BSPs are flexible enough to play around with room sizes, wall heigths, etc, that flexibilikty comes with a price.
BSPs are not really performant. Once you reach a decent number of them in your level (1000+), you will start seing incredible lag.
They require much more system resources than a real static mesh.
Im afraid there is no way around of making your car model (and all other game assets for that matter) in a real 3D application.
If you want to use a free software, then there is Blender for you.
If you want to go for a more professional solution and you find the Autodesk rate plans as usurious as I, then there is also MODO, etc…
So, BSPs are good enough as they are, if used for what they are for.