Good to see you're having fun!
With your cash I'd invest it in something that'll last a lifetime: Driver training.
Suggest an advanced driving course to your dad, he'll not only be proud of you for being mature and realizing that teens are inherently crappy drivers, but also for wanting to improve yourself before trying to make your car faster (A.K.A. responsible). I don't know of any in your neck of the woods offhand, but I know Team Oneill does a great job up north!
But welcome to the "my car is cheap, parts are cheap, and it's a tank" club!