What are the rules? Is the car only allowed to roll down the hill, or is the power source from the car only allowed to come from gravity?
If it's the latter, I'd use regenerative braking to charge a capacitor, then reverse the power on corner exit to accelerate.
...or just use a mini-blackhole as an electric power source and run down the hill in a 1000 hp electric rally car. "Powered by gravity"
