When you have a certain car you want to upgrade quickly, go to host your own event circuit or point to point Do one lap and your car's horsepower that you want upgraded. Choose to enter the event. Race and bet only if you can win. No matter where you finish, you will unlock a mod each time. Make sure you finish the race. Only do one lap, as it does not take as long. When you have your car fully done, race for pink slips.