In drag racing if you tap the nitrous button instead of holding the button down your car will accelerate quickly, but in a manner you can find easier to control.
Select the car and proper race level until you're goiing up against the boss car you want to unlock. Then beat the car at the PRO-level Boss Races. You'll find them available in your pocket garage.
Select the car and proper race level until you're goiing up against the boss car you want to unlock. Then beat the car at the MASTER-level Boss Races. You'll find them available in your pocket garage.
To unlock the Nissan Skyline R34 GTR, race against the Boss Car using an American car in a Trophy-level Boss Race.
To unlock the Subaru Impreza WRX STi, race against the Boss Car using an American car in a Cup-level Boss Race.