To unlock these cars, you have getting certain positons tournament and collect all flames on each mode to unlock them.

Car	How to unlock
16 Angels	Come Third Or Higher On Bowling Alley Tournament In Turbo Mode.
Battle Spec	Come Second Or Higher On Attic Tournament In Nitro Mode.
Bone Shaker	Come Second Or Higher On Mini Golf Tournament In Nitro Mode.
Da Kar	Collect All Flames In Nitro Mode.
Dieselboy	Come First On Mini Golf Tournament In Inferno Mode.
Hollowback	Come First On Bedroom Tournament In Inferno Mode.
Hot Wheels Drift King 24/Seven	Come Third Or Higher On Bedroom Tournament In Turbo Mode.
Muscle Tone	Come Second Or Higher On Bedroom Tournament In Nitro Mode.
Nitro Door Slammer	Come Second Or Higher On Bowling Alley Tournament In Nitro Mode.
Phantom Racer	Come Third Or Higher On Attic Tournament In Turbo Mode.
Power Bomb	Come First On Bowling Alley Tournament In Inferno Mode.
Shadow MKII	Collect All Flames In Every Mode.
Speed Bump	Collect All Flames In Inferno Mode.
Spine Buster	Come First On Attic Tournament In Inferno Mode.
Super Tsunami	Collect All Flames In Turbo Mode.
The Demon	Come Third Or Higher On Mini Golf Tournament In Turbo Mode.