Go to the extras screen from the main menu and enter codes to unlock the following.

Effect	Code
AAA Shell	Chet, Skidmark, Smooth Move, Skidmark.
Firestone Shell	Turbo, Whiplash, Turbo, White Shadow.
New Shell	Turbo, Chet, White Shadow, Turbo.
New Shell	Smooth Move, White Shadow, Smooth Move, Turbo.
New Shell	Smooth Move, Turbo, Burn, Smooth Move.
New Shell	Whiplash, Skidmark, Smooth Move, Whiplash.
New Shell	Skidmark, Whiplash, Turbo, Skidmark.
Reverse Mode	White Shadow, Turbo, Smooth Move, White Shadow.
Turbos Original Racing Shell	Burn, Smooth Move, Turbo, Burn.
Unlock Skidmark	Skidmark, Whiplash, Turbo, Burn.