Select the 'Extras' option at the 'Main' menu and enter the following codes to unlock 
the corresponding effect.

AAA Shell:
Enter Chet, Skidmark, Smooth Move, Skidmark.

Firestone Shell:
Enter Turbo, Whiplash, Turbo, White Shadow.

New Shell:
Enter Turbo, Chet, White Shadow, Turbo.

New Shell:
Enter Smooth Move, White Shadow, Smooth Move, Turbo.

New Shell:
Enter Smooth Move, Turbo, Burn, Smooth Move.

New Shell:
Enter Whiplash, Skidmark, Smooth Move, Whiplash.

New Shell:
Enter Skidmark, Whiplash, Turbo, Skidmark.

Reverse Mode:
Enter White Shadow, Turbo, Smooth Move, White Shadow.

Turbos Original Racing Shell:
Enter Burn, Smooth Move, Turbo, Burn.

Unlock Skidmark:
Enter Skidmark, Whiplash, Turbo, Burn.