To get unlimited turbo, go the the title screen. Press R1+L1, R2+L2, R1+L1, R2+L2. When the code is entered correctly, you get a message confirming entry.
Unlock all Tracks:
To unlock all of the tracks in the game, go to the main menu and press L1, L1, L2, L2, R2, R2, R1, R1, then L1+R1+L2+R2. If the code is entered correctly, a message will appear.
Unlock Infinite Ammo:
To unlock infinite ammo, pause the game and press L2, R2, L1, R1, L2+R2, L1+R1. This will give you unlimited ammo. Enter the same code to turn it off.
Unlock Infinite Shields:
To unlock infinite shields, pause the game and press L1+R1, L2+R2, L1+L2, R1+R2. Enter the code again to turn it off.
Unlock Fast Oval Race:
To unlock the oval for a mega-speedy race, enter this code at the title screen: L1, L2, L1, R1, L1, R2, L1+R1, L2+R2. Enjoy the speed on the Oval.
Get Lots of Money:
To use this code, start a new career and pick your driver, then enter the following code: L1, L1, L1, L1, L1, L2, L2, L2, L2, L2, R1, R1, R1, R1, R1, R2, R2, R2, R2, R2, L1+R1+L2+R2.