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Moon Gravity, Unlimited Turbo and a Hint.
For Moon gravity:
Press Left, Right, Up, Down, B, B, B at the main menu. The lower left corner of the screen will flash to confirm correct code entry.
Unlimited turbo:
Press Down, Down, Down, L, R, Z at the main menu. The lower left corner of the screen will flash to confirm correct code entry.
To get all bikes complete hardcore in career mode.
No Boundaries
At the main menu, enter
L,R,Z,L,R,Z
It should give you no boundaries, enter it again to turn it off