Enter the following cheats at the "Code Entry" screen under the Secrets option. Note: Each game disc has one of a few random "access codes"--only cheats that are made for the access code that matches your own disc's access code will work for you.

Access code 8426

NYOOTWCF - All cars and championships except 3 bonus Championships
NYOOTWCF - All cars and 3 bonus Championships.
LLNWUGXT - Double engine power for all cars (XBOX) or to increase the overall speed of your car (PC)
DBXLYJUU - Swap Front wheel drive cars to rear wheel drive.
GFNPIDBY - No car damage.
WVUMRQMJ - Unlock all cut scenes.