This trick requires some starting money, a private lobby on Xbox Live, and an online friend.

1.Purchase the 1969 Chevrolet Camaro Z28.
2.Purchase all engines for that Camaro.
3.Then, purchase every part for every engine, including all aspiration parts.
4.Exit to the main menu to save the engines and parts that were purchased.
5.Enter the engine purchase menu and sell every engine. Note: Do not leave the engine menu.
6.Press Xbox 360 logo and join your friend's private lobby.
7.Leave the lobby, then quickly press B to exit to the title screen, with the "Press A to Start" message.
8.Enter your engine purchase menu. You will still own all the previously purchased engines and parts.
9.Repeat steps 5 through 8 to get more engines that can be sold for unlimited money.