Code:
Buy a Camaro (or any car with multiple engine options really,
but there needs to be at least 3 including the stock one),
buy the second engine (the LS7 in the case of the Camaro),
add all of the parts to it,
go back to the engine selection, highlight the middle engine (the LS7)
and press the right trigger and the x button at the same time.
If you do it right you should notice that the C5.R engine is now selected,
and it's asking you if you want to sell the engine for 14,400 credits.
Press the a button and you will notice that you instead get 43,800 credits
and your engine is still in the car.
You can do this as many times as you like.
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