Forced Induction to Naturally Aspirated cars

"These swaps are accomplished by performing an aspiration conversion, then in the engine modification screen choosing the higher grade part (such as "Race") for the newly-converted induction style. Once equipped, simply "sell" the part and you'll be left with a naturally-aspirated engine in place of forced-induction.

The following is a list of the cars I have found which have the ability to perform this swap. If the engine of a given car can be swapped into other models, the FI to NA swap usually applies as well"

http://notsoepicepic.blogspot.ca/2012/03/forza-4-fi-to-na-swap-list.html