Got it.
Just make a copy of your whole cardynamics folder and leave that alone on your pc hard drive for a backup and to use as the default settings for the car physics.
on the xbox hard drive Just open the physics you want
example stingray.ini
edit /select all /copy
then scroll down and open the one you want to change
example mini.ini
edit /select all/ paste
Now the mini cooper is handeling like a stingray but the sounds are still for the mini cooper nothing like challenging a friend in split screen and using a mini cooper with a ferrari under the hood.
to return it back just open the mini.ini on your pc hard drive that is a backup and untouched and
edit /select all/ copy
then open the min.ini on the xbox
edit /select all/ paste.
!!! Has Not Been Tested With Sytem Link, Xbox Connect Etc !!! single system split screen is fine since the modified file is there for all players but unless 2 xboxes are modified with the same physics i don't know the turn out just yet.