Your save files are typically stored in Documents\NFS Most Wanted .
Scroll past "Ultimate" to the very end of the list to find the option. nfs+mw+junkman+editor
There is a known myth in the community: installing more than 9 Junkman Engine parts will permanently corrupt your save file. This is . The game’s floating-point physics variable has a signed 16-bit limit. Exceeding 2,100 hp breaks the torque calculation, resulting in negative acceleration (your car flings backwards at lightspeed). Your save files are typically stored in Documents\NFS
One a key contributor mw editor put in last 100 hp breaks the torque calculation