Is there a way to turn off the upgrades on AI cars? I remember seeing it somewhere but i cant find it nemore. Maybe im thinking of another forza game, but its pretty useful if you want to race your lower pp cars.
You could force that in FM4 but I don’t think there is a way to do so in FM5.
I loved racing stock vs stock - found it very interesting particularly to take a car that was lower in PI than the rest of the field.
Shame there is no way to force this in FM5 
This goes back to how I want free play to have the same options as setting up a multiplayer race. In multiplayer we have a boat-load of options and forcing cars to be stock was one of them.
In career no but I think you can in freeplay
I cant find it after being away from the game for a while, but i could of swore it was there. I triple checked basically everywhere n cant find the option for single player. Im probly thinkin of forza 4, which i rember i used to love racing against a custom grid.
Try online in private lobbies. One of the far right options is to disable upgrades and tuning.
Can’t be done in career.