Tuning on FORZA MOTORSPORT 7

Hello everyone.
I would like information on the possibility of upgrading cars in FM7. When I try to tune my cars, they come out of the race specifications, thus making tuning impossible as well as useless.ù
For this reason I would like to know:

  1. when it is possible to perform tuning on FM7 cars
  2. If it actually serves to complete the game (I would not like the tuning component to stop only in the ‘test car’ mode)
  3. are there races where the tuning, in short, is necessary ???

thx all for answer