Illegal Cars

Protip:

When buying a car from those recommended for a series, the game automatically upgrades it to the top of the allowable class, which means any additional upgrading will almost certainly increase the PI above the maximum allowable. However, you don't have
to buy the car automatically-upgraded. Just see what is eligible, and then back out to the main menu and buy that car from there instead.

Don't know of any illegal car parts that you can put in a car, unless you have stolen them or they do not comply with government regulations due to emission's.

if you like the car then you should, it could be a great looking car but just not fast enough, so if the opportunity comes along to have a bigger engine then why not?

Well, it was nice knowing you, Randver.

Everyone seems to love car analogies around here, so here's mine: just because you can put illegal parts into a car to make it better or go faster, doesn't mean that you should.

ok I'm playing forza motorsports 7, racing the rivals, for instance I was doing a race and it would only let me build a car to let say 500hp and sport tire. I set a lap time of 2:01. Couldnt get no faster time. Go out and go to the leaderboards and the number
1 guy has set a laptime of 1:30. It gives you the option of downloading his tune. Only that his tune has 1100hp. But tell me its illegal, you can upgrade you car to 1000hp and try to enter the race. What does it tell me, due to your cars upgrade, it is illegal
for the race. How are they able to race and I cant

Maybe you tried to buy an illegal car. Lot of guys mod this game. It's stupid, but MS will sort it out. They can tell who did or not do something intentionally. Then again, maybe a hub is down in your area and the servers are clogged.