Maybe a stupid question, but can I add any upgrades to the cars I use for spec races?

I've been doing a lot of Touring series and have started to notice a lot of other players have a lot higher PI than I do. I've been even pretty successful using the 2018 Audi Sport RS 3 LMS even though I'm often the lowest ranked PI in the race. It doesn't seem to explain the rules well. It just says "fine-tuned touring cars from around the world blah blah" I don't wanna spend the time upgrading and tuning just to find out I can't use it.