Changing how much tuning upgrades affect the performance?

Hi. I have been trying to figure out if and how its possible to alter how much each stage of tuning affects the vehicles. Im starting to think it cant be done. To clear this up, im not looking for the modifiers or boosters etc, I want to make original games upgrades have a bigger effect on the car. Can anyone confirm it cant be done or could someone lead me to the right files to do so? Thanks.