i bought my car with a stg 2 setup. when i got it, it was geting about 13-14psi but it was pulling real hard and my best time was a 14.2

my question is... if whoever owned this car before got a custom tune, how could i tell? (dumb question) and if i change the pulley to a 2.8 will it mess with the tune???

 

You would have to get someone to look at the tune with HPtuners to tell. And yes, slapping on a smaller pulley will mess with the tune. If it is a stock stage 2 tune, you should be fine with putting on a 2.9" pulley without retuning, but nothing smaller.