1986 Pontiac Fiero Huge vibration noise

I'm having a big vibration trouble when i press the gaz around 2500-2600 rpm and 3800-4000 the vibrations are loud from the interior but i don't know if people outside can hear it. and on the idle the entire carcshake , as if i have a hug v8 motor but it's a v6 ( 3.1reforged ) , someone told me it could be a exhaust vibration but it is really loud so i don't think it can only be an exhaust vibration.. and looks liek it's affecting my performance i have around 185 hp ( new cam and headers ) but when i press the gaz it's like if i had only 130-135 .. thanks for helping me