6503 Thunderbird SC 3.8 supercharged engines can increase the blower boost to 15lbs., that is a 5psi increase. The pulley is 2.81″ in diameter. The engine will pick 25-30 hp and 40-45 ft. lbs. of torque on a stock application simply by changing to this overdrive pulley and belt kit. Easy to install and one of the best “bang for the buck” items sold.