The short block engine in my 1994 GT 5-speed was swapped with a 1991 short block by the previous owner. I was told that the reason for the swap was that the engine was being built for a supercharger or a turbo. I was also told that the 1994 motors had cast cranks and weaker rods and pistons than the 1991 motors (1991 motors had forged cranks and better rods and pistons) and that was the reason for the swap. Now my problem, which I noticed before I traded for the car last week. I am experiencing quite a vibration in the motor. It is noticeable around 3000 rpm's and gets worse as the revs go up to 4000. At 4000 the vibration is severe and shakes the rearview mirror and dash. It then seems to get less and less as the revs approach 5000 rpm's. This is not a drive train vibration. The vibration is there when the car is parked and reved in neutral with the clutch pushed in. It is not any worse or gotten better since I owned the car and I want to fix it. Now my questions. I searched this site for engine vibration problems and found out that there were 28 ounce fly wheels and 50 ounce fly wheels. Could my vibration be caused by the previous owned using the 1994 fly wheel on the 1991 short block? Were the cast motors (1994) externally balanced different than the forged motors (1991) or were they all the same? What about the harmonic balancer, was that different also? Anything else I should look for?