My car (95 Firebird Formula) has a really weird deep vibration between 75-85 mph. It's not like anything I've felt before... feels like it's something in the drivetrain, but the alignment of the car is very good and stays straight when you let go of the wheel. The car has a similar vibration feel at times when cruising at a steady speed of around 40-50 mph, but not as deep and harsh feeling as around the higher speed. I am in the process of a clutch swap and was wondering if anyone has any suggestions for what I could check into while the drivetrain is apart. I'm going to change out the pilot bushing/bearing (whatever comes with the spec stage 2) and I took the driveshaft to be looked at at a local drivetrain place, told them to see if it was in balance and to change the u-joints if they thought it needed it (felt fine to me, no play in them). The rearend in the car is stock as far as i know (3.42s). Any suggestions anyone may have would be very appreciated, because the car feels like its going to drop something out the bottom cruising on the highway. Thanks in advance.

 

i don't know if this will help, but a friend of mine had the same problem with his Camaro at highway speed (70-80mph). he ended up swappin the stock driveshaft for an aluminum one, and he didnt get the vibration anymore. but then he blew a synchro in his tranny and ended up swappin it with a WC T5 a few weeks ago, then the vibration was back again. he came to the conclusion it was something with the alignment between the tranny and the driveshaft, since it's EXACTLY the same vibration, there was no vibration before the tranny swap. he figures it went away when he changed the driveshaft, but then after changing the tranny when he put the driveshaft back on it wasnt aligned right again. it's unlikely that changing the driveshaft fixed the problem and then changing the tranny brought it back. he drove it for about 2 years without any problems before the tranny swap. haven't had a chance to test his theory though cuz its gettin real cold now and no garage to work in. did you have the tranny or the driveshaft off before the vibration started?

 

LT1s came with a crappy (by comparison) steel driveshaft. they are VERY well known for having vibration at highway speeds.

your choices are to have it balanced, buy a used 1le or ls1 or buy an aftermarket shaft.