Does anyone have any experience with polyurethane motor mounts in a LT1 car? A customer brought in a 95 Trans Am that has a significant vibration at 2500 rpm the motor seems to run ok, but it has poly mounts in it, and I was wondering how smooth these cars are with these mounts in normally. Any thoughts?

Welcome! I'm assuming this vibration isn't under load, correct? How about under load? Any mods like an underdrive pulley kit?

I've run poly in the motor and trans - maybe a little more high frequency buzz gets into the cabin but nothing that RPM specific - I'd look elsewhere. A couple thoughts:

The OEM damper rubber can fail and start to separate
has the clutch been changed recently? OEM LT1 is externally balanced with an off balance flywheel
check the bellhousing for cracks - the upper bolt bosses can crack off
check plug wire routing from opti to plugs - swap a pair of cyls and it will run but not right

Allan, can you be more specific about the clutch? I know it was changed right before this guy bought the car, what would be screwed up in there to cause a vibration?

Thanks, for the welcome! I've been lurking on here for years, but this is my first post. You're correct the vibration is with no load, car seems to run fine, it still vibrates when you're driving it but it doesn't seem as bad, it doesn't appear to have under drive pulleys, but the balancer doesn't appear stock, I can't find any tags or stampings on it, but I've never seen a factory balancer that has a removable outer ring held in place with large snap rings, but honestly I've never worked on a LT1 before.

I'd think you could quickly determine if you have any plug wires crossed with an IR thermometer. Start the car, and point it at the individual primary exhaust tubes. Any mis-firing cylinders would warm much slower than the others. I'd expect crossed wires to show up worse at idle than just about any off idle RPM.

Its definitely not misfiring, the motor runs fine it just has a vibration.

It's been 10yrs or so since I dealt with a LT1 clutch, but I recall bring the old and new flywheels to a balance shop near an airport, and having the new match balanced to the old. The LT1 is externally balanced from the factory using the flywheel. When a motor is built with aftermarket parts the internals are internally balanced, and the flywheel needs to be neutral balanced. That is about the extent of my LT1 memories. Google 'lt1 clutch balance' for more info

The damper also is part of the motor balance - it needs to be nuetral balanced

I remember having to take the flywheel/pressure plate/clutch disk in to get the whole assembly balanced. That was after surfacing the existing flywheel.

Well, Ichanged motor mounts and trans mount back to rubber,and fixed where one of the headers was hitting the frame rail,vibration is better but still there, I'm pretty sure its the clutch like your all saying, but the owner has decided not to pursue it any further right now,thanks for the help!

Well, if you really think it's the clutch I would fix it ASAP. It's probably already hurt the bearings if it's been driven hard though.