On my 72 I installed an LS3 480, with a Keisler RS500, and a rebuilt diff with 3:55 gears. I've got about 3k miles on it after about 15 months of driving. One thing I've noticed is that I've got a loud Thud when I shift gears, but only when I'm getting on it a bit. The sound is mechanical in nature, as if it's a gear getting slammed into place. There's no grinding, just a nice solid slam or thud. You can actually feel it in the seat when it happens. It happens after I've already shifted gears, and then when I release the clutch and get on it at the same time I hear the thud. I imagine/visualize it like the drive shaft expanding and contracting in length, creating the effect, but I don't think that's it. It doesn't FEEL like it's coming from the tranny.

It's been there from day one of the conversion, but I just thought it was a break in issue. I have no leaks under the car, and I put in all the correct fluids. I also drained the tranny and replaced the fluids around 1000 miles. I don't drive the car hard at all so I'm sure it's not from any kind of abuse. I have ZERO vibration from the drivetrain.

It's still there and maybe it's getting worse (not sure because I don't drive it enough).

What are your thoughts for what it could be?
What things should I be investigating/checking?

Thanks,

Chris

 

You might want to check the engine mounts. When you torque an engine it will thump back down.

 

Be sure to check the front diff mount, a loose one can cause this.

 

Are the motor mounts and trans mounts new?
Also, I've heard the TKO trans swaps can have the yoke close to the tunnel. Any chance that's hitting?

 

All new mounts everywhere.

 

Yep, a broken engine mount is a good possibility but it usually causes the fan to hit the shroud too.

 

Sounds like a wore snubber bushing to me....?

Brian

 

Rear of transmission lifting up and hitting the underside of the tunnel?

 

This one gets my vote.

And for what it is worth.....call around for a shop that has a chassis dyno that is hopefully above ground. This way...you can strap the car down and run it and be able to see it happens....OR....use an in-ground dyno and record what is happening with a video and review it.

With good money being spent on your conversion...what is a few more bucks to get it right.

DUB

 

check shifter linkages .. could be hitting tunnel

 

Don't over look the obvious, since all mounts have been replaced...might be bolts that have loosened up, maybe even missing. If you feel it in your seat sounds like transmission mount or front differential mount to me.

 

Sounds like I've got some good ideas to follow up on. More to come once I've completed my honey-do list.

Thanks

 

Unless you replaced your old clutch fan and shroud with a modern electric setup

 

When I read the first post...for some reason I assumed the THUD was when he was releasing the clutch. Which would..at least for me...would say that the front differential mount bushing has an issues that needs to be looked at.

DUB

 

Is the sound essentially the same in every gear? Does it only happen after a shift? If yes to both, I would be doubtful about the differential bushing. If you place it in first and drive at about 2500 rpm, does it do it if you get on and off the gas suddenly a few times ("buck" the car)? I would expect a bad diff bushing to show up like that as well.

 

Just for my info is a snubber bushing and a front differential bushing one in the same....?

Brian

 

Same Same

 

Snubber bushing bolt was lose. I didn't replace the bushing. Should I?

 

If it tightened up ok and squished the mount a little bit, then all should be well.

 

Is the noise gone?