Strange & severe vibration from muffler brackets?

[Duc Hunter]

With my old exhaust (B&B headers & Magna-flow muffler) the car had no brace from the muffler to the engine console when I bought the car. I welded a bracket on the muffler, and it had a mounting plate to bolt it to the engine console. When I did this there was suddenly a vibration between 2k-3k RPM, and the vibration was much stronger on deceleration. Obviously it's also stronger in lower gears. The vibration is slightly high frequency like a buzz or harmonic, flows through he whole car and is very noticeable. As a result I removed the brace, and it went away. Fast forward to putting new headers and a GT3 muffler on, vibration is back.

The car is a ’75 911S w/964 engine, instead of a harmonic balancer it has a Rothsport single pulley kit. It also has a Patrick Motorsports light weight flywheel and Sachs clutch. It’s on a DME and a Steve Wong chip.

I thought of just taking the new brace off, but the new headers are slip jointed so I didn’t know if that was ok. Does anyone have any idea why it would vibrate like that? Any thoughts on a cure? I did some searching and have not found any talking about this issue?

[brianlay]

I’m sure you checked but maybe the system is touching the body somewhere?
Does it happen revving the engine while stationary or does it have to be under load?

[Tea Tray]

SWAG - maybe the issue described here?
Kinda interesting, last night I was thumbing through my Parts and Technical Reference Catalog (12/1995) and found this tip!

[spuggy]

Originally Posted by Duc Hunter

I thought of just taking the new brace off, but the new headers are slip jointed

Used to have GHL headers with slip joints on my 930. For a few seconds after starting when it was cold out, I'd hear the unmistakable raspy sound of open exhaust. Then the gaps would seal - sometimes with a little "pffft" noise.

I sometimes thought about how much hotter (and bigger) the headers would be after 20 minutes running. Or when they were that pretty orange color under boost. It never seemed to cause a problem.

I wonder if your design expands/wants to move more - but the brace prevents it somehow?

Just thinking out loud with too little coffee, so feel free to ignore But I think I'd lean towards disconnecting the brace to see if it changes the behavior and go from there.