Engine Vibration - What is causing it?

[mikedsilva]

During my quest to rebuild a 3.0 motor, I have started trying various mufflers.
Here is the situation.. 3.0 single plug motor, using 10.2 CP pistons, webcam 464/465 cams, EFI from rasant, and throttle bodies from RHD Engineering. Headers are 1 5/8 from GT- Racing.

First muffler was the M&K GT3. The engine runs smooth everywhere with this muffler. It's loud as hell, but smooth. I soon found that I could realise a lot more power with open pipes so I tried a different muffler set up.

Second muffler was a Dynomax 17552. This one has an internal x-pipe. I had a muffler shop install it and mate it up to the headers. Sounded good, had a bit more flow, but the first drive exhibited a vibration between 2000 and 3000. A strange vibration that didnt seem to be dependent or gear.. just rpm. When I say vibration, it's like sitting on top of a washing machine that is severely unbalanced and trying to buck you off. The whole car vibrated and the windows and door trims rattled etc. Also, the first drive home from the muffler shop, resulted in a bunch of fibraglass packing hanging out one of the tips.

Quite a bit huh? And that amount of packing should not have been able to get inside a tube unless it was compromised.. I decided to open up the muffler and found that the cross over was not very 'complete' in the welding. So I had a local shop weld it up more completely and repack, close and re mount.