I just saw this post. Thanks for sharing all that. Great post. I will save the link to refer to people in the future.

Drone reduced
 

With all of the things I have done to my Z the exhaust system which so sweet at first had become a drone monster. From 2K-3K rpm, the exhaust would drone at highway speeds and get really loud during regular driving. I did some searching on the forum and saw an article about adding Helmholtz resonators to reduce the 130-160Hz noise by pressure wave cancellation. Well, I added them in and they DO REDUCE the drone.

They are quite simple in design but a bear to install. The fab shop I use came up with a solution that was way easier than T-ing off the exhaust tip area. The system clears everything under the car and the drone has been reduced by at least 50%.

The resonators are 28 inches long and 2.25 inches in diameter. All 304 stainless.