For those experiencing an unwanted drone inside the car while cruising on the road between 50 and 70 mph (or between 1500 and 2000 rpm) a solution is as simple as installing 1/4 wave resonance tubes on the exhaust. This mod can be done on any system that has the normal amount of space (approx 2") between the pipes and the tunnel plate.

I have a 2000 C5 Coupe with an STS rear mount turbo kit. After the turbo install the interior resonance was a little too loud for me to enjoy cruising the hill country back roads.
After many "google" research evenings, I decided to try some practical engineering in sound reduction.


This method has been proven effective on many different cars in the past. The length of the tube is determined by the frequency of the drone. The higher the frequency, the shorter the tube.
The diameter of the tube determines the percent of reduction in the unwanted frequency. I chose 1 7/8" diameter for my application on a standard 2 1/2" diameter exhaust pipe
Since the available space is marginal, the design I employed uses the space between the exhaust pipes and the aluminum heat shield in the tunnel area. After much measuring and test fitting, I fashioned the tubes to start at the area on each side of the torque converter and run back along the existing exhaust pipes, toward the front of the car. (see pic)
A 1 7/8" hole was made in the exhaust where the 2" 90 degree elbo would just cover it. That provides the path for the sound to travel down the tube.

The difference in sound is very significant! Where I had a -11db 100Hz spike in sound level is now a -25db valley.
Overall exhaust sound between 1500 and 2000 rpm is much quieter in the car with no restriction in exhaust flow.

Test fitting prior to welding. The far end of the tubes are capped. Elbows are cut and ground to be a perfect saddle fit over the exhaust pipe.
A 1 7/8" hole is cut in the exhaust pipe where the 90 degree elbow will cover it. The elbow is 2" so the hole will be completely covered.


This shows a little over 2" between the exhaust and the tunnel plate.



This is before the install of the resonance tubes. The 100Hz peak is present at 1500rpm.


This is after the resonance tubes are installed. The 100Hz spike is gone and in its place is a -25db valley. This is also at 1500rpm.

 

That's impressive engineering on your part. I'm really surprised that you could squeeze the pipes into the tunnel and still get the tunnel plate on. Good work!

 

Yeah, how did that fit on the car? Do you have install pics? What's the deal with the 02 sensors? I would not think those are going on with ease...

 

The resonance tubes fit below the tunnel plate and basically rest on top of the existing exhaust pipes. The tubes are 1 7/8" diameter and there is about 2 1/4" between the original exhaust and the tunnel plate

 

It fit without modifying anything and there is room for the secondary O2 sensors if you have them.

 

The next time I have it up on the lift I will take some photos of the installed system.
During a test drive last night there was no discernible exhaust note inside the car while cruising at 60 to 70 mph.

 

Nicely done!

 

Very nice