How I got rid of the DRONE
 

I was going to use VT mufflers but found out they no longer make them. Had to try somethings else. I really like the exhaust sound just the drone between 1500rpm and 2000rpm was ridiculous. So bad I could not hear the radio when it got into that range and after driving the car for a while my ears would be ringing. I set myself a target of reducing the drone without changing the exhaust overall sound.
Car is a 67 GTO.
I am using the Pypes 2 1/2" X pipe system with the race pro mufflers. This is coming off Ram Air 2 1/4" manifolds.
The Pypes tailpipes ended just in front of the rear bumper lower lip so the sound was being transmitted up through the trunk and then into the cabin. Extending these a touch, only about 3” helped a bit but not enough.
Then I remembered a technique we used years ago on one of the Ford programs we were working on, I work at Ford BTW, where we fitted a so called 1/4 wave resonator, some call them J pipes, to a car that had too much drone for the chief Engineers liking. It worked on this so I thought I would make some and try.
These are just a calculated length of exhaust pipe, I used 1 1/2” diameter pipe, that is teed into each bank and is blanked off at the other end.
By knowing what engine rpm the drone occurs at you can calculate the sound frequency. In turn from this you can determine the sound wavelength. The aim then fabricate these pipes so that the sound wave travels down the pipe, bounces off the plugged end and is returned back into the main exhaust 180 degs out of phase to cancel out the drone sound.
I targeted the mid point rpm (1750rpm) and came up with a length of 850mm for my 1/4 wave resonators. Got these made and fitted.
I knew in theory these should have worked but was still amazed as to how well it worked. Mind you the drone is not 100% gone but subjectively I would say it’s about 80% gone.
Beauty of doing it this way there is no interference to the gas flow and overall sound.
They do look a bit odd under the car but hey, they work.
They can be any shape you want and in hindsight I could have bent them the other way around to they pointed to the back.

I got rid of mine when I took out the X pipe and put in a H.Tom

I read up on the J-pipe too when I was looking for solutions to my drone. Cool to see someone try it out. I wonder if it would have been more effective using same size 2.5 pipe?

Similar in theory, I guess, to what they did in '68?
Notice the pinched off portions that branch off the tail pipe-
https://assets.hemmings.com/story_im...00-0.jpg?rev=3

I was thinking the same thing!

From what I could find during looking at these things the diameter does not really play a big part. It is the length that is important. Having said that they still recommended no less than 1 1/2" dia for a 2.5" system.

Yep, looks like the same theory in that one. I would not have had the room to do that on the 67 GTO. I assume yours worked?

That wasn't mine, it's a photo from the Gardner system. From what I've read, it did work but caused problems down the road as they aged. The pinched off portion ended right in front of the tail lamp where rust-out and the resulting hot exhaust gases caused damage to the plastic lenses.
I believe that's why they went to a purpose built resonator on the '69s.

Thanks for posting. Glad you were successful.

I did the same thing but ran the j tubes above the mufflers. I have the x pipe setup too with the drone in the same rpm range. A little tight packaging all that with dougs cutouts also but the drone is mostly gone. Its a 70 GTO and this setup has been on there for 5 years or so.

On 2 of my cars,removing the X and replacing it with a H got rid of the drone.Tom

Or you can run Goerlich Xlerator mufflers - don't drone.

Can you post the equation for calculating length from the drone rpm?

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Why not just install resonators?

Resonators are broad band attenuation items. In my case I did not want to alter the overall sound of the exhaust, I just targeted the frequency where the drone was. Nothing wrong with resonators though.