Glad you had some sucess and part of the drone is reduced.
Exhaust tips that are small mufflers will help a bit more.
I do not understand how you decided on the 25 inch length for the stub without measuring the frequency in Hertz of the drone sound beforehand.
Changing your fuel chemistry will probably change the frequency of the drone a bit
and/or the rpm where it is maximum loudness.
You could try “pure gas” without ethanol,
or create a test blend by adding some E85 to regular E10 gasohol.
Hotter summer air temperatures will also change the drone a bit.
I did measure it on my drive over (133 Hz). The equation I used that involved Hertz gave me a J-tube that was like 22 ft long. I either did something wrong with my units or had the wrong equation. It's very difficult to calculate the speed of sound as well as it's a function of multiple variable. I assumed standard pressure but a temperature of about 450-500F. After I had done the math using the 1/4 wave equation, I checked on other forums and they seemed to have some sort of success (whether minor or dramatic) when dealing with drone at 2000 rpm. So, I figured I was in the ballpark.
The plan in the future is to change the exhaust tips. I'm not sure to what, yet as I want something a little unique. After I graduate from my Master's in June, the plan is to remove the entire fuel system and install a Tanks Inc tank with a Fitech throttle body. So, that may change everything.
