[HawgRyder]

Another old trick...if you disconnect the muffler from the header...and insert a piece of steel (thin bar stock?) into the pipe...so that it fits snuggly either in a vertical or horizontal position...imagine dividing the pipe in half by this....and try it again.
What this does is stop the circular flow of gasses in that section of pipe...and...it divides the pipe into 2 pipes so the the frequency would be twice the original.
The nice ting here...is that there is very little reduction in pipe flow capability.
This used to work on old aircraft engines and some older cars.
Bob