I will try to take a photo of the HC and post it tomorrow. I appreciate your test with the vacuum gauge, but since the HC is very easy to remove or put back, I will try it at the drag strip instead.
I have sort of used the gauge testing before, when I was trying to find a low restriction exhaust for my Raptor ATV. I used a low pressure tire gauge and shop vac
. I found one from a CBR600 crotch rocket that flowed almost as good as an open pipe but was as quiet as the stock exhaust.