Dual conical valve springs

I don't know where that dip in the torque comes from. All of the engines I test on this dyno have that little dip regardless of intake, carb, cam, headers, etc. that we use. It could be something in the dyno exhaust system. Maybe some sort of standing wave that hits right at that RPM. It is worse with some engines and better with others but every dyno sheet that I've looked at had a little bit of a dip around 5500 rpm.