[BMK]

Rob

Yes the Summit cones are the part you saw.

Seeing your work with the spiral insert and the later spiral coil I think you would have no problem making them up out of a piece of perforated sheet.

The total holes and size of holes add up to the internal pipe surface/opening so there is no restriction.

I spot welded them in and tried it with a meter at 45o at 1100mm. Stationary test.

The drive past was quoted with no evident HP drop. Then opened them and put three deeper spot welds in.

I have heard owners have used one at each end of the muffler for better results.