Dynomax race bullets on street

I have 10 or 12"(cant remember right off hand) x 3 1/2" bullet mufflers,
W/ 3 1/2" spiral inserts about 1/2 way up inside them tackwelded in place, no crossover ,fairly loud , no drone , very free flowing , dumped in front of the rear axle.


are those inserts something you made or is it off the shelf?