Having made an excellent NA Engine manifold, the next step was to make a silencer with a good noise reduction and a low back pressure.
At the Danish Technical University I found a PHD study from 1970. In this there were descriptions of silencer systems for the city buses.
The guy had achieved an excellent noise reduction and improved fuel economy of more than 10%.
It consisted of multiple offset resonances (organ pipe) chambers (low pass filters). And no damping materials was involved.
I tried to convert this into my configuration with a 2 chamber design. After some iterations, and a lot of stainless steel welding, I got a increased power curve and reasonable good noise reduction.
Altogether the improvement including the 4:2:1 manifold was a power increase of 15-20%.