The muffler itself is an open pass through to all four exit pipes so the rear valves would cut down the sound when closed leaving only two exit pipes. But what look like primary valves cut off half of the incoming flow to the muffler which I don't understand. Maybe the muffler can't handle the flow from two inlet pipes when only two exit pipes are open? There is an h-pipe up front past the secondary cats and an x-pipe at the rear in front of the primary valves. Is this balancing upstream scavenging and rear back pressure? However this was engineered would be interesting to understand. Sounds quite sophisticated. Any feedback or thoughts on this?