[vinylengraver]

I'm reviving this in hopes of getting an answer to the following question:
In the stock setup (looking at the picture of its innards) when the flapper valve is shut the exhaust from the longer pipe (the one with the flap) exits through 3 large holes and ends up inside the muffler/resonator chamber while the shorter, perforated pipe flows freely.
If I weld those three big holes shut (basically a straight pipe now) but leave the other pipe as is, with the valve open the exhaust will flow through both pipes freely but with added loudness. Like the PE, which has no valve. All good.
Now, if I leave the flap functional - when it's closed - the exhaust gas has nowhere to go now but through the short perforated pipe, nice and muffled.
Makes sense.
But the pressure from the longer pipe has nowhere to go now - since there are no holes in it and it's closed off with the flap... How is the pressure relieved without creating major turbulence inside?
Or do I have it all wrong?
FYI, I wanted to use the ExhaustMeister module for those quiet mornings when all neighbors are still asleep...